A Universal Serial Bus (USB) is used to connect a host, such as a PC or workstation, to a number of peripheral devices. USB uses a tree structure, with the host as the root (the system’s master), hubs as interior nodes, and peripherals as leaves (and slaves). Modern PCs support several such trees of USB devices, usually a few USB 3.0 (5 GBit/s) or USB 3.1 (10 GBit/s) and some legacy USB 2.0 (480 MBit/s) busses just in case.
That master/slave asymmetry was designed-in for a number of reasons, one being ease of use. It is not physically possible to mistake upstream and downstream or it does not matter with a type C plug (or they are built into the peripheral). Also, the host software doesn’t need to deal with distributed auto-configuration since the pre-designated master node manages all that.
Kernel developers added USB support to Linux early in the 2.2 kernel series and have been developing it further since then. Besides support for each new generation of USB, various host controllers gained support, new drivers for peripherals have been added and advanced features for latency measurement and improved power management introduced.
Linux can run inside USB devices as well as on the hosts that control the devices. But USB device drivers running inside those peripherals don’t do the same things as the ones running inside hosts, so they’ve been given a different name: gadget drivers. This document does not cover gadget drivers.
Host-side drivers for USB devices talk to the “usbcore” APIs. There are two. One is intended for general-purpose drivers (exposed through driver frameworks), and the other is for drivers that are part of the core. Such core drivers include the hub driver (which manages trees of USB devices) and several different kinds of host controller drivers, which control individual busses.
The device model seen by USB drivers is relatively complex.
USB supports four kinds of data transfers (control, bulk, interrupt, and isochronous). Two of them (control and bulk) use bandwidth as it’s available, while the other two (interrupt and isochronous) are scheduled to provide guaranteed bandwidth.
The device description model includes one or more “configurations” per device, only one of which is active at a time. Devices are supposed to be capable of operating at lower than their top speeds and may provide a BOS descriptor showing the lowest speed they remain fully operational at.
From USB 3.0 on configurations have one or more “functions”, which provide a common functionality and are grouped together for purposes of power management.
Configurations or functions have one or more “interfaces”, each of which may have “alternate settings”. Interfaces may be standardized by USB “Class” specifications, or may be specific to a vendor or device.
USB device drivers actually bind to interfaces, not devices. Think of them as “interface drivers”, though you may not see many devices where the distinction is important. Most USB devices are simple, with only one function, one configuration, one interface, and one alternate setting.
Interfaces have one or more “endpoints”, each of which supports one type and direction of data transfer such as “bulk out” or “interrupt in”. The entire configuration may have up to sixteen endpoints in each direction, allocated as needed among all the interfaces.
Data transfer on USB is packetized; each endpoint has a maximum packet size. Drivers must often be aware of conventions such as flagging the end of bulk transfers using “short” (including zero length) packets.
The Linux USB API supports synchronous calls for control and bulk messages. It also supports asynchronous calls for all kinds of data transfer, using request structures called “URBs” (USB Request Blocks).
Accordingly, the USB Core API exposed to device drivers covers quite a lot of territory. You’ll probably need to consult the USB 3.0 specification, available online from www.usb.org at no cost, as well as class or device specifications.
The only host-side drivers that actually touch hardware (reading/writing registers, handling IRQs, and so on) are the HCDs. In theory, all HCDs provide the same functionality through the same API. In practice, that’s becoming more true, but there are still differences that crop up especially with fault handling on the less common controllers. Different controllers don’t necessarily report the same aspects of failures, and recovery from faults (including software-induced ones like unlinking an URB) isn’t yet fully consistent. Device driver authors should make a point of doing disconnect testing (while the device is active) with each different host controller driver, to make sure drivers don’t have bugs of their own as well as to make sure they aren’t relying on some HCD-specific behavior.
In <linux/usb/ch9.h> you will find the USB data types defined in chapter 9 of the USB specification. These data types are used throughout USB, and in APIs including this host side API, gadget APIs, and usbfs.
Returns human readable-name of the speed.
Parameters
Get maximum requested speed for a given USB controller.
Parameters
Description
The function gets the maximum speed string from property “maximum-speed”, and returns the corresponding enum usb_device_speed.
Returns human readable name for the state.
Parameters
The host side API exposes several layers to drivers, some of which are more necessary than others. These support lifecycle models for host side drivers and devices, and support passing buffers through usbcore to some HCD that performs the I/O for the device driver.
host-side endpoint descriptor and queue
Definition
struct usb_host_endpoint {
struct usb_endpoint_descriptor desc;
struct usb_ss_ep_comp_descriptor ss_ep_comp;
struct usb_ssp_isoc_ep_comp_descriptor ssp_isoc_ep_comp;
struct list_head urb_list;
void * hcpriv;
struct ep_device * ep_dev;
unsigned char * extra;
int extralen;
int enabled;
int streams;
};
Members
Description
USB requests are always queued to a given endpoint, identified by a descriptor within an active interface in a given USB configuration.
what usb device drivers talk to
Definition
struct usb_interface {
struct usb_host_interface * altsetting;
struct usb_host_interface * cur_altsetting;
unsigned num_altsetting;
struct usb_interface_assoc_descriptor * intf_assoc;
int minor;
enum usb_interface_condition condition;
unsigned sysfs_files_created:1;
unsigned ep_devs_created:1;
unsigned unregistering:1;
unsigned needs_remote_wakeup:1;
unsigned needs_altsetting0:1;
unsigned needs_binding:1;
unsigned resetting_device:1;
unsigned authorized:1;
struct device dev;
struct device * usb_dev;
atomic_t pm_usage_cnt;
struct work_struct reset_ws;
};
Members
Description
USB device drivers attach to interfaces on a physical device. Each interface encapsulates a single high level function, such as feeding an audio stream to a speaker or reporting a change in a volume control. Many USB devices only have one interface. The protocol used to talk to an interface’s endpoints can be defined in a usb “class” specification, or by a product’s vendor. The (default) control endpoint is part of every interface, but is never listed among the interface’s descriptors.
The driver that is bound to the interface can use standard driver model calls such as dev_get_drvdata() on the dev member of this structure.
Each interface may have alternate settings. The initial configuration of a device sets altsetting 0, but the device driver can change that setting using usb_set_interface(). Alternate settings are often used to control the use of periodic endpoints, such as by having different endpoints use different amounts of reserved USB bandwidth. All standards-conformant USB devices that use isochronous endpoints will use them in non-default settings.
The USB specification says that alternate setting numbers must run from 0 to one less than the total number of alternate settings. But some devices manage to mess this up, and the structures aren’t necessarily stored in numerical order anyhow. Use usb_altnum_to_altsetting() to look up an alternate setting in the altsetting array based on its number.
long-term representation of a device interface
Definition
struct usb_interface_cache {
unsigned num_altsetting;
struct kref ref;
struct usb_host_interface altsetting[0];
};
Members
Description
These structures persist for the lifetime of a usb_device, unlike struct usb_interface (which persists only as long as its configuration is installed). The altsetting arrays can be accessed through these structures at any time, permitting comparison of configurations and providing support for the /proc/bus/usb/devices pseudo-file.
representation of a device’s configuration
Definition
struct usb_host_config {
struct usb_config_descriptor desc;
char * string;
struct usb_interface_assoc_descriptor * intf_assoc[USB_MAXIADS];
struct usb_interface * interface[USB_MAXINTERFACES];
struct usb_interface_cache * intf_cache[USB_MAXINTERFACES];
unsigned char * extra;
int extralen;
};
Members
Description
USB devices may have multiple configurations, but only one can be active at any time. Each encapsulates a different operational environment; for example, a dual-speed device would have separate configurations for full-speed and high-speed operation. The number of configurations available is stored in the device descriptor as bNumConfigurations.
A configuration can contain multiple interfaces. Each corresponds to a different function of the USB device, and all are available whenever the configuration is active. The USB standard says that interfaces are supposed to be numbered from 0 to desc.bNumInterfaces-1, but a lot of devices get this wrong. In addition, the interface array is not guaranteed to be sorted in numerical order. Use usb_ifnum_to_if() to look up an interface entry based on its number.
Device drivers should not attempt to activate configurations. The choice of which configuration to install is a policy decision based on such considerations as available power, functionality provided, and the user’s desires (expressed through userspace tools). However, drivers can call usb_reset_configuration() to reinitialize the current configuration and all its interfaces.
kernel’s representation of a USB device
Definition
struct usb_device {
int devnum;
char devpath[16];
u32 route;
enum usb_device_state state;
enum usb_device_speed speed;
struct usb_tt * tt;
int ttport;
unsigned int toggle[2];
struct usb_device * parent;
struct usb_bus * bus;
struct usb_host_endpoint ep0;
struct device dev;
struct usb_device_descriptor descriptor;
struct usb_host_bos * bos;
struct usb_host_config * config;
struct usb_host_config * actconfig;
struct usb_host_endpoint * ep_in[16];
struct usb_host_endpoint * ep_out[16];
char ** rawdescriptors;
unsigned short bus_mA;
u8 portnum;
u8 level;
unsigned can_submit:1;
unsigned persist_enabled:1;
unsigned have_langid:1;
unsigned authorized:1;
unsigned authenticated:1;
unsigned wusb:1;
unsigned lpm_capable:1;
unsigned usb2_hw_lpm_capable:1;
unsigned usb2_hw_lpm_besl_capable:1;
unsigned usb2_hw_lpm_enabled:1;
unsigned usb2_hw_lpm_allowed:1;
unsigned usb3_lpm_u1_enabled:1;
unsigned usb3_lpm_u2_enabled:1;
int string_langid;
char * product;
char * manufacturer;
char * serial;
struct list_head filelist;
int maxchild;
u32 quirks;
atomic_t urbnum;
unsigned long active_duration;
#ifdef CONFIG_PM
unsigned long connect_time;
unsigned do_remote_wakeup:1;
unsigned reset_resume:1;
unsigned port_is_suspended:1;
#endif
struct wusb_dev * wusb_dev;
int slot_id;
enum usb_device_removable removable;
struct usb2_lpm_parameters l1_params;
struct usb3_lpm_parameters u1_params;
struct usb3_lpm_parameters u2_params;
unsigned lpm_disable_count;
};
Members
Notes
Usbcore drivers should not set usbdev->state directly. Instead use usb_set_device_state().
iterate over all child devices on the hub
Parameters
returns true iff an interface is claimed
Parameters
Return
true (nonzero) iff the interface is claimed, else false (zero).
