usb.rst

   1.. _usb-hostside-api:
   2
   3===========================
   4The Linux-USB Host Side API
   5===========================
   6
   7Introduction to USB on Linux
   8============================
   9
  10A Universal Serial Bus (USB) is used to connect a host, such as a PC or
  11workstation, to a number of peripheral devices. USB uses a tree
  12structure, with the host as the root (the system's master), hubs as
  13interior nodes, and peripherals as leaves (and slaves). Modern PCs
  14support several such trees of USB devices, usually
  15a few USB 3.0 (5 GBit/s) or USB 3.1 (10 GBit/s) and some legacy
  16USB 2.0 (480 MBit/s) buses just in case.
  17
  18That master/slave asymmetry was designed-in for a number of reasons, one
  19being ease of use. It is not physically possible to mistake upstream and
  20downstream or it does not matter with a type C plug (or they are built into the
  21peripheral). Also, the host software doesn't need to deal with
  22distributed auto-configuration since the pre-designated master node
  23manages all that.
  24
  25Kernel developers added USB support to Linux early in the 2.2 kernel
  26series and have been developing it further since then. Besides support
  27for each new generation of USB, various host controllers gained support,
  28new drivers for peripherals have been added and advanced features for latency
  29measurement and improved power management introduced.
  30
  31Linux can run inside USB devices as well as on the hosts that control
  32the devices. But USB device drivers running inside those peripherals
  33don't do the same things as the ones running inside hosts, so they've
  34been given a different name: *gadget drivers*. This document does not
  35cover gadget drivers.
  36
  37USB Host-Side API Model
  38=======================
  39
  40Host-side drivers for USB devices talk to the "usbcore" APIs. There are
  41two. One is intended for *general-purpose* drivers (exposed through
  42driver frameworks), and the other is for drivers that are *part of the
  43core*. Such core drivers include the *hub* driver (which manages trees
  44of USB devices) and several different kinds of *host controller
  45drivers*, which control individual buses.
  46
  47The device model seen by USB drivers is relatively complex.
  48
  49-  USB supports four kinds of data transfers (control, bulk, interrupt,
  50   and isochronous). Two of them (control and bulk) use bandwidth as
  51   it's available, while the other two (interrupt and isochronous) are
  52   scheduled to provide guaranteed bandwidth.
  53
  54-  The device description model includes one or more "configurations"
  55   per device, only one of which is active at a time. Devices are supposed
  56   to be capable of operating at lower than their top
  57   speeds and may provide a BOS descriptor showing the lowest speed they
  58   remain fully operational at.
  59
  60-  From USB 3.0 on configurations have one or more "functions", which
  61   provide a common functionality and are grouped together for purposes
  62   of power management.
  63
  64-  Configurations or functions have one or more "interfaces", each of which may have
  65   "alternate settings". Interfaces may be standardized by USB "Class"
  66   specifications, or may be specific to a vendor or device.
  67
  68   USB device drivers actually bind to interfaces, not devices. Think of
  69   them as "interface drivers", though you may not see many devices
  70   where the distinction is important. *Most USB devices are simple,
  71   with only one function, one configuration, one interface, and one alternate
  72   setting.*
  73
  74-  Interfaces have one or more "endpoints", each of which supports one
  75   type and direction of data transfer such as "bulk out" or "interrupt
  76   in". The entire configuration may have up to sixteen endpoints in
  77   each direction, allocated as needed among all the interfaces.
  78
  79-  Data transfer on USB is packetized; each endpoint has a maximum
  80   packet size. Drivers must often be aware of conventions such as
  81   flagging the end of bulk transfers using "short" (including zero
  82   length) packets.
  83
  84-  The Linux USB API supports synchronous calls for control and bulk
  85   messages. It also supports asynchronous calls for all kinds of data
  86   transfer, using request structures called "URBs" (USB Request
  87   Blocks).
  88
  89Accordingly, the USB Core API exposed to device drivers covers quite a
  90lot of territory. You'll probably need to consult the USB 3.0
  91specification, available online from www.usb.org at no cost, as well as
  92class or device specifications.
  93
  94The only host-side drivers that actually touch hardware (reading/writing
  95registers, handling IRQs, and so on) are the HCDs. In theory, all HCDs
  96provide the same functionality through the same API. In practice, that's
  97becoming more true, but there are still differences
  98that crop up especially with fault handling on the less common controllers.
  99Different controllers don't
 100necessarily report the same aspects of failures, and recovery from
 101faults (including software-induced ones like unlinking an URB) isn't yet
 102fully consistent. Device driver authors should make a point of doing
 103disconnect testing (while the device is active) with each different host
 104controller driver, to make sure drivers don't have bugs of their own as
 105well as to make sure they aren't relying on some HCD-specific behavior.
 106
 107.. _usb_chapter9:
 108
 109USB-Standard Types
 110==================
 111
 112In ``include/uapi/linux/usb/ch9.h`` you will find the USB data types defined
 113in chapter 9 of the USB specification. These data types are used throughout
 114USB, and in APIs including this host side API, gadget APIs, usb character
 115devices and debugfs interfaces. That file is itself included by
 116``include/linux/usb/ch9.h``, which also contains declarations of a few
 117utility routines for manipulating these data types; the implementations
 118are in ``drivers/usb/common/common.c``.
 119
 120.. kernel-doc:: drivers/usb/common/common.c
 121   :export:
 122
 123In addition, some functions useful for creating debugging output are
 124defined in ``drivers/usb/common/debug.c``.
