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diff --git a/Documentation/driver-api/usb/usb.rst b/Documentation/driver-api/usb/usb.rst new file mode 100644 index 000000000..078e981e2 --- /dev/null +++ b/Documentation/driver-api/usb/usb.rst @@ -0,0 +1,1045 @@ +.. _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 ``<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. + +.. kernel-doc:: include/linux/usb/ch9.h + :internal: + +.. _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: |