Note
Callers must own the driver model’s usb bus readlock. So driver probe() entries don’t need extra locking, but other call contexts may need to explicitly claim that lock.
returns stable device path in the usb tree
Parameters
Return
Length of the string (> 0) or negative if size was too small.
Note
This identifier is intended to be “stable”, reflecting physical paths in hardware such as physical bus addresses for host controllers or ports on USB hubs. That makes it stay the same until systems are physically reconfigured, by re-cabling a tree of USB devices or by moving USB host controllers. Adding and removing devices, including virtual root hubs in host controller driver modules, does not change these path identifiers; neither does rebooting or re-enumerating. These are more useful identifiers than changeable (“unstable”) ones like bus numbers or device addresses.
With a partial exception for devices connected to USB 2.0 root hubs, these identifiers are also predictable. So long as the device tree isn’t changed, plugging any USB device into a given hub port always gives it the same path. Because of the use of “companion” controllers, devices connected to ports on USB 2.0 root hubs (EHCI host controllers) will get one path ID if they are high speed, and a different one if they are full or low speed.
macro used to describe a specific usb device
Parameters
Description
This macro is used to create a struct usb_device_id that matches a specific device.
describe a specific usb device with a version range
Parameters
Description
This macro is used to create a struct usb_device_id that matches a specific device, with a version range.
describe a usb device with a specific interface class
Parameters
Description
This macro is used to create a struct usb_device_id that matches a specific interface class of devices.
describe a usb device with a specific interface protocol
Parameters
Description
This macro is used to create a struct usb_device_id that matches a specific interface protocol of devices.
describe a usb device with a specific interface number
Parameters
Description
This macro is used to create a struct usb_device_id that matches a specific interface number of devices.
macro used to describe a class of usb devices
Parameters
Description
This macro is used to create a struct usb_device_id that matches a specific class of devices.
macro used to describe a class of usb interfaces
Parameters
Description
This macro is used to create a struct usb_device_id that matches a specific class of interfaces.
describe a specific usb device with a class of usb interfaces
Parameters
Description
This macro is used to create a struct usb_device_id that matches a specific device with a specific class of interfaces.
This is especially useful when explicitly matching devices that have vendor specific bDeviceClass values, but standards-compliant interfaces.
describe a specific usb vendor with a class of usb interfaces
Parameters
Description
This macro is used to create a struct usb_device_id that matches a specific vendor with a specific class of interfaces.
This is especially useful when explicitly matching devices that have vendor specific bDeviceClass values, but standards-compliant interfaces.
wrapper for driver-model structure
Definition
struct usbdrv_wrap {
struct device_driver driver;
int for_devices;
};
Members
identifies USB interface driver to usbcore
Definition
struct usb_driver {
const char * name;
int (* probe) (struct usb_interface *intf,const struct usb_device_id *id);
void (* disconnect) (struct usb_interface *intf);
int (* unlocked_ioctl) (struct usb_interface *intf, unsigned int code,void *buf);
int (* suspend) (struct usb_interface *intf, pm_message_t message);
int (* resume) (struct usb_interface *intf);
int (* reset_resume) (struct usb_interface *intf);
int (* pre_reset) (struct usb_interface *intf);
int (* post_reset) (struct usb_interface *intf);
const struct usb_device_id * id_table;
struct usb_dynids dynids;
struct usbdrv_wrap drvwrap;
unsigned int no_dynamic_id:1;
unsigned int supports_autosuspend:1;
unsigned int disable_hub_initiated_lpm:1;
unsigned int soft_unbind:1;
};
Members
Description
USB interface drivers must provide a name, probe() and disconnect() methods, and an id_table. Other driver fields are optional.
The id_table is used in hotplugging. It holds a set of descriptors, and specialized data may be associated with each entry. That table is used by both user and kernel mode hotplugging support.
The probe() and disconnect() methods are called in a context where they can sleep, but they should avoid abusing the privilege. Most work to connect to a device should be done when the device is opened, and undone at the last close. The disconnect code needs to address concurrency issues with respect to open() and close() methods, as well as forcing all pending I/O requests to complete (by unlinking them as necessary, and blocking until the unlinks complete).
identifies USB device driver to usbcore
Definition
struct usb_device_driver {
const char * name;
int (* probe) (struct usb_device *udev);
void (* disconnect) (struct usb_device *udev);
int (* suspend) (struct usb_device *udev, pm_message_t message);
int (* resume) (struct usb_device *udev, pm_message_t message);
struct usbdrv_wrap drvwrap;
unsigned int supports_autosuspend:1;
};
Members
Description
USB drivers must provide all the fields listed above except drvwrap.
identifies a USB driver that wants to use the USB major number
Definition
struct usb_class_driver {
char * name;
char *(* devnode) (struct device *dev, umode_t *mode);
const struct file_operations * fops;
int minor_base;
};
Members
Description
This structure is used for the usb_register_dev() and usb_deregister_dev() functions, to consolidate a number of the parameters used for them.
Helper macro for registering a USB driver
Parameters
Description
Helper macro for USB drivers which do not do anything special in module init/exit. This eliminates a lot of boilerplate. Each module may only use this macro once, and calling it replaces module_init() and module_exit()
USB Request Block
Definition
struct urb {
struct list_head urb_list;
struct list_head anchor_list;
struct usb_anchor * anchor;
struct usb_device * dev;
struct usb_host_endpoint * ep;
unsigned int pipe;
unsigned int stream_id;
int status;
unsigned int transfer_flags;
void * transfer_buffer;
dma_addr_t transfer_dma;
struct scatterlist * sg;
int num_mapped_sgs;
int num_sgs;
u32 transfer_buffer_length;
u32 actual_length;
unsigned char * setup_packet;
dma_addr_t setup_dma;
int start_frame;
int number_of_packets;
int interval;
int error_count;
void * context;
usb_complete_t complete;
struct usb_iso_packet_descriptor iso_frame_desc[0];
};
Members
Description
This structure identifies USB transfer requests. URBs must be allocated by calling usb_alloc_urb() and freed with a call to usb_free_urb(). Initialization may be done using various usb_fill_*:c:func:_urb() functions. URBs are submitted using usb_submit_urb(), and pending requests may be canceled using usb_unlink_urb() or usb_kill_urb().
Data Transfer Buffers:
Normally drivers provide I/O buffers allocated with kmalloc() or otherwise taken from the general page pool. That is provided by transfer_buffer (control requests also use setup_packet), and host controller drivers perform a dma mapping (and unmapping) for each buffer transferred. Those mapping operations can be expensive on some platforms (perhaps using a dma bounce buffer or talking to an IOMMU), although they’re cheap on commodity x86 and ppc hardware.
Alternatively, drivers may pass the URB_NO_TRANSFER_DMA_MAP transfer flag, which tells the host controller driver that no such mapping is needed for the transfer_buffer since the device driver is DMA-aware. For example, a device driver might allocate a DMA buffer with usb_alloc_coherent() or call usb_buffer_map(). When this transfer flag is provided, host controller drivers will attempt to use the dma address found in the transfer_dma field rather than determining a dma address themselves.
Note that transfer_buffer must still be set if the controller does not support DMA (as indicated by bus.uses_dma) and when talking to root hub. If you have to trasfer between highmem zone and the device on such controller, create a bounce buffer or bail out with an error. If transfer_buffer cannot be set (is in highmem) and the controller is DMA capable, assign NULL to it, so that usbmon knows not to use the value. The setup_packet must always be set, so it cannot be located in highmem.
Initialization:
All URBs submitted must initialize the dev, pipe, transfer_flags (may be zero), and complete fields. All URBs must also initialize transfer_buffer and transfer_buffer_length. They may provide the URB_SHORT_NOT_OK transfer flag, indicating that short reads are to be treated as errors; that flag is invalid for write requests.
Bulk URBs may use the URB_ZERO_PACKET transfer flag, indicating that bulk OUT transfers should always terminate with a short packet, even if it means adding an extra zero length packet.
Control URBs must provide a valid pointer in the setup_packet field. Unlike the transfer_buffer, the setup_packet may not be mapped for DMA beforehand.
Interrupt URBs must provide an interval, saying how often (in milliseconds or, for highspeed devices, 125 microsecond units) to poll for transfers. After the URB has been submitted, the interval field reflects how the transfer was actually scheduled. The polling interval may be more frequent than requested. For example, some controllers have a maximum interval of 32 milliseconds, while others support intervals of up to 1024 milliseconds. Isochronous URBs also have transfer intervals. (Note that for isochronous endpoints, as well as high speed interrupt endpoints, the encoding of the transfer interval in the endpoint descriptor is logarithmic. Device drivers must convert that value to linear units themselves.)
If an isochronous endpoint queue isn’t already running, the host controller will schedule a new URB to start as soon as bandwidth utilization allows. If the queue is running then a new URB will be scheduled to start in the first transfer slot following the end of the preceding URB, if that slot has not already expired. If the slot has expired (which can happen when IRQ delivery is delayed for a long time), the scheduling behavior depends on the URB_ISO_ASAP flag. If the flag is clear then the URB will be scheduled to start in the expired slot, implying that some of its packets will not be transferred; if the flag is set then the URB will be scheduled in the first unexpired slot, breaking the queue’s synchronization. Upon URB completion, the start_frame field will be set to the (micro)frame number in which the transfer was scheduled. Ranges for frame counter values are HC-specific and can go from as low as 256 to as high as 65536 frames.
Isochronous URBs have a different data transfer model, in part because the quality of service is only “best effort”. Callers provide specially allocated URBs, with number_of_packets worth of iso_frame_desc structures at the end. Each such packet is an individual ISO transfer. Isochronous URBs are normally queued, submitted by drivers to arrange that transfers are at least double buffered, and then explicitly resubmitted in completion handlers, so that data (such as audio or video) streams at as constant a rate as the host controller scheduler can support.
Completion Callbacks:
The completion callback is made in_interrupt(), and one of the first things that a completion handler should do is check the status field. The status field is provided for all URBs. It is used to report unlinked URBs, and status for all non-ISO transfers. It should not be examined before the URB is returned to the completion handler.