 125
 126.. _usb_header:
 127
 128Host-Side Data Types and Macros
 129===============================
 130
 131The host side API exposes several layers to drivers, some of which are
 132more necessary than others. These support lifecycle models for host side
 133drivers and devices, and support passing buffers through usbcore to some
 134HCD that performs the I/O for the device driver.
 135
 136.. kernel-doc:: include/linux/usb.h
 137   :internal:
 138
 139USB Core APIs
 140=============
 141
 142There are two basic I/O models in the USB API. The most elemental one is
 143asynchronous: drivers submit requests in the form of an URB, and the
 144URB's completion callback handles the next step. All USB transfer types
 145support that model, although there are special cases for control URBs
 146(which always have setup and status stages, but may not have a data
 147stage) and isochronous URBs (which allow large packets and include
 148per-packet fault reports). Built on top of that is synchronous API
 149support, where a driver calls a routine that allocates one or more URBs,
 150submits them, and waits until they complete. There are synchronous
 151wrappers for single-buffer control and bulk transfers (which are awkward
 152to use in some driver disconnect scenarios), and for scatterlist based
 153streaming i/o (bulk or interrupt).
 154
 155USB drivers need to provide buffers that can be used for DMA, although
 156they don't necessarily need to provide the DMA mapping themselves. There
 157are APIs to use used when allocating DMA buffers, which can prevent use
 158of bounce buffers on some systems. In some cases, drivers may be able to
 159rely on 64bit DMA to eliminate another kind of bounce buffer.
 160
 161.. kernel-doc:: drivers/usb/core/urb.c
 162   :export:
 163
 164.. c:namespace:: usb_core
 165.. kernel-doc:: drivers/usb/core/message.c
 166   :export:
 167
 168.. kernel-doc:: drivers/usb/core/file.c
 169   :export:
 170
 171.. kernel-doc:: drivers/usb/core/driver.c
 172   :export:
 173
 174.. kernel-doc:: drivers/usb/core/usb.c
 175   :export:
 176
 177.. kernel-doc:: drivers/usb/core/hub.c
 178   :export:
 179
 180Host Controller APIs
 181====================
 182
 183These APIs are only for use by host controller drivers, most of which
 184implement standard register interfaces such as XHCI, EHCI, OHCI, or UHCI. UHCI
 185was one of the first interfaces, designed by Intel and also used by VIA;
 186it doesn't do much in hardware. OHCI was designed later, to have the
 187hardware do more work (bigger transfers, tracking protocol state, and so
 188on). EHCI was designed with USB 2.0; its design has features that
 189resemble OHCI (hardware does much more work) as well as UHCI (some parts
 190of ISO support, TD list processing). XHCI was designed with USB 3.0. It
 191continues to shift support for functionality into hardware.
 192
 193There are host controllers other than the "big three", although most PCI
 194based controllers (and a few non-PCI based ones) use one of those
 195interfaces. Not all host controllers use DMA; some use PIO, and there is
 196also a simulator and a virtual host controller to pipe USB over the network.
 197
 198The same basic APIs are available to drivers for all those controllers.
 199For historical reasons they are in two layers: :c:type:`struct
 200usb_bus <usb_bus>` is a rather thin layer that became available
 201in the 2.2 kernels, while :c:type:`struct usb_hcd <usb_hcd>`
 202is a more featureful layer
 203that lets HCDs share common code, to shrink driver size and
 204significantly reduce hcd-specific behaviors.
 205
 206.. kernel-doc:: drivers/usb/core/hcd.c
 207   :export:
 208
 209.. kernel-doc:: drivers/usb/core/hcd-pci.c
 210   :export:
 211
 212.. kernel-doc:: drivers/usb/core/buffer.c
 213   :internal:
 214
 215The USB character device nodes
 216==============================
 217
 218This chapter presents the Linux character device nodes. You may prefer
 219to avoid writing new kernel code for your USB driver. User mode device
 220drivers are usually packaged as applications or libraries, and may use
 221character devices through some programming library that wraps it.
 222Such libraries include:
 223
 224 - `libusb <http://libusb.sourceforge.net>`__ for C/C++, and
 225 - `jUSB <http://jUSB.sourceforge.net>`__ for Java.
 226
 227Some old information about it can be seen at the "USB Device Filesystem"
 228section of the USB Guide. The latest copy of the USB Guide can be found
 229at http://www.linux-usb.org/
 230
 231.. note::
 232
 233  - They were used to be implemented via *usbfs*, but this is not part of
 234    the sysfs debug interface.
 235
 236   - This particular documentation is incomplete, especially with respect
 237     to the asynchronous mode. As of kernel 2.5.66 the code and this
 238     (new) documentation need to be cross-reviewed.
 239
 240What files are in "devtmpfs"?
 241-----------------------------
 242
 243Conventionally mounted at ``/dev/bus/usb/``, usbfs features include:
 244
 245-  ``/dev/bus/usb/BBB/DDD`` ... magic files exposing the each device's
 246   configuration descriptors, and supporting a series of ioctls for
 247   making device requests, including I/O to devices. (Purely for access
 248   by programs.)
 249
 250Each bus is given a number (``BBB``) based on when it was enumerated; within
 251each bus, each device is given a similar number (``DDD``). Those ``BBB/DDD``
 252paths are not "stable" identifiers; expect them to change even if you
 253always leave the devices plugged in to the same hub port. *Don't even
 254think of saving these in application configuration files.* Stable
 255identifiers are available, for user mode applications that want to use
 256them. HID and networking devices expose these stable IDs, so that for
 257example you can be sure that you told the right UPS to power down its
 258second server. Pleast note that it doesn't (yet) expose those IDs.
