Documentation: convert USB to new format

This is a conversion of the USB documentation to the Sphinx format.
No content was altered or reformatted.

Signed-off-by: Oliver <oneukum@suse.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

2 files changed, 749 insertions, 0 deletions

diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index e18135b513e2..743828ead665 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -20,6 +20,7 @@ available subsections can be seen below.
    sound
    frame-buffer
    input
+   usb
    spi
    i2c
    hsi
diff --git a/Documentation/driver-api/usb.rst b/Documentation/driver-api/usb.rst
new file mode 100644
index 000000000000..851cc40b66b5
--- /dev/null
+++ b/Documentation/driver-api/usb.rst
@@ -0,0 +1,748 @@
+===========================
+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-Standard Types
+==================
+
+In ``<linux/usb/ch9.h>`` you will find the USB data types defined in
+chapter 9 of the USB specification. These data types are used throughout
+USB, and in APIs including this host side API, gadget APIs, and usbfs.
+
+.. kernel-doc:: include/linux/usb/ch9.h
+   :internal:
+
+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 Filesystem (usbfs)
+==========================
+
+This chapter presents the Linux *usbfs*. You may prefer to avoid writing
+new kernel code for your USB driver; that's the problem that usbfs set
+out to solve. User mode device drivers are usually packaged as
+applications or libraries, and may use usbfs through some programming
+library that wraps it. Such libraries include
+`libusb <http://libusb.sourceforge.net>`__ for C/C++, and
+`jUSB <http://jUSB.sourceforge.net>`__ for Java.
+
+    **Note**
+
+    This particular documentation is incomplete, especially with respect
+    to the asynchronous mode. As of kernel 2.5.66 the code and this
+    (new) documentation need to be cross-reviewed.
+
+Configure usbfs into Linux kernels by enabling the *USB filesystem*
+option (CONFIG_USB_DEVICEFS), and you get basic support for user mode
+USB device drivers. Until relatively recently it was often (confusingly)
+called *usbdevfs* although it wasn't solving what *devfs* was. Every USB
+device will appear in usbfs, regardless of whether or not it has a
+kernel driver.
+
+What files are in "usbfs"?
+--------------------------
+
+Conventionally mounted at ``/proc/bus/usb``, usbfs features include:
+
+-  ``/proc/bus/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.
+
+-  ``/proc/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. "usbfs" doesn't (yet) expose those IDs.
+
+Mounting and Access Control
+---------------------------
+
+There are a number of mount options for usbfs, which will be of most
+interest to you if you need to override the default access control
+policy. That policy is that only root may read or write device files
+(``/proc/bus/BBB/DDD``) although anyone may read the ``devices`` or
+``drivers`` files. I/O requests to the device also need the
+CAP_SYS_RAWIO capability,
+
+The significance of that is that by default, all user mode device
+drivers need super-user privileges. You can change modes or ownership in
+a driver setup when the device hotplugs, or maye just start the driver
+right then, as a privileged server (or some activity within one). That's
+the most secure approach for multi-user systems, but for single user
+systems ("trusted" by that user) it's more convenient just to grant
+everyone all access (using the *devmode=0666* option) so the driver can
+start whenever it's needed.
+
+The mount options for usbfs, usable in /etc/fstab or in command line
+invocations of *mount*, are:
+
+*busgid*\ =NNNNN
+    Controls the GID used for the /proc/bus/usb/BBB directories.
+    (Default: 0)
+
+*busmode*\ =MMM
+    Controls the file mode used for the /proc/bus/usb/BBB directories.
+    (Default: 0555)
+
+*busuid*\ =NNNNN
+    Controls the UID used for the /proc/bus/usb/BBB directories.
+    (Default: 0)
+
+*devgid*\ =NNNNN
+    Controls the GID used for the /proc/bus/usb/BBB/DDD files. (Default:
+    0)
+
+*devmode*\ =MMM
+    Controls the file mode used for the /proc/bus/usb/BBB/DDD files.
+    (Default: 0644)
+
+*devuid*\ =NNNNN
+    Controls the UID used for the /proc/bus/usb/BBB/DDD files. (Default:
+    0)
+
+*listgid*\ =NNNNN
+    Controls the GID used for the /proc/bus/usb/devices and drivers
+    files. (Default: 0)
+
+*listmode*\ =MMM
+    Controls the file mode used for the /proc/bus/usb/devices and
+    drivers files. (Default: 0444)
+
+*listuid*\ =NNNNN
+    Controls the UID used for the /proc/bus/usb/devices and drivers
+    files. (Default: 0)
+
+Note that many Linux distributions hard-wire the mount options for usbfs
+in their init scripts, such as ``/etc/rc.d/rc.sysinit``, rather than
+making it easy to set this per-system policy in ``/etc/fstab``.
+
+/proc/bus/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 the
+``Documentation/usb/proc_usb_info.txt`` file in your Linux kernel
+sources.
+
+This file, in combination with the poll() system call, can also be used
+to detect when devices are added or removed:
+
+::
+
+    int fd;
+    struct pollfd pfd;
+
+    fd = open("/proc/bus/usb/devices", O_RDONLY);
+    pfd = { fd, POLLIN, 0 };
+    for (;;) {
+        /* The first time through, this call will return immediately. */
+        poll(&pfd, 1, -1);
+
+        /* To see what's changed, compare the file's previous and current
+           contents or scan the filesystem.  (Scanning is more precise.) */
+    }
+
+Note that this behavior is intended to be used for informational and
+debug purposes. It would be more appropriate to use programs such as
+udev or HAL to initialize a device or start a user-mode helper program,
+for instance.
+
+/proc/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.
+
+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 /proc/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
+``Documentation/usb/error-codes.txt`` in your kernel sources.)
+
+Each of these files multiplexes access to several I/O streams, one per
+endpoint. Each device has one control endpoint (endpoint zero) which
+supports a limited RPC style RPC access. Devices are configured by
+hub_wq (in the kernel) setting a device-wide *configuration* that
+affects things like power consumption and basic functionality. The
+endpoints are part of USB *interfaces*, which may have *altsettings*
+affecting things like which endpoints are available. Many devices only
+have a single configuration and interface, so drivers for them will
+ignore configurations and altsettings.
+
+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*