The context field is normally used to link URBs back to the relevant driver or request state.
When the completion callback is invoked for non-isochronous URBs, the actual_length field tells how many bytes were transferred. This field is updated even when the URB terminated with an error or was unlinked.
ISO transfer status is reported in the status and actual_length fields of the iso_frame_desc array, and the number of errors is reported in error_count. Completion callbacks for ISO transfers will normally (re)submit URBs to ensure a constant transfer rate.
Note that even fields marked “public” should not be touched by the driver when the urb is owned by the hcd, that is, since the call to usb_submit_urb() till the entry into the completion routine.
initializes a control urb
Parameters
Description
Initializes a control urb with the proper information needed to submit it to a device.
macro to help initialize a bulk urb
Parameters
Description
Initializes a bulk urb with the proper information needed to submit it to a device.
macro to help initialize a interrupt urb
Parameters
Description
Initializes a interrupt urb with the proper information needed to submit it to a device.
Note that High Speed and SuperSpeed(+) interrupt endpoints use a logarithmic encoding of the endpoint interval, and express polling intervals in microframes (eight per millisecond) rather than in frames (one per millisecond).
Wireless USB also uses the logarithmic encoding, but specifies it in units of 128us instead of 125us. For Wireless USB devices, the interval is passed through to the host controller, rather than being translated into microframe units.
Parameters
Return
1 if urb describes an IN transfer (device-to-host), otherwise 0.
Parameters
Return
1 if urb describes an OUT transfer (host-to-device), otherwise 0.
support for scatter/gather I/O
Definition
struct usb_sg_request {
int status;
size_t bytes;
};
Members
Description
These requests are initialized using usb_sg_init(), and then are used as request handles passed to usb_sg_wait() or usb_sg_cancel(). Most members of the request object aren’t for driver access.
The status and bytecount values are valid only after usb_sg_wait() returns. If the status is zero, then the bytecount matches the total from the request.
After an error completion, drivers may need to clear a halt condition on the endpoint.
There are two basic I/O models in the USB API. The most elemental one is asynchronous: drivers submit requests in the form of an URB, and the URB’s completion callback handles the next step. All USB transfer types support that model, although there are special cases for control URBs (which always have setup and status stages, but may not have a data stage) and isochronous URBs (which allow large packets and include per-packet fault reports). Built on top of that is synchronous API support, where a driver calls a routine that allocates one or more URBs, submits them, and waits until they complete. There are synchronous wrappers for single-buffer control and bulk transfers (which are awkward to use in some driver disconnect scenarios), and for scatterlist based streaming i/o (bulk or interrupt).
USB drivers need to provide buffers that can be used for DMA, although they don’t necessarily need to provide the DMA mapping themselves. There are APIs to use used when allocating DMA buffers, which can prevent use of bounce buffers on some systems. In some cases, drivers may be able to rely on 64bit DMA to eliminate another kind of bounce buffer.
Parameters
Description
Initializes a urb so that the USB subsystem can use it properly.
If a urb is created with a call to usb_alloc_urb() it is not necessary to call this function. Only use this if you allocate the space for a struct urb on your own. If you call this function, be careful when freeing the memory for your urb that it is no longer in use by the USB core.
Only use this function if you _really_ understand what you are doing.
creates a new urb for a USB driver to use
Parameters
Description
Creates an urb for the USB driver to use, initializes a few internal structures, increments the usage counter, and returns a pointer to it.
If the driver want to use this urb for interrupt, control, or bulk endpoints, pass ‘0’ as the number of iso packets.
The driver must call usb_free_urb() when it is finished with the urb.
Return
A pointer to the new urb, or NULL if no memory is available.
frees the memory used by a urb when all users of it are finished
Parameters
Description
Must be called when a user of a urb is finished with it. When the last user of the urb calls this function, the memory of the urb is freed.
Note
The transfer buffer associated with the urb is not freed unless the URB_FREE_BUFFER transfer flag is set.
Parameters
Description
This must be called whenever a urb is transferred from a device driver to a host controller driver. This allows proper reference counting to happen for urbs.
Return
A pointer to the urb with the incremented reference counter.
anchors an URB while it is processed
Parameters
Description
This can be called to have access to URBs which are to be executed without bothering to track them
Parameters
Description
Call this to stop the system keeping track of this URB
issue an asynchronous transfer request for an endpoint
Parameters
Description
This submits a transfer request, and transfers control of the URB describing that request to the USB subsystem. Request completion will be indicated later, asynchronously, by calling the completion handler. The three types of completion are success, error, and unlink (a software-induced fault, also called “request cancellation”).
URBs may be submitted in interrupt context.
The caller must have correctly initialized the URB before submitting it. Functions such as usb_fill_bulk_urb() and usb_fill_control_urb() are available to ensure that most fields are correctly initialized, for the particular kind of transfer, although they will not initialize any transfer flags.
If the submission is successful, the complete() callback from the URB will be called exactly once, when the USB core and Host Controller Driver (HCD) are finished with the URB. When the completion function is called, control of the URB is returned to the device driver which issued the request. The completion handler may then immediately free or reuse that URB.
With few exceptions, USB device drivers should never access URB fields provided by usbcore or the HCD until its complete() is called. The exceptions relate to periodic transfer scheduling. For both interrupt and isochronous urbs, as part of successful URB submission urb->interval is modified to reflect the actual transfer period used (normally some power of two units). And for isochronous urbs, urb->start_frame is modified to reflect when the URB’s transfers were scheduled to start.
Not all isochronous transfer scheduling policies will work, but most host controller drivers should easily handle ISO queues going from now until 10-200 msec into the future. Drivers should try to keep at least one or two msec of data in the queue; many controllers require that new transfers start at least 1 msec in the future when they are added. If the driver is unable to keep up and the queue empties out, the behavior for new submissions is governed by the URB_ISO_ASAP flag. If the flag is set, or if the queue is idle, then the URB is always assigned to the first available (and not yet expired) slot in the endpoint’s schedule. If the flag is not set and the queue is active then the URB is always assigned to the next slot in the schedule following the end of the endpoint’s previous URB, even if that slot is in the past. When a packet is assigned in this way to a slot that has already expired, the packet is not transmitted and the corresponding usb_iso_packet_descriptor’s status field will return -EXDEV. If this would happen to all the packets in the URB, submission fails with a -EXDEV error code.
For control endpoints, the synchronous usb_control_msg() call is often used (in non-interrupt context) instead of this call. That is often used through convenience wrappers, for the requests that are standardized in the USB 2.0 specification. For bulk endpoints, a synchronous usb_bulk_msg() call is available.
Return
0 on successful submissions. A negative error number otherwise.
Request Queuing:
URBs may be submitted to endpoints before previous ones complete, to minimize the impact of interrupt latencies and system overhead on data throughput. With that queuing policy, an endpoint’s queue would never be empty. This is required for continuous isochronous data streams, and may also be required for some kinds of interrupt transfers. Such queuing also maximizes bandwidth utilization by letting USB controllers start work on later requests before driver software has finished the completion processing for earlier (successful) requests.
As of Linux 2.6, all USB endpoint transfer queues support depths greater than one. This was previously a HCD-specific behavior, except for ISO transfers. Non-isochronous endpoint queues are inactive during cleanup after faults (transfer errors or cancellation).
Reserved Bandwidth Transfers:
Periodic transfers (interrupt or isochronous) are performed repeatedly, using the interval specified in the urb. Submitting the first urb to the endpoint reserves the bandwidth necessary to make those transfers. If the USB subsystem can’t allocate sufficient bandwidth to perform the periodic request, submitting such a periodic request should fail.
For devices under xHCI, the bandwidth is reserved at configuration time, or when the alt setting is selected. If there is not enough bus bandwidth, the configuration/alt setting request will fail. Therefore, submissions to periodic endpoints on devices under xHCI should never fail due to bandwidth constraints.
Device drivers must explicitly request that repetition, by ensuring that some URB is always on the endpoint’s queue (except possibly for short periods during completion callbacks). When there is no longer an urb queued, the endpoint’s bandwidth reservation is canceled. This means drivers can use their completion handlers to ensure they keep bandwidth they need, by reinitializing and resubmitting the just-completed urb until the driver longer needs that periodic bandwidth.
Memory Flags:
The general rules for how to decide which mem_flags to use are the same as for kmalloc. There are four different possible values; GFP_KERNEL, GFP_NOFS, GFP_NOIO and GFP_ATOMIC.
GFP_NOFS is not ever used, as it has not been implemented yet.
GFP_NOIO is used in the block io path and error handling of storage devices.
All other situations use GFP_KERNEL.
Parameters
Description
This routine cancels an in-progress request. URBs complete only once per submission, and may be canceled only once per submission. Successful cancellation means termination of urb will be expedited and the completion handler will be called with a status code indicating that the request has been canceled (rather than any other code).
Drivers should not call this routine or related routines, such as usb_kill_urb() or usb_unlink_anchored_urbs(), after their disconnect method has returned. The disconnect function should synchronize with a driver’s I/O routines to insure that all URB-related activity has completed before it returns.
This request is asynchronous, however the HCD might call the ->:c:func:complete() callback during unlink. Therefore when drivers call usb_unlink_urb(), they must not hold any locks that may be taken by the completion function. Success is indicated by returning -EINPROGRESS, at which time the URB will probably not yet have been given back to the device driver. When it is eventually called, the completion function will see urb->status == -ECONNRESET. Failure is indicated by usb_unlink_urb() returning any other value. Unlinking will fail when urb is not currently “linked” (i.e., it was never submitted, or it was unlinked before, or the hardware is already finished with it), even if the completion handler has not yet run.
The URB must not be deallocated while this routine is running. In particular, when a driver calls this routine, it must insure that the completion handler cannot deallocate the URB.
Return
-EINPROGRESS on success. See description for other values on failure.
Unlinking and Endpoint Queues:
[The behaviors and guarantees described below do not apply to virtual root hubs but only to endpoint queues for physical USB devices.]