 259
 260/dev/bus/usb/BBB/DDD
 261--------------------
 262
 263Use these files in one of these basic ways:
 264
 265- *They can be read,* producing first the device descriptor (18 bytes) and
 266  then the descriptors for the current configuration. See the USB 2.0 spec
 267  for details about those binary data formats. You'll need to convert most
 268  multibyte values from little endian format to your native host byte
 269  order, although a few of the fields in the device descriptor (both of
 270  the BCD-encoded fields, and the vendor and product IDs) will be
 271  byteswapped for you. Note that configuration descriptors include
 272  descriptors for interfaces, altsettings, endpoints, and maybe additional
 273  class descriptors.
 274
 275- *Perform USB operations* using *ioctl()* requests to make endpoint I/O
 276  requests (synchronously or asynchronously) or manage the device. These
 277  requests need the ``CAP_SYS_RAWIO`` capability, as well as filesystem
 278  access permissions. Only one ioctl request can be made on one of these
 279  device files at a time. This means that if you are synchronously reading
 280  an endpoint from one thread, you won't be able to write to a different
 281  endpoint from another thread until the read completes. This works for
 282  *half duplex* protocols, but otherwise you'd use asynchronous i/o
 283  requests.
 284
 285Each connected USB device has one file.  The ``BBB`` indicates the bus
 286number.  The ``DDD`` indicates the device address on that bus.  Both
 287of these numbers are assigned sequentially, and can be reused, so
 288you can't rely on them for stable access to devices.  For example,
 289it's relatively common for devices to re-enumerate while they are
 290still connected (perhaps someone jostled their power supply, hub,
 291or USB cable), so a device might be ``002/027`` when you first connect
 292it and ``002/048`` sometime later.
 293
 294These files can be read as binary data.  The binary data consists
 295of first the device descriptor, then the descriptors for each
 296configuration of the device.  Multi-byte fields in the device descriptor
 297are converted to host endianness by the kernel.  The configuration
 298descriptors are in bus endian format! The configuration descriptor
 299are wTotalLength bytes apart. If a device returns less configuration
 300descriptor data than indicated by wTotalLength there will be a hole in
 301the file for the missing bytes.  This information is also shown
 302in text form by the ``/sys/kernel/debug/usb/devices`` file, described later.
 303
 304These files may also be used to write user-level drivers for the USB
 305devices.  You would open the ``/dev/bus/usb/BBB/DDD`` file read/write,
 306read its descriptors to make sure it's the device you expect, and then
 307bind to an interface (or perhaps several) using an ioctl call.  You
 308would issue more ioctls to the device to communicate to it using
 309control, bulk, or other kinds of USB transfers.  The IOCTLs are
 310listed in the ``<linux/usbdevice_fs.h>`` file, and at this writing the
 311source code (``linux/drivers/usb/core/devio.c``) is the primary reference
 312for how to access devices through those files.
 313
 314Note that since by default these ``BBB/DDD`` files are writable only by
 315root, only root can write such user mode drivers.  You can selectively
 316grant read/write permissions to other users by using ``chmod``.  Also,
 317usbfs mount options such as ``devmode=0666`` may be helpful.
 318
 319
 320Life Cycle of User Mode Drivers
 321-------------------------------
 322
 323Such a driver first needs to find a device file for a device it knows
 324how to handle. Maybe it was told about it because a ``/sbin/hotplug``
 325event handling agent chose that driver to handle the new device. Or
 326maybe it's an application that scans all the ``/dev/bus/usb`` device files,
 327and ignores most devices. In either case, it should :c:func:`read()`
 328all the descriptors from the device file, and check them against what it
 329knows how to handle. It might just reject everything except a particular
 330vendor and product ID, or need a more complex policy.
 331
 332Never assume there will only be one such device on the system at a time!
 333If your code can't handle more than one device at a time, at least
 334detect when there's more than one, and have your users choose which
 335device to use.
 336
 337Once your user mode driver knows what device to use, it interacts with
 338it in either of two styles. The simple style is to make only control
 339requests; some devices don't need more complex interactions than those.
 340(An example might be software using vendor-specific control requests for
 341some initialization or configuration tasks, with a kernel driver for the
 342rest.)
 343
 344More likely, you need a more complex style driver: one using non-control
 345endpoints, reading or writing data and claiming exclusive use of an
 346interface. *Bulk* transfers are easiest to use, but only their sibling
 347*interrupt* transfers work with low speed devices. Both interrupt and
 348*isochronous* transfers offer service guarantees because their bandwidth
 349is reserved. Such "periodic" transfers are awkward to use through usbfs,
 350unless you're using the asynchronous calls. However, interrupt transfers
 351can also be used in a synchronous "one shot" style.
 352
 353Your user-mode driver should never need to worry about cleaning up
 354request state when the device is disconnected, although it should close
 355its open file descriptors as soon as it starts seeing the ENODEV errors.
 356
 357The ioctl() Requests
 358--------------------
 359
 360To use these ioctls, you need to include the following headers in your
 361userspace program::
 362
 363    #include <linux/usb.h>
 364    #include <linux/usbdevice_fs.h>
 365    #include <asm/byteorder.h>
 366
 367The standard USB device model requests, from "Chapter 9" of the USB 2.0
 368specification, are automatically included from the ``<linux/usb/ch9.h>``
 369header.
 370
 371Unless noted otherwise, the ioctl requests described here will update
 372the modification time on the usbfs file to which they are applied
 373(unless they fail). A return of zero indicates success; otherwise, a
 374standard USB error code is returned (These are documented in
 375:ref:`usb-error-codes`).
