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 .. _usb-hostside-api:
 
 ===========================
 The Linux-USB Host Side API
 ===========================
 
 Introduction to USB on Linux
 ============================
 
 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.
 
 USB Host-Side API Model
 =======================
 
 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.
 
 .. _usb_chapter9:
 
 USB-Standard Types
 ==================
 
 In ``include/uapi/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, usb character
 devices and debugfs interfaces. That file is itself included by
 ``include/linux/usb/ch9.h``, which also contains declarations of a few
 utility routines for manipulating these data types; the implementations
 are in ``drivers/usb/common/common.c``.
 
 .. kernel-doc:: drivers/usb/common/common.c
    :export:
 
 In addition, some functions useful for creating debugging output are
 defined in ``drivers/usb/common/debug.c``.
 
 .. _usb_header:
 
 Host-Side Data Types and Macros
 ===============================
 
 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.
 
 .. kernel-doc:: include/linux/usb.h
    :internal:
 
 USB Core APIs
 =============
 
 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.
 
 .. kernel-doc:: drivers/usb/core/urb.c
    :export:
 
 .. kernel-doc:: drivers/usb/core/message.c
    :export:
 
 .. kernel-doc:: drivers/usb/core/file.c
    :export:
 
 .. kernel-doc:: drivers/usb/core/driver.c
    :export:
 
 .. kernel-doc:: drivers/usb/core/usb.c
    :export:
 
 .. kernel-doc:: drivers/usb/core/hub.c
    :export:
 
 Host Controller APIs
 ====================
 
 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: :c:type:`struct
 usb_bus <usb_bus>` is a rather thin layer that became available
 in the 2.2 kernels, while :c:type:`struct usb_hcd <usb_hcd>`
 is a more featureful layer
 that lets HCDs share common code, to shrink driver size and
 significantly reduce hcd-specific behaviors.
 
 .. kernel-doc:: drivers/usb/core/hcd.c
    :export:
 
 .. kernel-doc:: drivers/usb/core/hcd-pci.c
    :export:
 
 .. kernel-doc:: drivers/usb/core/buffer.c
    :internal:
 
 The USB character device nodes
 ==============================
 
 This chapter presents the Linux character device nodes. You may prefer
 to avoid writing new kernel code for your USB driver. User mode device
 drivers are usually packaged as applications or libraries, and may use
 character devices through some programming library that wraps it.
 Such libraries include:
 
  - `libusb <http://libusb.sourceforge.net>`__ for C/C++, and
  - `jUSB <http://jUSB.sourceforge.net>`__ for Java.
 
 Some old information about it can be seen at the "USB Device Filesystem"
 section of the USB Guide. The latest copy of the USB Guide can be found
 at http://www.linux-usb.org/
 
 .. note::
 
   - They were used to be implemented via *usbfs*, but this is not part of
     the sysfs debug interface.
 
    - 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.
 
 What files are in "devtmpfs"?
 -----------------------------
 
 Conventionally mounted at ``/dev/bus/usb/``, usbfs features include:
 
 -  ``/dev/bus/usb/BBB/DDD`` ... magic files exposing the each device's
    configuration descriptors, and supporting a series of ioctls for
    making device requests, including I/O to devices. (Purely for access
    by programs.)
 
 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. Pleast note that it doesn't (yet) expose those IDs.
 
 /dev/bus/usb/BBB/DDD
 --------------------
 
 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.
 
 Each connected USB device has one file.  The ``BBB`` indicates the bus
 number.  The ``DDD`` indicates the device address on that bus.  Both
 of these numbers are assigned sequentially, and can be reused, so
 you can't rely on them for stable access to devices.  For example,
 it's relatively common for devices to re-enumerate while they are
 still connected (perhaps someone jostled their power supply, hub,
 or USB cable), so a device might be ``002/027`` when you first connect
 it and ``002/048`` sometime later.
 