Host Controller Drivers (HCDs) place all the URBs for a particular endpoint in a queue. Normally the queue advances as the controller hardware processes each request. But when an URB terminates with an error its queue generally stops (see below), at least until that URB’s completion routine returns. It is guaranteed that a stopped queue will not restart until all its unlinked URBs have been fully retired, with their completion routines run, even if that’s not until some time after the original completion handler returns. The same behavior and guarantee apply when an URB terminates because it was unlinked.
Bulk and interrupt endpoint queues are guaranteed to stop whenever an URB terminates with any sort of error, including -ECONNRESET, -ENOENT, and -EREMOTEIO. Control endpoint queues behave the same way except that they are not guaranteed to stop for -EREMOTEIO errors. Queues for isochronous endpoints are treated differently, because they must advance at fixed rates. Such queues do not stop when an URB encounters an error or is unlinked. An unlinked isochronous URB may leave a gap in the stream of packets; it is undefined whether such gaps can be filled in.
Note that early termination of an URB because a short packet was received will generate a -EREMOTEIO error if and only if the URB_SHORT_NOT_OK flag is set. By setting this flag, USB device drivers can build deep queues for large or complex bulk transfers and clean them up reliably after any sort of aborted transfer by unlinking all pending URBs at the first fault.
When a control URB terminates with an error other than -EREMOTEIO, it is quite likely that the status stage of the transfer will not take place.
Parameters
Description
This routine cancels an in-progress request. It is guaranteed that upon return all completion handlers will have finished and the URB will be totally idle and available for reuse. These features make this an ideal way to stop I/O in a disconnect() callback or close() function. If the request has not already finished or been unlinked the completion handler will see urb->status == -ENOENT.
While the routine is running, attempts to resubmit the URB will fail with error -EPERM. Thus even if the URB’s completion handler always tries to resubmit, it will not succeed and the URB will become idle.
The URB must not be deallocated while this routine is running. In particular, when a driver calls this routine, it must insure that the completion handler cannot deallocate the URB.
This routine may not be used in an interrupt context (such as a bottom half or a completion handler), or when holding a spinlock, or in other situations where the caller can’t schedule().
This routine should not be called by a driver after its disconnect method has returned.
Parameters
Description
This routine cancels an in-progress request. It is guaranteed that upon return all completion handlers will have finished and the URB will be totally idle and cannot be reused. These features make this an ideal way to stop I/O in a disconnect() callback. If the request has not already finished or been unlinked the completion handler will see urb->status == -ENOENT.
After and while the routine runs, attempts to resubmit the URB will fail with error -EPERM. Thus even if the URB’s completion handler always tries to resubmit, it will not succeed and the URB will become idle.
The URB must not be deallocated while this routine is running. In particular, when a driver calls this routine, it must insure that the completion handler cannot deallocate the URB.
This routine may not be used in an interrupt context (such as a bottom half or a completion handler), or when holding a spinlock, or in other situations where the caller can’t schedule().
This routine should not be called by a driver after its disconnect method has returned.
Parameters
Description
After the routine has run, attempts to resubmit the URB will fail with error -EPERM. Thus even if the URB’s completion handler always tries to resubmit, it will not succeed and the URB will become idle.
The URB must not be deallocated while this routine is running. In particular, when a driver calls this routine, it must insure that the completion handler cannot deallocate the URB.
cancel transfer requests en masse
Parameters
Description
this allows all outstanding URBs to be killed starting from the back of the queue
This routine should not be called by a driver after its disconnect method has returned.
cease all traffic from an anchor
Parameters
Description
this allows all outstanding URBs to be poisoned starting from the back of the queue. Newly added URBs will also be poisoned
This routine should not be called by a driver after its disconnect method has returned.
let an anchor be used successfully again
Parameters
Description
Reverses the effect of usb_poison_anchored_urbs the anchor can be used normally after it returns
asynchronously cancel transfer requests en masse
Parameters
Description
this allows all outstanding URBs to be unlinked starting from the back of the queue. This function is asynchronous. The unlinking is just triggered. It may happen after this function has returned.
This routine should not be called by a driver after its disconnect method has returned.
Parameters
Description
Call this to stop the last urb being unanchored from waking up any usb_wait_anchor_empty_timeout waiters. This is used in the hcd urb give- back path to delay waking up until after the completion handler has run.
Parameters
Description
Allow usb_wait_anchor_empty_timeout waiters to be woken up again, and wake up any current waiters if the anchor is empty.
wait for an anchor to be unused
Parameters
Description
Call this is you want to be sure all an anchor’s URBs have finished
Return
Non-zero if the anchor became unused. Zero on timeout.
Parameters
Description
This will take the oldest urb from an anchor, unanchor and return it
Return
The oldest urb from anchor, or NULL if anchor has no urbs associated with it.
unanchor all an anchor’s urbs
Parameters
Description
use this to get rid of all an anchor’s urbs
is an anchor empty
Parameters
Return
1 if the anchor has no urbs associated with it.
Builds a control urb, sends it off and waits for completion
Parameters
Context
!in_interrupt ()
Description
This function sends a simple control message to a specified endpoint and waits for the message to complete, or timeout.
Don’t use this function from within an interrupt context. If you need an asynchronous message, or need to send a message from within interrupt context, use usb_submit_urb(). If a thread in your driver uses this call, make sure your disconnect() method can wait for it to complete. Since you don’t have a handle on the URB used, you can’t cancel the request.
Return
If successful, the number of bytes transferred. Otherwise, a negative error number.
Builds an interrupt urb, sends it off and waits for completion
Parameters
Context
!in_interrupt ()
Description
This function sends a simple interrupt message to a specified endpoint and waits for the message to complete, or timeout.
Don’t use this function from within an interrupt context. If you need an asynchronous message, or need to send a message from within interrupt context, use usb_submit_urb() If a thread in your driver uses this call, make sure your disconnect() method can wait for it to complete. Since you don’t have a handle on the URB used, you can’t cancel the request.
Return
If successful, 0. Otherwise a negative error number. The number of actual bytes transferred will be stored in the actual_length parameter.
Builds a bulk urb, sends it off and waits for completion
Parameters
Context
!in_interrupt ()
Description
This function sends a simple bulk message to a specified endpoint and waits for the message to complete, or timeout.
Don’t use this function from within an interrupt context. If you need an asynchronous message, or need to send a message from within interrupt context, use usb_submit_urb() If a thread in your driver uses this call, make sure your disconnect() method can wait for it to complete. Since you don’t have a handle on the URB used, you can’t cancel the request.
Because there is no usb_interrupt_msg() and no USBDEVFS_INTERRUPT ioctl, users are forced to abuse this routine by using it to submit URBs for interrupt endpoints. We will take the liberty of creating an interrupt URB (with the default interval) if the target is an interrupt endpoint.
Return
If successful, 0. Otherwise a negative error number. The number of actual bytes transferred will be stored in the actual_length parameter.
initializes scatterlist-based bulk/interrupt I/O request
Parameters
Description
This initializes a scatter/gather request, allocating resources such as I/O mappings and urb memory (except maybe memory used by USB controller drivers).
The request must be issued using usb_sg_wait(), which waits for the I/O to complete (or to be canceled) and then cleans up all resources allocated by usb_sg_init().
The request may be canceled with usb_sg_cancel(), either before or after usb_sg_wait() is called.
Return
Zero for success, else a negative errno value.
synchronously execute scatter/gather request
Parameters
Context
!in_interrupt ()
Description
This function blocks until the specified I/O operation completes. It leverages the grouping of the related I/O requests to get good transfer rates, by queueing the requests. At higher speeds, such queuing can significantly improve USB throughput.
There are three kinds of completion for this function. (1) success, where io->status is zero. The number of io->bytes
transferred is as requested.
When this function returns, all memory allocated through usb_sg_init() or this call will have been freed. The request block parameter may still be passed to usb_sg_cancel(), or it may be freed. It could also be reinitialized and then reused.
Data Transfer Rates:
Bulk transfers are valid for full or high speed endpoints. The best full speed data rate is 19 packets of 64 bytes each per frame, or 1216 bytes per millisecond. The best high speed data rate is 13 packets of 512 bytes each per microframe, or 52 KBytes per millisecond.
The reason to use interrupt transfers through this API would most likely be to reserve high speed bandwidth, where up to 24 KBytes per millisecond could be transferred. That capability is less useful for low or full speed interrupt endpoints, which allow at most one packet per millisecond, of at most 8 or 64 bytes (respectively).
It is not necessary to call this function to reserve bandwidth for devices under an xHCI host controller, as the bandwidth is reserved when the configuration or interface alt setting is selected.
stop scatter/gather i/o issued by usb_sg_wait()
Parameters
Description
This stops a request after it has been started by usb_sg_wait(). It can also prevents one initialized by usb_sg_init() from starting, so that call just frees resources allocated to the request.
issues a generic GET_DESCRIPTOR request
Parameters
Context
!in_interrupt ()
Description
Gets a USB descriptor. Convenience functions exist to simplify getting some types of descriptors. Use usb_get_string() or usb_string() for USB_DT_STRING. Device (USB_DT_DEVICE) and configuration descriptors (USB_DT_CONFIG) are part of the device structure. In addition to a number of USB-standard descriptors, some devices also use class-specific or vendor-specific descriptors.
This call is synchronous, and may not be used in an interrupt context.
Return
The number of bytes received on success, or else the status code returned by the underlying usb_control_msg() call.
returns UTF-8 version of a string descriptor
Parameters
Context
!in_interrupt ()
Description
This converts the UTF-16LE encoded strings returned by devices, from usb_get_string_descriptor(), to null-terminated UTF-8 encoded ones that are more usable in most kernel contexts. Note that this function chooses strings in the first language supported by the device.
This call is synchronous, and may not be used in an interrupt context.
Return
length of the string (>= 0) or usb_control_msg status (< 0).
issues a GET_STATUS call
Parameters
Context
!in_interrupt ()
Description
Returns device, interface, or endpoint status. Normally only of interest to see if the device is self powered, or has enabled the remote wakeup facility; or whether a bulk or interrupt endpoint is halted (“stalled”).
Bits in these status bitmaps are set using the SET_FEATURE request, and cleared using the CLEAR_FEATURE request. The usb_clear_halt() function should be used to clear halt (“stall”) status.
This call is synchronous, and may not be used in an interrupt context.