 376
 377Each of these files multiplexes access to several I/O streams, one per
 378endpoint. Each device has one control endpoint (endpoint zero) which
 379supports a limited RPC style RPC access. Devices are configured by
 380hub_wq (in the kernel) setting a device-wide *configuration* that
 381affects things like power consumption and basic functionality. The
 382endpoints are part of USB *interfaces*, which may have *altsettings*
 383affecting things like which endpoints are available. Many devices only
 384have a single configuration and interface, so drivers for them will
 385ignore configurations and altsettings.
 386
 387Management/Status Requests
 388~~~~~~~~~~~~~~~~~~~~~~~~~~
 389
 390A number of usbfs requests don't deal very directly with device I/O.
 391They mostly relate to device management and status. These are all
 392synchronous requests.
 393
 394USBDEVFS_CLAIMINTERFACE
 395    This is used to force usbfs to claim a specific interface, which has
 396    not previously been claimed by usbfs or any other kernel driver. The
 397    ioctl parameter is an integer holding the number of the interface
 398    (bInterfaceNumber from descriptor).
 399
 400    Note that if your driver doesn't claim an interface before trying to
 401    use one of its endpoints, and no other driver has bound to it, then
 402    the interface is automatically claimed by usbfs.
 403
 404    This claim will be released by a RELEASEINTERFACE ioctl, or by
 405    closing the file descriptor. File modification time is not updated
 406    by this request.
 407
 408USBDEVFS_CONNECTINFO
 409    Says whether the device is lowspeed. The ioctl parameter points to a
 410    structure like this::
 411
 412	struct usbdevfs_connectinfo {
 413		unsigned int   devnum;
 414		unsigned char  slow;
 415	};
 416
 417    File modification time is not updated by this request.
 418
 419    *You can't tell whether a "not slow" device is connected at high
 420    speed (480 MBit/sec) or just full speed (12 MBit/sec).* You should
 421    know the devnum value already, it's the DDD value of the device file
 422    name.
 423
 424USBDEVFS_GET_SPEED
 425    Returns the speed of the device. The speed is returned as a
 426    numerical value in accordance with enum usb_device_speed
 427
 428    File modification time is not updated by this request.
 429
 430USBDEVFS_GETDRIVER
 431    Returns the name of the kernel driver bound to a given interface (a
 432    string). Parameter is a pointer to this structure, which is
 433    modified::
 434
 435	struct usbdevfs_getdriver {
 436		unsigned int  interface;
 437		char          driver[USBDEVFS_MAXDRIVERNAME + 1];
 438	};
 439
 440    File modification time is not updated by this request.
 441
 442USBDEVFS_IOCTL
 443    Passes a request from userspace through to a kernel driver that has
 444    an ioctl entry in the *struct usb_driver* it registered::
 445
 446	struct usbdevfs_ioctl {
 447		int     ifno;
 448		int     ioctl_code;
 449		void    *data;
 450	};
 451
 452	/* user mode call looks like this.
 453	 * 'request' becomes the driver->ioctl() 'code' parameter.
 454	 * the size of 'param' is encoded in 'request', and that data
 455	 * is copied to or from the driver->ioctl() 'buf' parameter.
 456	 */
 457	static int
 458	usbdev_ioctl (int fd, int ifno, unsigned request, void *param)
 459	{
 460		struct usbdevfs_ioctl   wrapper;
 461
 462		wrapper.ifno = ifno;
 463		wrapper.ioctl_code = request;
 464		wrapper.data = param;
 465
 466		return ioctl (fd, USBDEVFS_IOCTL, &wrapper);
 467	}
 468
 469    File modification time is not updated by this request.
 470
 471    This request lets kernel drivers talk to user mode code through
 472    filesystem operations even when they don't create a character or
 473    block special device. It's also been used to do things like ask
 474    devices what device special file should be used. Two pre-defined
 475    ioctls are used to disconnect and reconnect kernel drivers, so that
 476    user mode code can completely manage binding and configuration of
 477    devices.
 478
 479USBDEVFS_RELEASEINTERFACE
 480    This is used to release the claim usbfs made on interface, either
 481    implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the
 482    file descriptor is closed. The ioctl parameter is an integer holding
 483    the number of the interface (bInterfaceNumber from descriptor); File
 484    modification time is not updated by this request.
 485
 486    .. warning::
 487
 488	*No security check is made to ensure that the task which made
 489	the claim is the one which is releasing it. This means that user
 490	mode driver may interfere other ones.*
 491
 492USBDEVFS_RESETEP
 493    Resets the data toggle value for an endpoint (bulk or interrupt) to
 494    DATA0. The ioctl parameter is an integer endpoint number (1 to 15,
 495    as identified in the endpoint descriptor), with USB_DIR_IN added
 496    if the device's endpoint sends data to the host.
 497
 498    .. Warning::
 499
 500	*Avoid using this request. It should probably be removed.* Using
 501	it typically means the device and driver will lose toggle
 502	synchronization. If you really lost synchronization, you likely
 503	need to completely handshake with the device, using a request
 504	like CLEAR_HALT or SET_INTERFACE.
 505
 506USBDEVFS_DROP_PRIVILEGES
 507    This is used to relinquish the ability to do certain operations
 508    which are considered to be privileged on a usbfs file descriptor.
 509    This includes claiming arbitrary interfaces, resetting a device on
 510    which there are currently claimed interfaces from other users, and
 511    issuing USBDEVFS_IOCTL calls. The ioctl parameter is a 32 bit mask
 512    of interfaces the user is allowed to claim on this file descriptor.