 These files can be read as binary data.  The binary data consists
 of first the device descriptor, then the descriptors for each
 configuration of the device.  Multi-byte fields in the device descriptor
 are converted to host endianness by the kernel.  The configuration
 descriptors are in bus endian format! The configuration descriptor
 are wTotalLength bytes apart. If a device returns less configuration
 descriptor data than indicated by wTotalLength there will be a hole in
 the file for the missing bytes.  This information is also shown
 in text form by the ``/sys/kernel/debug/usb/devices`` file, described later.
 
 These files may also be used to write user-level drivers for the USB
 devices.  You would open the ``/dev/bus/usb/BBB/DDD`` file read/write,
 read its descriptors to make sure it's the device you expect, and then
 bind to an interface (or perhaps several) using an ioctl call.  You
 would issue more ioctls to the device to communicate to it using
 control, bulk, or other kinds of USB transfers.  The IOCTLs are
 listed in the ``<linux/usbdevice_fs.h>`` file, and at this writing the
 source code (``linux/drivers/usb/core/devio.c``) is the primary reference
 for how to access devices through those files.
 
 Note that since by default these ``BBB/DDD`` files are writable only by
 root, only root can write such user mode drivers.  You can selectively
 grant read/write permissions to other users by using ``chmod``.  Also,
 usbfs mount options such as ``devmode=0666`` may be helpful.
 
 
 Life Cycle of User Mode Drivers
 -------------------------------
 
 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 ``/dev/bus/usb`` device files,
 and ignores most devices. In either case, it should :c:func:`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.
 
 The ioctl() Requests
 --------------------
 
 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
 :ref:`usb-error-codes`).
 
 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.
 
 Management/Status Requests
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 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.
 
 USBDEVFS_CLAIMINTERFACE
     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.
 
 USBDEVFS_CONNECTINFO
     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.
 
 USBDEVFS_GETDRIVER
     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.
 
 USBDEVFS_IOCTL
     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.
 
 USBDEVFS_RELEASEINTERFACE
     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.*
 
 USBDEVFS_RESETEP
     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.
 
 USBDEVFS_DROP_PRIVILEGES
     This is used to relinquish the ability to do certain operations
     which are considered to be privileged on a usbfs file descriptor.
     This includes claiming arbitrary interfaces, resetting a device on
     which there are currently claimed interfaces from other users, and
     issuing USBDEVFS_IOCTL calls. The ioctl parameter is a 32 bit mask
     of interfaces the user is allowed to claim on this file descriptor.
     You may issue this ioctl more than one time to narrow said mask.
 