Returns 0 and the status value in *data (in host byte order) on success, or else the status code from the underlying usb_control_msg() call.
tells device to clear endpoint halt/stall condition
Parameters
Context
!in_interrupt ()
Description
This is used to clear halt conditions for bulk and interrupt endpoints, as reported by URB completion status. Endpoints that are halted are sometimes referred to as being “stalled”. Such endpoints are unable to transmit or receive data until the halt status is cleared. Any URBs queued for such an endpoint should normally be unlinked by the driver before clearing the halt condition, as described in sections 5.7.5 and 5.8.5 of the USB 2.0 spec.
Note that control and isochronous endpoints don’t halt, although control endpoints report “protocol stall” (for unsupported requests) using the same status code used to report a true stall.
This call is synchronous, and may not be used in an interrupt context.
Return
Zero on success, or else the status code returned by the underlying usb_control_msg() call.
Reset an endpoint’s state.
Parameters
Description
Resets any host-side endpoint state such as the toggle bit, sequence number or current window.
Makes a particular alternate setting be current
Parameters
Context
!in_interrupt ()
Description
This is used to enable data transfers on interfaces that may not be enabled by default. Not all devices support such configurability. Only the driver bound to an interface may change its setting.
Within any given configuration, each interface may have several alternative settings. These are often used to control levels of bandwidth consumption. For example, the default setting for a high speed interrupt endpoint may not send more than 64 bytes per microframe, while interrupt transfers of up to 3KBytes per microframe are legal. Also, isochronous endpoints may never be part of an interface’s default setting. To access such bandwidth, alternate interface settings must be made current.
Note that in the Linux USB subsystem, bandwidth associated with an endpoint in a given alternate setting is not reserved until an URB is submitted that needs that bandwidth. Some other operating systems allocate bandwidth early, when a configuration is chosen.
This call is synchronous, and may not be used in an interrupt context. Also, drivers must not change altsettings while urbs are scheduled for endpoints in that interface; all such urbs must first be completed (perhaps forced by unlinking).
Return
Zero on success, or else the status code returned by the underlying usb_control_msg() call.
lightweight device reset
Parameters
Description
This issues a standard SET_CONFIGURATION request to the device using the current configuration. The effect is to reset most USB-related state in the device, including interface altsettings (reset to zero), endpoint halts (cleared), and endpoint state (only for bulk and interrupt endpoints). Other usbcore state is unchanged, including bindings of usb device drivers to interfaces.
Because this affects multiple interfaces, avoid using this with composite (multi-interface) devices. Instead, the driver for each interface may use usb_set_interface() on the interfaces it claims. Be careful though; some devices don’t support the SET_INTERFACE request, and others won’t reset all the interface state (notably endpoint state). Resetting the whole configuration would affect other drivers’ interfaces.
The caller must own the device lock.
Return
Zero on success, else a negative error code.
Provide a way for drivers to change device configurations
Parameters
Context
In process context, must be able to sleep
Description
Device interface drivers are not allowed to change device configurations. This is because changing configurations will destroy the interface the driver is bound to and create new ones; it would be like a floppy-disk driver telling the computer to replace the floppy-disk drive with a tape drive!
Still, in certain specialized circumstances the need may arise. This routine gets around the normal restrictions by using a work thread to submit the change-config request.
Return
0 if the request was successfully queued, error code otherwise. The caller has no way to know whether the queued request will eventually succeed.
parse the extra headers present in CDC devices
Parameters
Description
This evaluates the extra headers present in CDC devices which bind the interfaces for data and control and provide details about the capabilities of the device.
Return
number of descriptors parsed or -EINVAL if the header is contradictory beyond salvage
register a USB device, and ask for a minor number
Parameters
Description
This should be called by all USB drivers that use the USB major number. If CONFIG_USB_DYNAMIC_MINORS is enabled, the minor number will be dynamically allocated out of the list of available ones. If it is not enabled, the minor number will be based on the next available free minor, starting at the class_driver->minor_base.
This function also creates a usb class device in the sysfs tree.
usb_deregister_dev() must be called when the driver is done with the minor numbers given out by this function.
Return
-EINVAL if something bad happens with trying to register a device, and 0 on success.
deregister a USB device’s dynamic minor.
Parameters
Description
Used in conjunction with usb_register_dev(). This function is called when the USB driver is finished with the minor numbers gotten from a call to usb_register_dev() (usually when the device is disconnected from the system.)
This function also removes the usb class device from the sysfs tree.
This should be called by all drivers that use the USB major number.
bind a driver to an interface
Parameters
Description
This is used by usb device drivers that need to claim more than one interface on a device when probing (audio and acm are current examples). No device driver should directly modify internal usb_interface or usb_device structure members.
Few drivers should need to use this routine, since the most natural way to bind to an interface is to return the private data from the driver’s probe() method.
Callers must own the device lock, so driver probe() entries don’t need extra locking, but other call contexts may need to explicitly claim that lock.
Return
0 on success.
unbind a driver from an interface
Parameters
Description
This can be used by drivers to release an interface without waiting for their disconnect() methods to be called. In typical cases this also causes the driver disconnect() method to be called.
This call is synchronous, and may not be used in an interrupt context. Callers must own the device lock, so driver disconnect() entries don’t need extra locking, but other call contexts may need to explicitly claim that lock.
find first usb_device_id matching device or interface
Parameters
Description
usb_match_id searches an array of usb_device_id’s and returns the first one matching the device or interface, or null. This is used when binding (or rebinding) a driver to an interface. Most USB device drivers will use this indirectly, through the usb core, but some layered driver frameworks use it directly. These device tables are exported with MODULE_DEVICE_TABLE, through modutils, to support the driver loading functionality of USB hotplugging.
Return
The first matching usb_device_id, or NULL.
What Matches:
The “match_flags” element in a usb_device_id controls which members are used. If the corresponding bit is set, the value in the device_id must match its corresponding member in the device or interface descriptor, or else the device_id does not match.
“driver_info” is normally used only by device drivers, but you can create a wildcard “matches anything” usb_device_id as a driver’s “modules.usbmap” entry if you provide an id with only a nonzero “driver_info” field. If you do this, the USB device driver’s probe() routine should use additional intelligence to decide whether to bind to the specified interface.
What Makes Good usb_device_id Tables:
The match algorithm is very simple, so that intelligence in driver selection must come from smart driver id records. Unless you have good reasons to use another selection policy, provide match elements only in related groups, and order match specifiers from specific to general. Use the macros provided for that purpose if you can.
The most specific match specifiers use device descriptor data. These are commonly used with product-specific matches; the USB_DEVICE macro lets you provide vendor and product IDs, and you can also match against ranges of product revisions. These are widely used for devices with application or vendor specific bDeviceClass values.
Matches based on device class/subclass/protocol specifications are slightly more general; use the USB_DEVICE_INFO macro, or its siblings. These are used with single-function devices where bDeviceClass doesn’t specify that each interface has its own class.
Matches based on interface class/subclass/protocol are the most general; they let drivers bind to any interface on a multiple-function device. Use the USB_INTERFACE_INFO macro, or its siblings, to match class-per-interface style devices (as recorded in bInterfaceClass).
Note that an entry created by USB_INTERFACE_INFO won’t match any interface if the device class is set to Vendor-Specific. This is deliberate; according to the USB spec the meanings of the interface class/subclass/protocol for these devices are also vendor-specific, and hence matching against a standard product class wouldn’t work anyway. If you really want to use an interface-based match for such a device, create a match record that also specifies the vendor ID. (Unforunately there isn’t a standard macro for creating records like this.)
Within those groups, remember that not all combinations are meaningful. For example, don’t give a product version range without vendor and product IDs; or specify a protocol without its associated class and subclass.
register a USB device (not interface) driver
Parameters
Description
Registers a USB device driver with the USB core. The list of unattached devices will be rescanned whenever a new driver is added, allowing the new driver to attach to any recognized devices.
Return
A negative error code on failure and 0 on success.
unregister a USB device (not interface) driver
Parameters
Context
must be able to sleep
Description
Unlinks the specified driver from the internal USB driver list.
register a USB interface driver
Parameters
Description
Registers a USB interface driver with the USB core. The list of unattached interfaces will be rescanned whenever a new driver is added, allowing the new driver to attach to any recognized interfaces.
Return
A negative error code on failure and 0 on success.
NOTE
if you want your driver to use the USB major number, you must call usb_register_dev() to enable that functionality. This function no longer takes care of that.
unregister a USB interface driver
Parameters
Context
must be able to sleep
Description
Unlinks the specified driver from the internal USB driver list.
NOTE
If you called usb_register_dev(), you still need to call usb_deregister_dev() to clean up your driver’s allocated minor numbers, this * call will no longer do it for you.
allow a USB device to be autosuspended
Parameters
Description
This routine allows udev to be autosuspended. An autosuspend won’t take place until the autosuspend_delay has elapsed and all the other necessary conditions are satisfied.
The caller must hold udev‘s device lock.
prevent a USB device from being autosuspended
Parameters
Description
This routine prevents udev from being autosuspended and wakes it up if it is already autosuspended.
The caller must hold udev‘s device lock.
decrement a USB interface’s PM-usage counter
Parameters
Description
This routine should be called by an interface driver when it is finished using intf and wants to allow it to autosuspend. A typical example would be a character-device driver when its device file is closed.
The routine decrements intf‘s usage counter. When the counter reaches 0, a delayed autosuspend request for intf‘s device is attempted. The attempt may fail (see autosuspend_check()).
This routine can run only in process context.
decrement a USB interface’s PM-usage counter
Parameters
Description
This routine does much the same thing as usb_autopm_put_interface(): It decrements intf‘s usage counter and schedules a delayed autosuspend request if the counter is <= 0. The difference is that it does not perform any synchronization; callers should hold a private lock and handle all synchronization issues themselves.
Typically a driver would call this routine during an URB’s completion handler, if no more URBs were pending.
This routine can run in atomic context.
decrement a USB interface’s PM-usage counter
Parameters
Description
This routine decrements intf‘s usage counter but does not carry out an autosuspend.
This routine can run in atomic context.
increment a USB interface’s PM-usage counter
Parameters
Description
This routine should be called by an interface driver when it wants to use intf and needs to guarantee that it is not suspended. In addition, the routine prevents intf from being autosuspended subsequently. (Note that this will not prevent suspend events originating in the PM core.) This prevention will persist until usb_autopm_put_interface() is called or intf is unbound. A typical example would be a character-device driver when its device file is opened.
intf‘s usage counter is incremented to prevent subsequent autosuspends. However if the autoresume fails then the counter is re-decremented.