 513    You may issue this ioctl more than one time to narrow said mask.
 514
 515Synchronous I/O Support
 516~~~~~~~~~~~~~~~~~~~~~~~
 517
 518Synchronous requests involve the kernel blocking until the user mode
 519request completes, either by finishing successfully or by reporting an
 520error. In most cases this is the simplest way to use usbfs, although as
 521noted above it does prevent performing I/O to more than one endpoint at
 522a time.
 523
 524USBDEVFS_BULK
 525    Issues a bulk read or write request to the device. The ioctl
 526    parameter is a pointer to this structure::
 527
 528	struct usbdevfs_bulktransfer {
 529		unsigned int  ep;
 530		unsigned int  len;
 531		unsigned int  timeout; /* in milliseconds */
 532		void          *data;
 533	};
 534
 535    The ``ep`` value identifies a bulk endpoint number (1 to 15, as
 536    identified in an endpoint descriptor), masked with USB_DIR_IN when
 537    referring to an endpoint which sends data to the host from the
 538    device. The length of the data buffer is identified by ``len``; Recent
 539    kernels support requests up to about 128KBytes. *FIXME say how read
 540    length is returned, and how short reads are handled.*.
 541
 542USBDEVFS_CLEAR_HALT
 543    Clears endpoint halt (stall) and resets the endpoint toggle. This is
 544    only meaningful for bulk or interrupt endpoints. The ioctl parameter
 545    is an integer endpoint number (1 to 15, as identified in an endpoint
 546    descriptor), masked with USB_DIR_IN when referring to an endpoint
 547    which sends data to the host from the device.
 548
 549    Use this on bulk or interrupt endpoints which have stalled,
 550    returning ``-EPIPE`` status to a data transfer request. Do not issue
 551    the control request directly, since that could invalidate the host's
 552    record of the data toggle.
 553
 554USBDEVFS_CONTROL
 555    Issues a control request to the device. The ioctl parameter points
 556    to a structure like this::
 557
 558	struct usbdevfs_ctrltransfer {
 559		__u8   bRequestType;
 560		__u8   bRequest;
 561		__u16  wValue;
 562		__u16  wIndex;
 563		__u16  wLength;
 564		__u32  timeout;  /* in milliseconds */
 565		void   *data;
 566	};
 567
 568    The first eight bytes of this structure are the contents of the
 569    SETUP packet to be sent to the device; see the USB 2.0 specification
 570    for details. The bRequestType value is composed by combining a
 571    ``USB_TYPE_*`` value, a ``USB_DIR_*`` value, and a ``USB_RECIP_*``
 572    value (from ``linux/usb.h``). If wLength is nonzero, it describes
 573    the length of the data buffer, which is either written to the device
 574    (USB_DIR_OUT) or read from the device (USB_DIR_IN).
 575
 576    At this writing, you can't transfer more than 4 KBytes of data to or
 577    from a device; usbfs has a limit, and some host controller drivers
 578    have a limit. (That's not usually a problem.) *Also* there's no way
 579    to say it's not OK to get a short read back from the device.
 580
 581USBDEVFS_RESET
 582    Does a USB level device reset. The ioctl parameter is ignored. After
 583    the reset, this rebinds all device interfaces. File modification
 584    time is not updated by this request.
 585
 586.. warning::
 587
 588	*Avoid using this call* until some usbcore bugs get fixed, since
 589	it does not fully synchronize device, interface, and driver (not
 590	just usbfs) state.
 591
 592USBDEVFS_SETINTERFACE
 593    Sets the alternate setting for an interface. The ioctl parameter is
 594    a pointer to a structure like this::
 595
 596	struct usbdevfs_setinterface {
 597		unsigned int  interface;
 598		unsigned int  altsetting;
 599	};
 600
 601    File modification time is not updated by this request.
 602
 603    Those struct members are from some interface descriptor applying to
 604    the current configuration. The interface number is the
 605    bInterfaceNumber value, and the altsetting number is the
 606    bAlternateSetting value. (This resets each endpoint in the
 607    interface.)
 608
 609USBDEVFS_SETCONFIGURATION
 610    Issues the :c:func:`usb_set_configuration()` call for the
 611    device. The parameter is an integer holding the number of a
 612    configuration (bConfigurationValue from descriptor). File
 613    modification time is not updated by this request.
 614
 615.. warning::
 616
 617	*Avoid using this call* until some usbcore bugs get fixed, since
 618	it does not fully synchronize device, interface, and driver (not
 619	just usbfs) state.
 620
 621Asynchronous I/O Support
 622~~~~~~~~~~~~~~~~~~~~~~~~
 623
 624As mentioned above, there are situations where it may be important to
 625initiate concurrent operations from user mode code. This is particularly
 626important for periodic transfers (interrupt and isochronous), but it can
 627be used for other kinds of USB requests too. In such cases, the
 628asynchronous requests described here are essential. Rather than
 629submitting one request and having the kernel block until it completes,
 630the blocking is separate.
 631
 632These requests are packaged into a structure that resembles the URB used
 633by kernel device drivers. (No POSIX Async I/O support here, sorry.) It
 634identifies the endpoint type (``USBDEVFS_URB_TYPE_*``), endpoint
 635(number, masked with USB_DIR_IN as appropriate), buffer and length,
 636and a user "context" value serving to uniquely identify each request.
 637(It's usually a pointer to per-request data.) Flags can modify requests
 638(not as many as supported for kernel drivers).
 639
 640Each request can specify a realtime signal number (between SIGRTMIN and
 641SIGRTMAX, inclusive) to request a signal be sent when the request
 642completes.