 Synchronous I/O Support
 ~~~~~~~~~~~~~~~~~~~~~~~
 
 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.
 
 USBDEVFS_BULK
     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.*.
 
 USBDEVFS_CLEAR_HALT
     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.
 
 USBDEVFS_CONTROL
     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.
 
 USBDEVFS_RESET
     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.
 
 USBDEVFS_SETINTERFACE
     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.)
 
 USBDEVFS_SETCONFIGURATION
     Issues the :c:func:`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.
 
 Asynchronous I/O Support
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 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.
 
 USBDEVFS_DISCARDURB
     *TBS* File modification time is not updated by this request.
 
 USBDEVFS_DISCSIGNAL
     *TBS* File modification time is not updated by this request.
 
 USBDEVFS_REAPURB
     *TBS* File modification time is not updated by this request.
 
 USBDEVFS_REAPURBNDELAY
     *TBS* File modification time is not updated by this request.
 
 USBDEVFS_SUBMITURB
     *TBS*
 
 The USB devices
 ===============
 
 The USB devices are now exported via debugfs:
 
 -  ``/sys/kernel/debug/usb/devices`` ... a text file showing each of the USB
    devices on known to the kernel, and their configuration descriptors.
    You can also poll() this to learn about new devices.
 
 /sys/kernel/debug/usb/devices
 -----------------------------
 
 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 below.
 
 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("/sys/kernel/debug/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.
 
 In this file, each device's output has multiple lines of ASCII output.
 
 I made it ASCII instead of binary on purpose, so that someone
 can obtain some useful data from it without the use of an
 auxiliary program.  However, with an auxiliary program, the numbers
 in the first 4 columns of each ``T:`` line (topology info:
 Lev, Prnt, Port, Cnt) can be used to build a USB topology diagram.
 
 Each line is tagged with a one-character ID for that line::
 
         T = Topology (etc.)
         B = Bandwidth (applies only to USB host controllers, which are
         virtualized as root hubs)
         D = Device descriptor info.
         P = Product ID info. (from Device descriptor, but they won't fit
         together on one line)
         S = String descriptors.
         C = Configuration descriptor info. (* = active configuration)
         I = Interface descriptor info.
         E = Endpoint descriptor info.
 
 /sys/kernel/debug/usb/devices output format
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Legend::
   d = decimal number (may have leading spaces or 0's)
   x = hexadecimal number (may have leading spaces or 0's)
   s = string
 
 
 
 Topology info
 ^^^^^^^^^^^^^
 
 ::
 
         T:  Bus=dd Lev=dd Prnt=dd Port=dd Cnt=dd Dev#=ddd Spd=dddd MxCh=dd
         |   |      |      |       |       |      |        |        |__MaxChildren
         |   |      |      |       |       |      |        |__Device Speed in Mbps
         |   |      |      |       |       |      |__DeviceNumber
         |   |      |      |       |       |__Count of devices at this level
         |   |      |      |       |__Connector/Port on Parent for this device
         |   |      |      |__Parent DeviceNumber
         |   |      |__Level in topology for this bus
         |   |__Bus number
         |__Topology info tag
 
 Speed may be:
 
         ======= ======================================================
         1.5     Mbit/s for low speed USB
         12      Mbit/s for full speed USB
         480     Mbit/s for high speed USB (added for USB 2.0);
                 also used for Wireless USB, which has no fixed speed
         5000    Mbit/s for SuperSpeed USB (added for USB 3.0)
         ======= ======================================================
 
 For reasons lost in the mists of time, the Port number is always
 too low by 1.  For example, a device plugged into port 4 will
 show up with ``Port=03``.
 
 Bandwidth info
 ^^^^^^^^^^^^^^
 
 ::
 
         B:  Alloc=ddd/ddd us (xx%), #Int=ddd, #Iso=ddd
         |   |                       |         |__Number of isochronous requests
         |   |                       |__Number of interrupt requests
         |   |__Total Bandwidth allocated to this bus
         |__Bandwidth info tag
 
 Bandwidth allocation is an approximation of how much of one frame
 (millisecond) is in use.  It reflects only periodic transfers, which
 are the only transfers that reserve bandwidth.  Control and bulk
 transfers use all other bandwidth, including reserved bandwidth that
 is not used for transfers (such as for short packets).
 
 The percentage is how much of the "reserved" bandwidth is scheduled by
 those transfers.  For a low or full speed bus (loosely, "USB 1.1"),
 90% of the bus bandwidth is reserved.  For a high speed bus (loosely,
 "USB 2.0") 80% is reserved.
 
 
 Device descriptor info & Product ID info
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 ::
 
         D:  Ver=x.xx Cls=xx(s) Sub=xx Prot=xx MxPS=dd #Cfgs=dd
         P:  Vendor=xxxx ProdID=xxxx Rev=xx.xx
 
 where::
 
         D:  Ver=x.xx Cls=xx(sssss) Sub=xx Prot=xx MxPS=dd #Cfgs=dd
         |   |        |             |      |       |       |__NumberConfigurations
         |   |        |             |      |       |__MaxPacketSize of Default Endpoint
         |   |        |             |      |__DeviceProtocol
         |   |        |             |__DeviceSubClass
         |   |        |__DeviceClass
         |   |__Device USB version
         |__Device info tag #1
 
 where::
 
         P:  Vendor=xxxx ProdID=xxxx Rev=xx.xx
         |   |           |           |__Product revision number
         |   |           |__Product ID code
         |   |__Vendor ID code
         |__Device info tag #2
 
 
 String descriptor info
 ^^^^^^^^^^^^^^^^^^^^^^
 ::
 
         S:  Manufacturer=ssss
         |   |__Manufacturer of this device as read from the device.
         |      For USB host controller drivers (virtual root hubs) this may
         |      be omitted, or (for newer drivers) will identify the kernel
         |      version and the driver which provides this hub emulation.
         |__String info tag
 
         S:  Product=ssss
         |   |__Product description of this device as read from the device.
         |      For older USB host controller drivers (virtual root hubs) this
         |      indicates the driver; for newer ones, it's a product (and vendor)
         |      description that often comes from the kernel's PCI ID database.
         |__String info tag
 
         S:  SerialNumber=ssss
         |   |__Serial Number of this device as read from the device.
         |      For USB host controller drivers (virtual root hubs) this is
         |      some unique ID, normally a bus ID (address or slot name) that
         |      can't be shared with any other device.
         |__String info tag
 
 
 
 Configuration descriptor info
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 ::
 
         C:* #Ifs=dd Cfg#=dd Atr=xx MPwr=dddmA
         | | |       |       |      |__MaxPower in mA
         | | |       |       |__Attributes
         | | |       |__ConfiguratioNumber
         | | |__NumberOfInterfaces
         | |__ "*" indicates the active configuration (others are " ")
         |__Config info tag
 
 USB devices may have multiple configurations, each of which act
 rather differently.  For example, a bus-powered configuration
 might be much less capable than one that is self-powered.  Only
 one device configuration can be active at a time; most devices
 have only one configuration.
 
 Each configuration consists of one or more interfaces.  Each
 interface serves a distinct "function", which is typically bound
 to a different USB device driver.  One common example is a USB
 speaker with an audio interface for playback, and a HID interface
 for use with software volume control.
 
 Interface descriptor info (can be multiple per Config)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 ::
 
         I:* If#=dd Alt=dd #EPs=dd Cls=xx(sssss) Sub=xx Prot=xx Driver=ssss
         | | |      |      |       |             |      |       |__Driver name
         | | |      |      |       |             |      |          or "(none)"
         | | |      |      |       |             |      |__InterfaceProtocol
         | | |      |      |       |             |__InterfaceSubClass
         | | |      |      |       |__InterfaceClass
         | | |      |      |__NumberOfEndpoints
         | | |      |__AlternateSettingNumber
         | | |__InterfaceNumber
         | |__ "*" indicates the active altsetting (others are " ")
         |__Interface info tag
 
 A given interface may have one or more "alternate" settings.
 For example, default settings may not use more than a small
 amount of periodic bandwidth.  To use significant fractions
 of bus bandwidth, drivers must select a non-default altsetting.
 
 Only one setting for an interface may be active at a time, and
 only one driver may bind to an interface at a time.  Most devices
 have only one alternate setting per interface.
 
 
 Endpoint descriptor info (can be multiple per Interface)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 ::
 
         E:  Ad=xx(s) Atr=xx(ssss) MxPS=dddd Ivl=dddss
         |   |        |            |         |__Interval (max) between transfers
         |   |        |            |__EndpointMaxPacketSize
         |   |        |__Attributes(EndpointType)
         |   |__EndpointAddress(I=In,O=Out)
         |__Endpoint info tag
 
 The interval is nonzero for all periodic (interrupt or isochronous)
 endpoints.  For high speed endpoints the transfer interval may be
 measured in microseconds rather than milliseconds.
 
 For high speed periodic endpoints, the ``EndpointMaxPacketSize`` reflects
 the per-microframe data transfer size.  For "high bandwidth"
 endpoints, that can reflect two or three packets (for up to
 3KBytes every 125 usec) per endpoint.
 
 With the Linux-USB stack, periodic bandwidth reservations use the
 transfer intervals and sizes provided by URBs, which can be less
 than those found in endpoint descriptor.
 
 Usage examples
 ~~~~~~~~~~~~~~
 
 If a user or script is interested only in Topology info, for
 example, use something like ``grep ^T: /sys/kernel/debug/usb/devices``
 for only the Topology lines.  A command like
 ``grep -i ^[tdp]: /sys/kernel/debug/usb/devices`` can be used to list
 only the lines that begin with the characters in square brackets,
 where the valid characters are TDPCIE.  With a slightly more able
 script, it can display any selected lines (for example, only T, D,
 and P lines) and change their output format.  (The ``procusb``
 Perl script is the beginning of this idea.  It will list only
 selected lines [selected from TBDPSCIE] or "All" lines from
 ``/sys/kernel/debug/usb/devices``.)
 
 The Topology lines can be used to generate a graphic/pictorial
 of the USB devices on a system's root hub.  (See more below
 on how to do this.)
 
 The Interface lines can be used to determine what driver is
 being used for each device, and which altsetting it activated.
 
 The Configuration lines could be used to list maximum power
 (in milliamps) that a system's USB devices are using.
 For example, ``grep ^C: /sys/kernel/debug/usb/devices``.
 
 
 Here's an example, from a system which has a UHCI root hub,
 an external hub connected to the root hub, and a mouse and
 a serial converter connected to the external hub.
 
 ::
 
         T:  Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12   MxCh= 2
         B:  Alloc= 28/900 us ( 3%), #Int=  2, #Iso=  0
         D:  Ver= 1.00 Cls=09(hub  ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
         P:  Vendor=0000 ProdID=0000 Rev= 0.00
         S:  Product=USB UHCI Root Hub
         S:  SerialNumber=dce0
         C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr=  0mA
         I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
         E:  Ad=81(I) Atr=03(Int.) MxPS=   8 Ivl=255ms
 
         T:  Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12   MxCh= 4
         D:  Ver= 1.00 Cls=09(hub  ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
         P:  Vendor=0451 ProdID=1446 Rev= 1.00
         C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=100mA
         I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
         E:  Ad=81(I) Atr=03(Int.) MxPS=   1 Ivl=255ms
 
         T:  Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#=  3 Spd=1.5  MxCh= 0
         D:  Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
         P:  Vendor=04b4 ProdID=0001 Rev= 0.00
         C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=100mA
         I:  If#= 0 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=01 Prot=02 Driver=mouse
         E:  Ad=81(I) Atr=03(Int.) MxPS=   3 Ivl= 10ms
 
         T:  Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#=  4 Spd=12   MxCh= 0
         D:  Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
         P:  Vendor=0565 ProdID=0001 Rev= 1.08
         S:  Manufacturer=Peracom Networks, Inc.
         S:  Product=Peracom USB to Serial Converter
         C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=100mA
         I:  If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial
         E:  Ad=81(I) Atr=02(Bulk) MxPS=  64 Ivl= 16ms
         E:  Ad=01(O) Atr=02(Bulk) MxPS=  16 Ivl= 16ms
         E:  Ad=82(I) Atr=03(Int.) MxPS=   8 Ivl=  8ms
 
 
 Selecting only the ``T:`` and ``I:`` lines from this (for example, by using
 ``procusb ti``), we have
 
 ::
 
         T:  Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12   MxCh= 2
         T:  Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12   MxCh= 4
         I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
         T:  Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#=  3 Spd=1.5  MxCh= 0
         I:  If#= 0 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=01 Prot=02 Driver=mouse
         T:  Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#=  4 Spd=12   MxCh= 0
         I:  If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial
 
 
 Physically this looks like (or could be converted to)::
 
                       +------------------+
                       |  PC/root_hub (12)|   Dev# = 1
                       +------------------+   (nn) is Mbps.
     Level 0           |  CN.0   |  CN.1  |   [CN = connector/port #]
                       +------------------+
                           /
                          /
             +-----------------------+
   Level 1   | Dev#2: 4-port hub (12)|
             +-----------------------+
             |CN.0 |CN.1 |CN.2 |CN.3 |
             +-----------------------+
                 \           \____________________
                  \_____                          \
                        \                          \
                +--------------------+      +--------------------+
   Level 2      | Dev# 3: mouse (1.5)|      | Dev# 4: serial (12)|
                +--------------------+      +--------------------+
 
 
 
 Or, in a more tree-like structure (ports [Connectors] without
 connections could be omitted)::
 
         PC:  Dev# 1, root hub, 2 ports, 12 Mbps
         |_ CN.0:  Dev# 2, hub, 4 ports, 12 Mbps
              |_ CN.0:  Dev #3, mouse, 1.5 Mbps
              |_ CN.1:
              |_ CN.2:  Dev #4, serial, 12 Mbps
              |_ CN.3:
         |_ CN.1:

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