This routine can run only in process context.
Return
0 on success.
increment a USB interface’s PM-usage counter
Parameters
Description
This routine does much the same thing as usb_autopm_get_interface(): It increments intf‘s usage counter and queues an autoresume request if the device is suspended. The differences are that it does not perform any synchronization (callers should hold a private lock and handle all synchronization issues themselves), and it does not autoresume the device directly (it only queues a request). After a successful call, the device may not yet be resumed.
This routine can run in atomic context.
Return
0 on success. A negative error code otherwise.
increment a USB interface’s PM-usage counter
Parameters
Description
This routine increments intf‘s usage counter but does not carry out an autoresume.
This routine can run in atomic context.
Given a configuration, find the alternate setting for the given interface.
Parameters
Description
Search the configuration’s interface cache for the given alt setting.
Return
The alternate setting, if found. NULL otherwise.
get the interface object with a given interface number
Parameters
Description
This walks the device descriptor for the currently active configuration to find the interface object with the particular interface number.
Note that configuration descriptors are not required to assign interface numbers sequentially, so that it would be incorrect to assume that the first interface in that descriptor corresponds to interface zero. This routine helps device drivers avoid such mistakes. However, you should make sure that you do the right thing with any alternate settings available for this interfaces.
Don’t call this function unless you are bound to one of the interfaces on this device or you have locked the device!
Return
A pointer to the interface that has ifnum as interface number, if found. NULL otherwise.
get the altsetting structure with a given alternate setting number.
Parameters
Description
This searches the altsetting array of the specified interface for an entry with the correct bAlternateSetting value.
Note that altsettings need not be stored sequentially by number, so it would be incorrect to assume that the first altsetting entry in the array corresponds to altsetting zero. This routine helps device drivers avoid such mistakes.
Don’t call this function unless you are bound to the intf interface or you have locked the device!
Return
A pointer to the entry of the altsetting array of intf that has altnum as the alternate setting number. NULL if not found.
find usb_interface pointer for driver and device
Parameters
Description
This walks the bus device list and returns a pointer to the interface with the matching minor and driver. Note, this only works for devices that share the USB major number.
Return
A pointer to the interface with the matching major and minor.
iterate over all USB devices in the system
Parameters
Description
Iterate over all USB devices and call fn for each, passing it data. If it returns anything other than 0, we break the iteration prematurely and return that value.
usb device constructor (usbcore-internal)
Parameters
Context
!:c:func:in_interrupt()
Description
Only hub drivers (including virtual root hub drivers for host controllers) should ever call this.
This call may not be used in a non-sleeping context.
Return
On success, a pointer to the allocated usb device. NULL on failure.
increments the reference count of the usb device structure
Parameters
Description
Each live reference to a device should be refcounted.
Drivers for USB interfaces should normally record such references in their probe() methods, when they bind to an interface, and release them by calling usb_put_dev(), in their disconnect() methods.
Return
A pointer to the device with the incremented reference counter.
release a use of the usb device structure
Parameters
Description
Must be called when a user of a device is finished with it. When the last user of the device calls this function, the memory of the device is freed.
increments the reference count of the usb interface structure
Parameters
Description
Each live reference to a interface must be refcounted.
Drivers for USB interfaces should normally record such references in their probe() methods, when they bind to an interface, and release them by calling usb_put_intf(), in their disconnect() methods.
Return
A pointer to the interface with the incremented reference counter.
release a use of the usb interface structure
Parameters
Description
Must be called when a user of an interface is finished with it. When the last user of the interface calls this function, the memory of the interface is freed.
cautiously acquire the lock for a usb device structure
Parameters
Description
Attempts to acquire the device lock, but fails if the device is NOTATTACHED or SUSPENDED, or if iface is specified and the interface is neither BINDING nor BOUND. Rather than sleeping to wait for the lock, the routine polls repeatedly. This is to prevent deadlock with disconnect; in some drivers (such as usb-storage) the disconnect() or suspend() method will block waiting for a device reset to complete.
Return
A negative error code for failure, otherwise 0.
return current bus frame number
Parameters
Return
The current frame number for the USB host controller used with the given USB device. This can be used when scheduling isochronous requests.
Note
Different kinds of host controller have different “scheduling horizons”. While one type might support scheduling only 32 frames into the future, others could support scheduling up to 1024 frames into the future.
allocate dma-consistent buffer for URB_NO_xxx_DMA_MAP
Parameters
Return
Either null (indicating no buffer could be allocated), or the cpu-space pointer to a buffer that may be used to perform DMA to the specified device. Such cpu-space buffers are returned along with the DMA address (through the pointer provided).
Note
These buffers are used with URB_NO_xxx_DMA_MAP set in urb->transfer_flags to avoid behaviors like using “DMA bounce buffers”, or thrashing IOMMU hardware during URB completion/resubmit. The implementation varies between platforms, depending on details of how DMA will work to this device. Using these buffers also eliminates cacheline sharing problems on architectures where CPU caches are not DMA-coherent. On systems without bus-snooping caches, these buffers are uncached.
When the buffer is no longer used, free it with usb_free_coherent().
free memory allocated with usb_alloc_coherent()
Parameters
Description
This reclaims an I/O buffer, letting it be reused. The memory must have been allocated using usb_alloc_coherent(), and the parameters must match those provided in that allocation request.
Parameters
Description
URB_NO_TRANSFER_DMA_MAP is added to urb->transfer_flags if the operation succeeds. If the device is connected to this system through a non-DMA controller, this operation always succeeds.
This call would normally be used for an urb which is reused, perhaps as the target of a large periodic transfer, with usb_buffer_dmasync() calls to synchronize memory and dma state.
Reverse the effect of this call with usb_buffer_unmap().
Return
Either NULL (indicating no buffer could be mapped), or urb.
Parameters
Parameters
Description
Reverses the effect of usb_buffer_map().
create scatterlist DMA mapping(s) for an endpoint
Parameters
Return
Either < 0 (indicating no buffers could be mapped), or the number of DMA mapping array entries in the scatterlist.
Note
The caller is responsible for placing the resulting DMA addresses from the scatterlist into URB transfer buffer pointers, and for setting the URB_NO_TRANSFER_DMA_MAP transfer flag in each of those URBs.
Top I/O rates come from queuing URBs, instead of waiting for each one to complete before starting the next I/O. This is particularly easy to do with scatterlists. Just allocate and submit one URB for each DMA mapping entry returned, stopping on the first error or when all succeed. Better yet, use the usb_sg_*() calls, which do that (and more) for you.
This call would normally be used when translating scatterlist requests, rather than usb_buffer_map(), since on some hardware (with IOMMUs) it may be able to coalesce mappings for improved I/O efficiency.
Reverse the effect of this call with usb_buffer_unmap_sg().
synchronize DMA and CPU view of scatterlist buffer(s)
Parameters
Description
Use this when you are re-using a scatterlist’s data buffers for another USB request.
free DMA mapping(s) for a scatterlist
Parameters
Description
Reverses the effect of usb_buffer_map_sg().
Parameters
Description
High speed HCDs use this to tell the hub driver that some split control or bulk transaction failed in a way that requires clearing internal state of a transaction translator. This is normally detected (and reported) from interrupt context.
It may not be possible for that hub to handle additional full (or low) speed transactions until that state is fully cleared out.
Return
0 if successful. A negative error code otherwise.
change a device’s current state (usbcore, hcds)
Parameters
Description
udev->state is _not_ fully protected by the device lock. Although most transitions are made only while holding the lock, the state can can change to USB_STATE_NOTATTACHED at almost any time. This is so that devices can be marked as disconnected as soon as possible, without having to wait for any semaphores to be released. As a result, all changes to any device’s state must be protected by the device_state_lock spinlock.
Once a device has been added to the device tree, all changes to its state should be made using this routine. The state should _not_ be set directly.
If udev->state is already USB_STATE_NOTATTACHED then no change is made. Otherwise udev->state is set to new_state, and if new_state is USB_STATE_NOTATTACHED then all of udev’s descendants’ states are also set to USB_STATE_NOTATTACHED.
called by HCD if the root hub lost Vbus power
Parameters
Description
The USB host controller driver calls this function when its root hub is resumed and Vbus power has been interrupted or the controller has been reset. The routine marks rhdev as having lost power. When the hub driver is resumed it will take notice and carry out power-session recovery for all the “USB-PERSIST”-enabled child devices; the others will be disconnected.
warn interface drivers and perform a USB port reset
Parameters
Description
Warns all drivers bound to registered interfaces (using their pre_reset method), performs the port reset, and then lets the drivers know that the reset is over (using their post_reset method).
Return
The same as for usb_reset_and_verify_device().
Note
The caller must own the device lock. For example, it’s safe to use this from a driver probe() routine after downloading new firmware. For calls that might not occur during probe(), drivers should lock the device using usb_lock_device_for_reset().
If an interface is currently being probed or disconnected, we assume its driver knows how to handle resets. For all other interfaces, if the driver doesn’t have pre_reset and post_reset methods then we attempt to unbind it and rebind afterward.
Reset a USB device from an atomic context
Parameters
Description
This function can be used to reset a USB device from an atomic context, where usb_reset_device() won’t work (as it blocks).
Doing a reset via this method is functionally equivalent to calling usb_reset_device(), except for the fact that it is delayed to a workqueue. This means that any drivers bound to other interfaces might be unbound, as well as users from usbfs in user space.
Corner cases:
Get the pointer of child device attached to the port which is specified by port1.
Parameters
Description
USB drivers call this function to get hub’s child device pointer.
Return
NULL if input param is invalid and child’s usb_device pointer if non-NULL.
These APIs are only for use by host controller drivers, most of which implement standard register interfaces such as XHCI, EHCI, OHCI, or UHCI. UHCI was one of the first interfaces, designed by Intel and also used by VIA; it doesn’t do much in hardware. OHCI was designed later, to have the hardware do more work (bigger transfers, tracking protocol state, and so on). EHCI was designed with USB 2.0; its design has features that resemble OHCI (hardware does much more work) as well as UHCI (some parts of ISO support, TD list processing). XHCI was designed with USB 3.0. It continues to shift support for functionality into hardware.