 643
 644When usbfs returns these urbs, the status value is updated, and the
 645buffer may have been modified. Except for isochronous transfers, the
 646actual_length is updated to say how many bytes were transferred; if the
 647USBDEVFS_URB_DISABLE_SPD flag is set ("short packets are not OK"), if
 648fewer bytes were read than were requested then you get an error report::
 649
 650    struct usbdevfs_iso_packet_desc {
 651	    unsigned int                     length;
 652	    unsigned int                     actual_length;
 653	    unsigned int                     status;
 654    };
 655
 656    struct usbdevfs_urb {
 657	    unsigned char                    type;
 658	    unsigned char                    endpoint;
 659	    int                              status;
 660	    unsigned int                     flags;
 661	    void                             *buffer;
 662	    int                              buffer_length;
 663	    int                              actual_length;
 664	    int                              start_frame;
 665	    int                              number_of_packets;
 666	    int                              error_count;
 667	    unsigned int                     signr;
 668	    void                             *usercontext;
 669	    struct usbdevfs_iso_packet_desc  iso_frame_desc[];
 670    };
 671
 672For these asynchronous requests, the file modification time reflects
 673when the request was initiated. This contrasts with their use with the
 674synchronous requests, where it reflects when requests complete.
 675
 676USBDEVFS_DISCARDURB
 677    *TBS* File modification time is not updated by this request.
 678
 679USBDEVFS_DISCSIGNAL
 680    *TBS* File modification time is not updated by this request.
 681
 682USBDEVFS_REAPURB
 683    *TBS* File modification time is not updated by this request.
 684
 685USBDEVFS_REAPURBNDELAY
 686    *TBS* File modification time is not updated by this request.
 687
 688USBDEVFS_SUBMITURB
 689    *TBS*
 690
 691The USB devices
 692===============
 693
 694The USB devices are now exported via debugfs:
 695
 696-  ``/sys/kernel/debug/usb/devices`` ... a text file showing each of the USB
 697   devices on known to the kernel, and their configuration descriptors.
 698   You can also poll() this to learn about new devices.
 699
 700/sys/kernel/debug/usb/devices
 701-----------------------------
 702
 703This file is handy for status viewing tools in user mode, which can scan
 704the text format and ignore most of it. More detailed device status
 705(including class and vendor status) is available from device-specific
 706files. For information about the current format of this file, see below.
 707
 708This file, in combination with the poll() system call, can also be used
 709to detect when devices are added or removed::
 710
 711    int fd;
 712    struct pollfd pfd;
 713
 714    fd = open("/sys/kernel/debug/usb/devices", O_RDONLY);
 715    pfd = { fd, POLLIN, 0 };
 716    for (;;) {
 717	/* The first time through, this call will return immediately. */
 718	poll(&pfd, 1, -1);
 719
 720	/* To see what's changed, compare the file's previous and current
 721	   contents or scan the filesystem.  (Scanning is more precise.) */
 722    }
 723
 724Note that this behavior is intended to be used for informational and
 725debug purposes. It would be more appropriate to use programs such as
 726udev or HAL to initialize a device or start a user-mode helper program,
 727for instance.
 728
 729In this file, each device's output has multiple lines of ASCII output.
 730
 731I made it ASCII instead of binary on purpose, so that someone
 732can obtain some useful data from it without the use of an
 733auxiliary program.  However, with an auxiliary program, the numbers
 734in the first 4 columns of each ``T:`` line (topology info:
 735Lev, Prnt, Port, Cnt) can be used to build a USB topology diagram.
 736
 737Each line is tagged with a one-character ID for that line::
 738
 739	T = Topology (etc.)
 740	B = Bandwidth (applies only to USB host controllers, which are
 741	virtualized as root hubs)
 742	D = Device descriptor info.
 743	P = Product ID info. (from Device descriptor, but they won't fit
 744	together on one line)
 745	S = String descriptors.
 746	C = Configuration descriptor info. (* = active configuration)
 747	I = Interface descriptor info.
 748	E = Endpoint descriptor info.
 749
 750/sys/kernel/debug/usb/devices output format
 751~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 752
 753Legend::
 754  d = decimal number (may have leading spaces or 0's)
 755  x = hexadecimal number (may have leading spaces or 0's)
 756  s = string
 757
 758
 759
 760Topology info
 761^^^^^^^^^^^^^
 762
 763::
 764
 765	T:  Bus=dd Lev=dd Prnt=dd Port=dd Cnt=dd Dev#=ddd Spd=dddd MxCh=dd
 766	|   |      |      |       |       |      |        |        |__MaxChildren
 767	|   |      |      |       |       |      |        |__Device Speed in Mbps
 768	|   |      |      |       |       |      |__DeviceNumber
 769	|   |      |      |       |       |__Count of devices at this level
 770	|   |      |      |       |__Connector/Port on Parent for this device
 771	|   |      |      |__Parent DeviceNumber
 772	|   |      |__Level in topology for this bus
 773	|   |__Bus number
 774	|__Topology info tag
 775
 776Speed may be:
 777
 778	======= ======================================================
 779	1.5	Mbit/s for low speed USB
 780	12	Mbit/s for full speed USB
 781	480	Mbit/s for high speed USB (added for USB 2.0)
 782	5000	Mbit/s for SuperSpeed USB (added for USB 3.0)
 783	======= ======================================================
 784
 785For reasons lost in the mists of time, the Port number is always
 786too low by 1.  For example, a device plugged into port 4 will
 787show up with ``Port=03``.