There are host controllers other than the “big three”, although most PCI based controllers (and a few non-PCI based ones) use one of those interfaces. Not all host controllers use DMA; some use PIO, and there is also a simulator and a virtual host controller to pipe USB over the network.
The same basic APIs are available to drivers for all those controllers. For historical reasons they are in two layers: struct usb_bus is a rather thin layer that became available in the 2.2 kernels, while struct usb_hcd is a more featureful layer that lets HCDs share common code, to shrink driver size and significantly reduce hcd-specific behaviors.
approximate periodic transaction time in nanoseconds
Parameters
Return
Approximate bus time in nanoseconds for a periodic transaction.
Note
See USB 2.0 spec section 5.11.3; only periodic transfers need to be scheduled in software, this function is only used for such scheduling.
add an URB to its endpoint queue
Parameters
Description
Host controller drivers should call this routine in their enqueue() method. The HCD’s private spinlock must be held and interrupts must be disabled. The actions carried out here are required for URB submission, as well as for endpoint shutdown and for usb_kill_urb.
Return
0 for no error, otherwise a negative error code (in which case the enqueue() method must fail). If no error occurs but enqueue() fails anyway, it must call usb_hcd_unlink_urb_from_ep() before releasing the private spinlock and returning.
check whether an URB may be unlinked
Parameters
Description
Host controller drivers should call this routine in their dequeue() method. The HCD’s private spinlock must be held and interrupts must be disabled. The actions carried out here are required for making sure than an unlink is valid.
Return
0 for no error, otherwise a negative error code (in which case the dequeue() method must fail). The possible error codes are:
- -EIDRM: urb was not submitted or has already completed.
- The completion function may not have been called yet.
-EBUSY: urb has already been unlinked.
remove an URB from its endpoint queue
Parameters
Description
Host controller drivers should call this routine before calling usb_hcd_giveback_urb(). The HCD’s private spinlock must be held and interrupts must be disabled. The actions carried out here are required for URB completion.
return URB from HCD to device driver
Parameters
Context
in_interrupt()
Description
This hands the URB from HCD to its USB device driver, using its completion function. The HCD has freed all per-urb resources (and is done using urb->hcpriv). It also released all HCD locks; the device driver won’t cause problems if it frees, modifies, or resubmits this URB.
If urb was unlinked, the value of status will be overridden by urb->unlinked. Erroneous short transfers are detected in case the HCD hasn’t checked for them.
allocate bulk endpoint stream IDs.
Parameters
Description
Sets up a group of bulk endpoints to have num_streams stream IDs available. Drivers may queue multiple transfers to different stream IDs, which may complete in a different order than they were queued.
Return
On success, the number of allocated streams. On failure, a negative error code.
free bulk endpoint stream IDs.
Parameters
Description
Reverts a group of bulk endpoints back to not using stream IDs. Can fail if we are given bad arguments, or HCD is broken.
Return
0 on success. On failure, a negative error code.
called by HCD to resume its root hub
Parameters
Description
The USB host controller calls this function when its root hub is suspended (with the remote wakeup feature enabled) and a remote wakeup request is received. The routine submits a workqueue request to resume the root hub (that is, manage its downstream ports again).
start immediate enumeration (for OTG)
Parameters
Context
in_interrupt()
Description
Starts enumeration, with an immediate reset followed later by hub_wq identifying and possibly configuring the device. This is needed by OTG controller drivers, where it helps meet HNP protocol timing requirements for starting a port reset.
Return
0 if successful.
hook IRQs to HCD framework (bus glue)
Parameters
Description
If the controller isn’t HALTed, calls the driver’s irq handler. Checks whether the controller is now dead.
Return
IRQ_HANDLED if the IRQ was handled. IRQ_NONE otherwise.
report abnormal shutdown of a host controller (bus glue)
Parameters
Description
This is called by bus glue to report a USB host controller that died while operations may still have been pending. It’s called automatically by the PCI glue, so only glue for non-PCI busses should need to call it.
Only call this function with the primary HCD.
create and initialize an HCD structure
Parameters
Context
!:c:func:in_interrupt()
Description
Allocate a struct usb_hcd, with extra space at the end for the HC driver’s private data. Initialize the generic members of the hcd structure.
Return
On success, a pointer to the created and initialized HCD structure. On failure (e.g. if memory is unavailable), NULL.
create and initialize an HCD structure
Parameters
Context
!:c:func:in_interrupt()
Description
Allocate a struct usb_hcd, with extra space at the end for the HC driver’s private data. Initialize the generic members of the hcd structure.
Return
On success, a pointer to the created and initialized HCD structure. On failure (e.g. if memory is unavailable), NULL.
finish generic HCD structure initialization and register
Parameters
Description
Finish the remaining parts of generic HCD initialization: allocate the buffers of consistent memory, register the bus, request the IRQ line, and call the driver’s reset() and start() routines.
shutdown processing for generic HCDs
Parameters
Context
!:c:func:in_interrupt()
Description
Disconnects the root hub, then reverses the effects of usb_add_hcd(), invoking the HCD’s stop() method.
initialize PCI-based HCDs
Parameters
Context
!:c:func:in_interrupt()
Description
Allocates basic PCI resources for this USB host controller, and then invokes the start() method for the HCD associated with it through the hotplug entry’s driver_data.
Store this function in the HCD’s struct pci_driver as probe().
Return
0 if successful.
shutdown processing for PCI-based HCDs
Parameters
Context
!:c:func:in_interrupt()
Description
Reverses the effect of usb_hcd_pci_probe(), first invoking the HCD’s stop() method. It is always called from a thread context, normally “rmmod”, “apmd”, or something similar.
Store this function in the HCD’s struct pci_driver as remove().
shutdown host controller
Parameters
initialize buffer pools
Parameters
Context
!:c:func:in_interrupt()
Description
Call this as part of initializing a host controller that uses the dma memory allocators. It initializes some pools of dma-coherent memory that will be shared by all drivers using that controller.
Call hcd_buffer_destroy() to clean up after using those pools.
Return
0 if successful. A negative errno value otherwise.
deallocate buffer pools
Parameters
Context
!:c:func:in_interrupt()
Description
This frees the buffer pools created by hcd_buffer_create().
This chapter presents the Linux usbfs. You may prefer to avoid writing new kernel code for your USB driver; that’s the problem that usbfs set out to solve. User mode device drivers are usually packaged as applications or libraries, and may use usbfs through some programming library that wraps it. Such libraries include libusb for C/C++, and jUSB for Java.
Note
This particular documentation is incomplete, especially with respect to the asynchronous mode. As of kernel 2.5.66 the code and this (new) documentation need to be cross-reviewed.
Configure usbfs into Linux kernels by enabling the USB filesystem option (CONFIG_USB_DEVICEFS), and you get basic support for user mode USB device drivers. Until relatively recently it was often (confusingly) called usbdevfs although it wasn’t solving what devfs was. Every USB device will appear in usbfs, regardless of whether or not it has a kernel driver.
Conventionally mounted at /proc/bus/usb, usbfs features include:
Each bus is given a number (BBB) based on when it was enumerated; within each bus, each device is given a similar number (DDD). Those BBB/DDD paths are not “stable” identifiers; expect them to change even if you always leave the devices plugged in to the same hub port. Don’t even think of saving these in application configuration files. Stable identifiers are available, for user mode applications that want to use them. HID and networking devices expose these stable IDs, so that for example you can be sure that you told the right UPS to power down its second server. “usbfs” doesn’t (yet) expose those IDs.
There are a number of mount options for usbfs, which will be of most interest to you if you need to override the default access control policy. That policy is that only root may read or write device files (/proc/bus/BBB/DDD) although anyone may read the devices or drivers files. I/O requests to the device also need the CAP_SYS_RAWIO capability,
The significance of that is that by default, all user mode device drivers need super-user privileges. You can change modes or ownership in a driver setup when the device hotplugs, or maye just start the driver right then, as a privileged server (or some activity within one). That’s the most secure approach for multi-user systems, but for single user systems (“trusted” by that user) it’s more convenient just to grant everyone all access (using the devmode=0666 option) so the driver can start whenever it’s needed.
The mount options for usbfs, usable in /etc/fstab or in command line invocations of mount, are:
Note that many Linux distributions hard-wire the mount options for usbfs in their init scripts, such as /etc/rc.d/rc.sysinit, rather than making it easy to set this per-system policy in /etc/fstab.
This file is handy for status viewing tools in user mode, which can scan the text format and ignore most of it. More detailed device status (including class and vendor status) is available from device-specific files. For information about the current format of this file, see the Documentation/usb/proc_usb_info.txt file in your Linux kernel sources.
This file, in combination with the poll() system call, can also be used to detect when devices are added or removed:
int fd;
struct pollfd pfd;
fd = open("/proc/bus/usb/devices", O_RDONLY);
pfd = { fd, POLLIN, 0 };
for (;;) {
/* The first time through, this call will return immediately. */
poll(&pfd, 1, -1);
/* To see what's changed, compare the file's previous and current
contents or scan the filesystem. (Scanning is more precise.) */
}
Note that this behavior is intended to be used for informational and debug purposes. It would be more appropriate to use programs such as udev or HAL to initialize a device or start a user-mode helper program, for instance.
Use these files in one of these basic ways:
They can be read, producing first the device descriptor (18 bytes) and then the descriptors for the current configuration. See the USB 2.0 spec for details about those binary data formats. You’ll need to convert most multibyte values from little endian format to your native host byte order, although a few of the fields in the device descriptor (both of the BCD-encoded fields, and the vendor and product IDs) will be byteswapped for you. Note that configuration descriptors include descriptors for interfaces, altsettings, endpoints, and maybe additional class descriptors.
Perform USB operations using ioctl() requests to make endpoint I/O requests (synchronously or asynchronously) or manage the device. These requests need the CAP_SYS_RAWIO capability, as well as filesystem access permissions. Only one ioctl request can be made on one of these device files at a time. This means that if you are synchronously reading an endpoint from one thread, you won’t be able to write to a different endpoint from another thread until the read completes. This works for half duplex protocols, but otherwise you’d use asynchronous i/o requests.