 788
 789Bandwidth info
 790^^^^^^^^^^^^^^
 791
 792::
 793
 794	B:  Alloc=ddd/ddd us (xx%), #Int=ddd, #Iso=ddd
 795	|   |                       |         |__Number of isochronous requests
 796	|   |                       |__Number of interrupt requests
 797	|   |__Total Bandwidth allocated to this bus
 798	|__Bandwidth info tag
 799
 800Bandwidth allocation is an approximation of how much of one frame
 801(millisecond) is in use.  It reflects only periodic transfers, which
 802are the only transfers that reserve bandwidth.  Control and bulk
 803transfers use all other bandwidth, including reserved bandwidth that
 804is not used for transfers (such as for short packets).
 805
 806The percentage is how much of the "reserved" bandwidth is scheduled by
 807those transfers.  For a low or full speed bus (loosely, "USB 1.1"),
 80890% of the bus bandwidth is reserved.  For a high speed bus (loosely,
 809"USB 2.0") 80% is reserved.
 810
 811
 812Device descriptor info & Product ID info
 813^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 814
 815::
 816
 817	D:  Ver=x.xx Cls=xx(s) Sub=xx Prot=xx MxPS=dd #Cfgs=dd
 818	P:  Vendor=xxxx ProdID=xxxx Rev=xx.xx
 819
 820where::
 821
 822	D:  Ver=x.xx Cls=xx(sssss) Sub=xx Prot=xx MxPS=dd #Cfgs=dd
 823	|   |        |             |      |       |       |__NumberConfigurations
 824	|   |        |             |      |       |__MaxPacketSize of Default Endpoint
 825	|   |        |             |      |__DeviceProtocol
 826	|   |        |             |__DeviceSubClass
 827	|   |        |__DeviceClass
 828	|   |__Device USB version
 829	|__Device info tag #1
 830
 831where::
 832
 833	P:  Vendor=xxxx ProdID=xxxx Rev=xx.xx
 834	|   |           |           |__Product revision number
 835	|   |           |__Product ID code
 836	|   |__Vendor ID code
 837	|__Device info tag #2
 838
 839
 840String descriptor info
 841^^^^^^^^^^^^^^^^^^^^^^
 842::
 843
 844	S:  Manufacturer=ssss
 845	|   |__Manufacturer of this device as read from the device.
 846	|      For USB host controller drivers (virtual root hubs) this may
 847	|      be omitted, or (for newer drivers) will identify the kernel
 848	|      version and the driver which provides this hub emulation.
 849	|__String info tag
 850
 851	S:  Product=ssss
 852	|   |__Product description of this device as read from the device.
 853	|      For older USB host controller drivers (virtual root hubs) this
 854	|      indicates the driver; for newer ones, it's a product (and vendor)
 855	|      description that often comes from the kernel's PCI ID database.
 856	|__String info tag
 857
 858	S:  SerialNumber=ssss
 859	|   |__Serial Number of this device as read from the device.
 860	|      For USB host controller drivers (virtual root hubs) this is
 861	|      some unique ID, normally a bus ID (address or slot name) that
 862	|      can't be shared with any other device.
 863	|__String info tag
 864
 865
 866
 867Configuration descriptor info
 868^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 869::
 870
 871	C:* #Ifs=dd Cfg#=dd Atr=xx MPwr=dddmA
 872	| | |       |       |      |__MaxPower in mA
 873	| | |       |       |__Attributes
 874	| | |       |__ConfiguratioNumber
 875	| | |__NumberOfInterfaces
 876	| |__ "*" indicates the active configuration (others are " ")
 877	|__Config info tag
 878
 879USB devices may have multiple configurations, each of which act
 880rather differently.  For example, a bus-powered configuration
 881might be much less capable than one that is self-powered.  Only
 882one device configuration can be active at a time; most devices
 883have only one configuration.
 884
 885Each configuration consists of one or more interfaces.  Each
 886interface serves a distinct "function", which is typically bound
 887to a different USB device driver.  One common example is a USB
 888speaker with an audio interface for playback, and a HID interface
 889for use with software volume control.
 890
 891Interface descriptor info (can be multiple per Config)
 892^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 893::
 894
 895	I:* If#=dd Alt=dd #EPs=dd Cls=xx(sssss) Sub=xx Prot=xx Driver=ssss
 896	| | |      |      |       |             |      |       |__Driver name
 897	| | |      |      |       |             |      |          or "(none)"
 898	| | |      |      |       |             |      |__InterfaceProtocol
 899	| | |      |      |       |             |__InterfaceSubClass
 900	| | |      |      |       |__InterfaceClass
 901	| | |      |      |__NumberOfEndpoints
 902	| | |      |__AlternateSettingNumber
 903	| | |__InterfaceNumber
 904	| |__ "*" indicates the active altsetting (others are " ")
 905	|__Interface info tag
 906
 907A given interface may have one or more "alternate" settings.
 908For example, default settings may not use more than a small
 909amount of periodic bandwidth.  To use significant fractions
 910of bus bandwidth, drivers must select a non-default altsetting.
 911
 912Only one setting for an interface may be active at a time, and
 913only one driver may bind to an interface at a time.  Most devices
 914have only one alternate setting per interface.
 915
 916
 917Endpoint descriptor info (can be multiple per Interface)
 918^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 919
 920::
 921
 922	E:  Ad=xx(s) Atr=xx(ssss) MxPS=dddd Ivl=dddss
 923	|   |        |            |         |__Interval (max) between transfers
 924	|   |        |            |__EndpointMaxPacketSize
 925	|   |        |__Attributes(EndpointType)
 926	|   |__EndpointAddress(I=In,O=Out)
 927	|__Endpoint info tag
 928
 929The interval is nonzero for all periodic (interrupt or isochronous)
 930endpoints.  For high speed endpoints the transfer interval may be
 931measured in microseconds rather than milliseconds.