Such a driver first needs to find a device file for a device it knows how to handle. Maybe it was told about it because a /sbin/hotplug event handling agent chose that driver to handle the new device. Or maybe it’s an application that scans all the /proc/bus/usb device files, and ignores most devices. In either case, it should read() all the descriptors from the device file, and check them against what it knows how to handle. It might just reject everything except a particular vendor and product ID, or need a more complex policy.
Never assume there will only be one such device on the system at a time! If your code can’t handle more than one device at a time, at least detect when there’s more than one, and have your users choose which device to use.
Once your user mode driver knows what device to use, it interacts with it in either of two styles. The simple style is to make only control requests; some devices don’t need more complex interactions than those. (An example might be software using vendor-specific control requests for some initialization or configuration tasks, with a kernel driver for the rest.)
More likely, you need a more complex style driver: one using non-control endpoints, reading or writing data and claiming exclusive use of an interface. Bulk transfers are easiest to use, but only their sibling interrupt transfers work with low speed devices. Both interrupt and isochronous transfers offer service guarantees because their bandwidth is reserved. Such “periodic” transfers are awkward to use through usbfs, unless you’re using the asynchronous calls. However, interrupt transfers can also be used in a synchronous “one shot” style.
Your user-mode driver should never need to worry about cleaning up request state when the device is disconnected, although it should close its open file descriptors as soon as it starts seeing the ENODEV errors.
To use these ioctls, you need to include the following headers in your userspace program:
#include <linux/usb.h>
#include <linux/usbdevice_fs.h>
#include <asm/byteorder.h>
The standard USB device model requests, from “Chapter 9” of the USB 2.0 specification, are automatically included from the <linux/usb/ch9.h> header.
Unless noted otherwise, the ioctl requests described here will update the modification time on the usbfs file to which they are applied (unless they fail). A return of zero indicates success; otherwise, a standard USB error code is returned. (These are documented in Documentation/usb/error-codes.txt in your kernel sources.)
Each of these files multiplexes access to several I/O streams, one per endpoint. Each device has one control endpoint (endpoint zero) which supports a limited RPC style RPC access. Devices are configured by hub_wq (in the kernel) setting a device-wide configuration that affects things like power consumption and basic functionality. The endpoints are part of USB interfaces, which may have altsettings affecting things like which endpoints are available. Many devices only have a single configuration and interface, so drivers for them will ignore configurations and altsettings.
A number of usbfs requests don’t deal very directly with device I/O. They mostly relate to device management and status. These are all synchronous requests.
This is used to force usbfs to claim a specific interface, which has not previously been claimed by usbfs or any other kernel driver. The ioctl parameter is an integer holding the number of the interface (bInterfaceNumber from descriptor).
Note that if your driver doesn’t claim an interface before trying to use one of its endpoints, and no other driver has bound to it, then the interface is automatically claimed by usbfs.
This claim will be released by a RELEASEINTERFACE ioctl, or by closing the file descriptor. File modification time is not updated by this request.
Says whether the device is lowspeed. The ioctl parameter points to a structure like this:
struct usbdevfs_connectinfo {
unsigned int devnum;
unsigned char slow;
};
File modification time is not updated by this request.
You can’t tell whether a “not slow” device is connected at high speed (480 MBit/sec) or just full speed (12 MBit/sec). You should know the devnum value already, it’s the DDD value of the device file name.
Returns the name of the kernel driver bound to a given interface (a string). Parameter is a pointer to this structure, which is modified:
struct usbdevfs_getdriver {
unsigned int interface;
char driver[USBDEVFS_MAXDRIVERNAME + 1];
};
File modification time is not updated by this request.
Passes a request from userspace through to a kernel driver that has an ioctl entry in the struct usb_driver it registered.
struct usbdevfs_ioctl {
int ifno;
int ioctl_code;
void *data;
};
/* user mode call looks like this.
* 'request' becomes the driver->ioctl() 'code' parameter.
* the size of 'param' is encoded in 'request', and that data
* is copied to or from the driver->ioctl() 'buf' parameter.
*/
static int
usbdev_ioctl (int fd, int ifno, unsigned request, void *param)
{
struct usbdevfs_ioctl wrapper;
wrapper.ifno = ifno;
wrapper.ioctl_code = request;
wrapper.data = param;
return ioctl (fd, USBDEVFS_IOCTL, &wrapper);
}
File modification time is not updated by this request.
This request lets kernel drivers talk to user mode code through filesystem operations even when they don’t create a character or block special device. It’s also been used to do things like ask devices what device special file should be used. Two pre-defined ioctls are used to disconnect and reconnect kernel drivers, so that user mode code can completely manage binding and configuration of devices.
This is used to release the claim usbfs made on interface, either implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the file descriptor is closed. The ioctl parameter is an integer holding the number of the interface (bInterfaceNumber from descriptor); File modification time is not updated by this request.
Warning
No security check is made to ensure that the task which made the claim is the one which is releasing it. This means that user mode driver may interfere other ones.
Resets the data toggle value for an endpoint (bulk or interrupt) to DATA0. The ioctl parameter is an integer endpoint number (1 to 15, as identified in the endpoint descriptor), with USB_DIR_IN added if the device’s endpoint sends data to the host.
Warning
Avoid using this request. It should probably be removed. Using it typically means the device and driver will lose toggle synchronization. If you really lost synchronization, you likely need to completely handshake with the device, using a request like CLEAR_HALT or SET_INTERFACE.
Synchronous requests involve the kernel blocking until the user mode request completes, either by finishing successfully or by reporting an error. In most cases this is the simplest way to use usbfs, although as noted above it does prevent performing I/O to more than one endpoint at a time.
Issues a bulk read or write request to the device. The ioctl parameter is a pointer to this structure:
struct usbdevfs_bulktransfer {
unsigned int ep;
unsigned int len;
unsigned int timeout; /* in milliseconds */
void *data;
};
The “ep” value identifies a bulk endpoint number (1 to 15, as identified in an endpoint descriptor), masked with USB_DIR_IN when referring to an endpoint which sends data to the host from the device. The length of the data buffer is identified by “len”; Recent kernels support requests up to about 128KBytes. FIXME say how read length is returned, and how short reads are handled..
Clears endpoint halt (stall) and resets the endpoint toggle. This is only meaningful for bulk or interrupt endpoints. The ioctl parameter is an integer endpoint number (1 to 15, as identified in an endpoint descriptor), masked with USB_DIR_IN when referring to an endpoint which sends data to the host from the device.
Use this on bulk or interrupt endpoints which have stalled, returning -EPIPE status to a data transfer request. Do not issue the control request directly, since that could invalidate the host’s record of the data toggle.
Issues a control request to the device. The ioctl parameter points to a structure like this:
struct usbdevfs_ctrltransfer {
__u8 bRequestType;
__u8 bRequest;
__u16 wValue;
__u16 wIndex;
__u16 wLength;
__u32 timeout; /* in milliseconds */
void *data;
};
The first eight bytes of this structure are the contents of the SETUP packet to be sent to the device; see the USB 2.0 specification for details. The bRequestType value is composed by combining a USB_TYPE_* value, a USB_DIR_* value, and a USB_RECIP_* value (from <linux/usb.h>). If wLength is nonzero, it describes the length of the data buffer, which is either written to the device (USB_DIR_OUT) or read from the device (USB_DIR_IN).
At this writing, you can’t transfer more than 4 KBytes of data to or from a device; usbfs has a limit, and some host controller drivers have a limit. (That’s not usually a problem.) Also there’s no way to say it’s not OK to get a short read back from the device.
Does a USB level device reset. The ioctl parameter is ignored. After the reset, this rebinds all device interfaces. File modification time is not updated by this request.
Warning
Avoid using this call until some usbcore bugs get fixed, since it does not fully synchronize device, interface, and driver (not just usbfs) state.
Sets the alternate setting for an interface. The ioctl parameter is a pointer to a structure like this:
struct usbdevfs_setinterface {
unsigned int interface;
unsigned int altsetting;
};
File modification time is not updated by this request.
Those struct members are from some interface descriptor applying to the current configuration. The interface number is the bInterfaceNumber value, and the altsetting number is the bAlternateSetting value. (This resets each endpoint in the interface.)
Issues the usb_set_configuration() call for the device. The parameter is an integer holding the number of a configuration (bConfigurationValue from descriptor). File modification time is not updated by this request.
Warning
Avoid using this call until some usbcore bugs get fixed, since it does not fully synchronize device, interface, and driver (not just usbfs) state.
As mentioned above, there are situations where it may be important to initiate concurrent operations from user mode code. This is particularly important for periodic transfers (interrupt and isochronous), but it can be used for other kinds of USB requests too. In such cases, the asynchronous requests described here are essential. Rather than submitting one request and having the kernel block until it completes, the blocking is separate.
These requests are packaged into a structure that resembles the URB used by kernel device drivers. (No POSIX Async I/O support here, sorry.) It identifies the endpoint type (USBDEVFS_URB_TYPE_*), endpoint (number, masked with USB_DIR_IN as appropriate), buffer and length, and a user “context” value serving to uniquely identify each request. (It’s usually a pointer to per-request data.) Flags can modify requests (not as many as supported for kernel drivers).
Each request can specify a realtime signal number (between SIGRTMIN and SIGRTMAX, inclusive) to request a signal be sent when the request completes.
When usbfs returns these urbs, the status value is updated, and the buffer may have been modified. Except for isochronous transfers, the actual_length is updated to say how many bytes were transferred; if the USBDEVFS_URB_DISABLE_SPD flag is set (“short packets are not OK”), if fewer bytes were read than were requested then you get an error report.
struct usbdevfs_iso_packet_desc {
unsigned int length;
unsigned int actual_length;
unsigned int status;
};
struct usbdevfs_urb {
unsigned char type;
unsigned char endpoint;
int status;
unsigned int flags;
void *buffer;
int buffer_length;
int actual_length;
int start_frame;
int number_of_packets;
int error_count;
unsigned int signr;
void *usercontext;
struct usbdevfs_iso_packet_desc iso_frame_desc[];
};
For these asynchronous requests, the file modification time reflects when the request was initiated. This contrasts with their use with the synchronous requests, where it reflects when requests complete.