 932
 933For high speed periodic endpoints, the ``EndpointMaxPacketSize`` reflects
 934the per-microframe data transfer size.  For "high bandwidth"
 935endpoints, that can reflect two or three packets (for up to
 9363KBytes every 125 usec) per endpoint.
 937
 938With the Linux-USB stack, periodic bandwidth reservations use the
 939transfer intervals and sizes provided by URBs, which can be less
 940than those found in endpoint descriptor.
 941
 942Usage examples
 943~~~~~~~~~~~~~~
 944
 945If a user or script is interested only in Topology info, for
 946example, use something like ``grep ^T: /sys/kernel/debug/usb/devices``
 947for only the Topology lines.  A command like
 948``grep -i ^[tdp]: /sys/kernel/debug/usb/devices`` can be used to list
 949only the lines that begin with the characters in square brackets,
 950where the valid characters are TDPCIE.  With a slightly more able
 951script, it can display any selected lines (for example, only T, D,
 952and P lines) and change their output format.  (The ``procusb``
 953Perl script is the beginning of this idea.  It will list only
 954selected lines [selected from TBDPSCIE] or "All" lines from
 955``/sys/kernel/debug/usb/devices``.)
 956
 957The Topology lines can be used to generate a graphic/pictorial
 958of the USB devices on a system's root hub.  (See more below
 959on how to do this.)
 960
 961The Interface lines can be used to determine what driver is
 962being used for each device, and which altsetting it activated.
 963
 964The Configuration lines could be used to list maximum power
 965(in milliamps) that a system's USB devices are using.
 966For example, ``grep ^C: /sys/kernel/debug/usb/devices``.
 967
 968
 969Here's an example, from a system which has a UHCI root hub,
 970an external hub connected to the root hub, and a mouse and
 971a serial converter connected to the external hub.
 972
 973::
 974
 975	T:  Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12   MxCh= 2
 976	B:  Alloc= 28/900 us ( 3%), #Int=  2, #Iso=  0
 977	D:  Ver= 1.00 Cls=09(hub  ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
 978	P:  Vendor=0000 ProdID=0000 Rev= 0.00
 979	S:  Product=USB UHCI Root Hub
 980	S:  SerialNumber=dce0
 981	C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr=  0mA
 982	I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
 983	E:  Ad=81(I) Atr=03(Int.) MxPS=   8 Ivl=255ms
 984
 985	T:  Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12   MxCh= 4
 986	D:  Ver= 1.00 Cls=09(hub  ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
 987	P:  Vendor=0451 ProdID=1446 Rev= 1.00
 988	C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=100mA
 989	I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
 990	E:  Ad=81(I) Atr=03(Int.) MxPS=   1 Ivl=255ms
 991
 992	T:  Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#=  3 Spd=1.5  MxCh= 0
 993	D:  Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
 994	P:  Vendor=04b4 ProdID=0001 Rev= 0.00
 995	C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=100mA
 996	I:  If#= 0 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=01 Prot=02 Driver=mouse
 997	E:  Ad=81(I) Atr=03(Int.) MxPS=   3 Ivl= 10ms
 998
 999	T:  Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#=  4 Spd=12   MxCh= 0
1000	D:  Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
1001	P:  Vendor=0565 ProdID=0001 Rev= 1.08
1002	S:  Manufacturer=Peracom Networks, Inc.
1003	S:  Product=Peracom USB to Serial Converter
1004	C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=100mA
1005	I:  If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial
1006	E:  Ad=81(I) Atr=02(Bulk) MxPS=  64 Ivl= 16ms
1007	E:  Ad=01(O) Atr=02(Bulk) MxPS=  16 Ivl= 16ms
1008	E:  Ad=82(I) Atr=03(Int.) MxPS=   8 Ivl=  8ms
1009
1010
1011Selecting only the ``T:`` and ``I:`` lines from this (for example, by using
1012``procusb ti``), we have
1013
1014::
1015
1016	T:  Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12   MxCh= 2
1017	T:  Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12   MxCh= 4
1018	I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
1019	T:  Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#=  3 Spd=1.5  MxCh= 0
1020	I:  If#= 0 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=01 Prot=02 Driver=mouse
1021	T:  Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#=  4 Spd=12   MxCh= 0
1022	I:  If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial
1023
1024
1025Physically this looks like (or could be converted to)::
1026
1027                      +------------------+
1028                      |  PC/root_hub (12)|   Dev# = 1
1029                      +------------------+   (nn) is Mbps.
1030    Level 0           |  CN.0   |  CN.1  |   [CN = connector/port #]
1031                      +------------------+
1032                          /
1033                         /
1034            +-----------------------+
1035  Level 1   | Dev#2: 4-port hub (12)|
1036            +-----------------------+
1037            |CN.0 |CN.1 |CN.2 |CN.3 |
1038            +-----------------------+
1039                \           \____________________
1040                 \_____                          \
1041                       \                          \
1042               +--------------------+      +--------------------+
1043  Level 2      | Dev# 3: mouse (1.5)|      | Dev# 4: serial (12)|
1044               +--------------------+      +--------------------+
1045
1046
1047
1048Or, in a more tree-like structure (ports [Connectors] without
1049connections could be omitted)::
1050
1051	PC:  Dev# 1, root hub, 2 ports, 12 Mbps
1052	|_ CN.0:  Dev# 2, hub, 4 ports, 12 Mbps
1053	     |_ CN.0:  Dev #3, mouse, 1.5 Mbps
1054	     |_ CN.1:
1055	     |_ CN.2:  Dev #4, serial, 12 Mbps
1056	     |_ CN.3:
1057	|_ CN.1: