1.. _usb-hostside-api: 2 3=========================== 4The Linux-USB Host Side API 5=========================== 6 7Introduction to USB on Linux 8============================ 9 10A Universal Serial Bus (USB) is used to connect a host, such as a PC or 11workstation, to a number of peripheral devices. USB uses a tree 12structure, with the host as the root (the system's master), hubs as 13interior nodes, and peripherals as leaves (and slaves). Modern PCs 14support several such trees of USB devices, usually 15a few USB 3.0 (5 GBit/s) or USB 3.1 (10 GBit/s) and some legacy 16USB 2.0 (480 MBit/s) busses just in case. 17 18That master/slave asymmetry was designed-in for a number of reasons, one 19being ease of use. It is not physically possible to mistake upstream and 20downstream or it does not matter with a type C plug (or they are built into the 21peripheral). Also, the host software doesn't need to deal with 22distributed auto-configuration since the pre-designated master node 23manages all that. 24 25Kernel developers added USB support to Linux early in the 2.2 kernel 26series and have been developing it further since then. Besides support 27for each new generation of USB, various host controllers gained support, 28new drivers for peripherals have been added and advanced features for latency 29measurement and improved power management introduced. 30 31Linux can run inside USB devices as well as on the hosts that control 32the devices. But USB device drivers running inside those peripherals 33don't do the same things as the ones running inside hosts, so they've 34been given a different name: *gadget drivers*. This document does not 35cover gadget drivers. 36 37USB Host-Side API Model 38======================= 39 40Host-side drivers for USB devices talk to the "usbcore" APIs. There are 41two. One is intended for *general-purpose* drivers (exposed through 42driver frameworks), and the other is for drivers that are *part of the 43core*. Such core drivers include the *hub* driver (which manages trees 44of USB devices) and several different kinds of *host controller 45drivers*, which control individual busses. 46 47The device model seen by USB drivers is relatively complex. 48 49- USB supports four kinds of data transfers (control, bulk, interrupt, 50 and isochronous). Two of them (control and bulk) use bandwidth as 51 it's available, while the other two (interrupt and isochronous) are 52 scheduled to provide guaranteed bandwidth. 53 54- The device description model includes one or more "configurations" 55 per device, only one of which is active at a time. Devices are supposed 56 to be capable of operating at lower than their top 57 speeds and may provide a BOS descriptor showing the lowest speed they 58 remain fully operational at. 59 60- From USB 3.0 on configurations have one or more "functions", which 61 provide a common functionality and are grouped together for purposes 62 of power management. 63 64- Configurations or functions have one or more "interfaces", each of which may have 65 "alternate settings". Interfaces may be standardized by USB "Class" 66 specifications, or may be specific to a vendor or device. 67 68 USB device drivers actually bind to interfaces, not devices. Think of 69 them as "interface drivers", though you may not see many devices 70 where the distinction is important. *Most USB devices are simple, 71 with only one function, one configuration, one interface, and one alternate 72 setting.* 73 74- Interfaces have one or more "endpoints", each of which supports one 75 type and direction of data transfer such as "bulk out" or "interrupt 76 in". The entire configuration may have up to sixteen endpoints in 77 each direction, allocated as needed among all the interfaces. 78 79- Data transfer on USB is packetized; each endpoint has a maximum 80 packet size. Drivers must often be aware of conventions such as 81 flagging the end of bulk transfers using "short" (including zero 82 length) packets. 83 84- The Linux USB API supports synchronous calls for control and bulk 85 messages. It also supports asynchronous calls for all kinds of data 86 transfer, using request structures called "URBs" (USB Request 87 Blocks). 88 89Accordingly, the USB Core API exposed to device drivers covers quite a 90lot of territory. You'll probably need to consult the USB 3.0 91specification, available online from www.usb.org at no cost, as well as 92class or device specifications. 93 94The only host-side drivers that actually touch hardware (reading/writing 95registers, handling IRQs, and so on) are the HCDs. In theory, all HCDs 96provide the same functionality through the same API. In practice, that's 97becoming more true, but there are still differences 98that crop up especially with fault handling on the less common controllers. 99Different controllers don't 100necessarily report the same aspects of failures, and recovery from 101faults (including software-induced ones like unlinking an URB) isn't yet 102fully consistent. Device driver authors should make a point of doing 103disconnect testing (while the device is active) with each different host 104controller driver, to make sure drivers don't have bugs of their own as 105well as to make sure they aren't relying on some HCD-specific behavior. 106 107.. _usb_chapter9: 108 109USB-Standard Types 110================== 111 112In ``include/uapi/linux/usb/ch9.h`` you will find the USB data types defined 113in chapter 9 of the USB specification. These data types are used throughout 114USB, and in APIs including this host side API, gadget APIs, usb character 115devices and debugfs interfaces. That file is itself included by 116``include/linux/usb/ch9.h``, which also contains declarations of a few 117utility routines for manipulating these data types; the implementations 118are in ``drivers/usb/common/common.c``. 119 120.. kernel-doc:: drivers/usb/common/common.c 121 :export: 122 123In addition, some functions useful for creating debugging output are 124defined in ``drivers/usb/common/debug.c``. 125 126Host-Side Data Types and Macros 127=============================== 128 129The host side API exposes several layers to drivers, some of which are 130more necessary than others. These support lifecycle models for host side 131drivers and devices, and support passing buffers through usbcore to some 132HCD that performs the I/O for the device driver. 133 134.. kernel-doc:: include/linux/usb.h 135 :internal: 136 137USB Core APIs 138============= 139 140There are two basic I/O models in the USB API. The most elemental one is 141asynchronous: drivers submit requests in the form of an URB, and the 142URB's completion callback handles the next step. All USB transfer types 143support that model, although there are special cases for control URBs 144(which always have setup and status stages, but may not have a data 145stage) and isochronous URBs (which allow large packets and include 146per-packet fault reports). Built on top of that is synchronous API 147support, where a driver calls a routine that allocates one or more URBs, 148submits them, and waits until they complete. There are synchronous 149wrappers for single-buffer control and bulk transfers (which are awkward 150to use in some driver disconnect scenarios), and for scatterlist based 151streaming i/o (bulk or interrupt). 152 153USB drivers need to provide buffers that can be used for DMA, although 154they don't necessarily need to provide the DMA mapping themselves. There 155are APIs to use used when allocating DMA buffers, which can prevent use 156of bounce buffers on some systems. In some cases, drivers may be able to 157rely on 64bit DMA to eliminate another kind of bounce buffer. 158 159.. kernel-doc:: drivers/usb/core/urb.c 160 :export: 161 162.. kernel-doc:: drivers/usb/core/message.c 163 :export: 164 165.. kernel-doc:: drivers/usb/core/file.c 166 :export: 167 168.. kernel-doc:: drivers/usb/core/driver.c 169 :export: 170 171.. kernel-doc:: drivers/usb/core/usb.c 172 :export: 173 174.. kernel-doc:: drivers/usb/core/hub.c 175 :export: 176 177Host Controller APIs 178==================== 179 180These APIs are only for use by host controller drivers, most of which 181implement standard register interfaces such as XHCI, EHCI, OHCI, or UHCI. UHCI 182was one of the first interfaces, designed by Intel and also used by VIA; 183it doesn't do much in hardware. OHCI was designed later, to have the 184hardware do more work (bigger transfers, tracking protocol state, and so 185on). EHCI was designed with USB 2.0; its design has features that 186resemble OHCI (hardware does much more work) as well as UHCI (some parts 187of ISO support, TD list processing). XHCI was designed with USB 3.0. It 188continues to shift support for functionality into hardware. 189 190There are host controllers other than the "big three", although most PCI 191based controllers (and a few non-PCI based ones) use one of those 192interfaces. Not all host controllers use DMA; some use PIO, and there is 193also a simulator and a virtual host controller to pipe USB over the network. 194 195The same basic APIs are available to drivers for all those controllers. 196For historical reasons they are in two layers: :c:type:`struct 197usb_bus <usb_bus>` is a rather thin layer that became available 198in the 2.2 kernels, while :c:type:`struct usb_hcd <usb_hcd>` 199is a more featureful layer 200that lets HCDs share common code, to shrink driver size and 201significantly reduce hcd-specific behaviors. 202 203.. kernel-doc:: drivers/usb/core/hcd.c 204 :export: 205 206.. kernel-doc:: drivers/usb/core/hcd-pci.c 207 :export: 208 209.. kernel-doc:: drivers/usb/core/buffer.c 210 :internal: 211 212The USB character device nodes 213============================== 214 215This chapter presents the Linux character device nodes. You may prefer 216to avoid writing new kernel code for your USB driver. User mode device 217drivers are usually packaged as applications or libraries, and may use 218character devices through some programming library that wraps it. 219Such libraries include: 220 221 - `libusb <http://libusb.sourceforge.net>`__ for C/C++, and 222 - `jUSB <http://jUSB.sourceforge.net>`__ for Java. 223 224Some old information about it can be seen at the "USB Device Filesystem" 225section of the USB Guide. The latest copy of the USB Guide can be found 226at http://www.linux-usb.org/ 227 228.. note:: 229 230 - They were used to be implemented via *usbfs*, but this is not part of 231 the sysfs debug interface. 232 233 - This particular documentation is incomplete, especially with respect 234 to the asynchronous mode. As of kernel 2.5.66 the code and this 235 (new) documentation need to be cross-reviewed. 236 237What files are in "devtmpfs"? 238----------------------------- 239 240Conventionally mounted at ``/dev/bus/usb/``, usbfs features include: 241 242- ``/dev/bus/usb/BBB/DDD`` ... magic files exposing the each device's 243 configuration descriptors, and supporting a series of ioctls for 244 making device requests, including I/O to devices. (Purely for access 245 by programs.) 246 247Each bus is given a number (``BBB``) based on when it was enumerated; within 248each bus, each device is given a similar number (``DDD``). Those ``BBB/DDD`` 249paths are not "stable" identifiers; expect them to change even if you 250always leave the devices plugged in to the same hub port. *Don't even 251think of saving these in application configuration files.* Stable 252identifiers are available, for user mode applications that want to use 253them. HID and networking devices expose these stable IDs, so that for 254example you can be sure that you told the right UPS to power down its 255second server. Pleast note that it doesn't (yet) expose those IDs. 256 257/dev/bus/usb/BBB/DDD 258-------------------- 259 260Use these files in one of these basic ways: 261 262- *They can be read,* producing first the device descriptor (18 bytes) and 263 then the descriptors for the current configuration. See the USB 2.0 spec 264 for details about those binary data formats. You'll need to convert most 265 multibyte values from little endian format to your native host byte 266 order, although a few of the fields in the device descriptor (both of 267 the BCD-encoded fields, and the vendor and product IDs) will be 268 byteswapped for you. Note that configuration descriptors include 269 descriptors for interfaces, altsettings, endpoints, and maybe additional 270 class descriptors. 271 272- *Perform USB operations* using *ioctl()* requests to make endpoint I/O 273 requests (synchronously or asynchronously) or manage the device. These 274 requests need the ``CAP_SYS_RAWIO`` capability, as well as filesystem 275 access permissions. Only one ioctl request can be made on one of these 276 device files at a time. This means that if you are synchronously reading 277 an endpoint from one thread, you won't be able to write to a different 278 endpoint from another thread until the read completes. This works for 279 *half duplex* protocols, but otherwise you'd use asynchronous i/o 280 requests. 281 282Each connected USB device has one file. The ``BBB`` indicates the bus 283number. The ``DDD`` indicates the device address on that bus. Both 284of these numbers are assigned sequentially, and can be reused, so 285you can't rely on them for stable access to devices. For example, 286it's relatively common for devices to re-enumerate while they are 287still connected (perhaps someone jostled their power supply, hub, 288or USB cable), so a device might be ``002/027`` when you first connect 289it and ``002/048`` sometime later. 290 291These files can be read as binary data. The binary data consists 292of first the device descriptor, then the descriptors for each 293configuration of the device. Multi-byte fields in the device descriptor 294are converted to host endianness by the kernel. The configuration 295descriptors are in bus endian format! The configuration descriptor 296are wTotalLength bytes apart. If a device returns less configuration 297descriptor data than indicated by wTotalLength there will be a hole in 298the file for the missing bytes. This information is also shown 299in text form by the ``/sys/kernel/debug/usb/devices`` file, described later. 300 301These files may also be used to write user-level drivers for the USB 302devices. You would open the ``/dev/bus/usb/BBB/DDD`` file read/write, 303read its descriptors to make sure it's the device you expect, and then 304bind to an interface (or perhaps several) using an ioctl call. You 305would issue more ioctls to the device to communicate to it using 306control, bulk, or other kinds of USB transfers. The IOCTLs are 307listed in the ``<linux/usbdevice_fs.h>`` file, and at this writing the 308source code (``linux/drivers/usb/core/devio.c``) is the primary reference 309for how to access devices through those files. 310 311Note that since by default these ``BBB/DDD`` files are writable only by 312root, only root can write such user mode drivers. You can selectively 313grant read/write permissions to other users by using ``chmod``. Also, 314usbfs mount options such as ``devmode=0666`` may be helpful. 315 316 317Life Cycle of User Mode Drivers 318------------------------------- 319 320Such a driver first needs to find a device file for a device it knows 321how to handle. Maybe it was told about it because a ``/sbin/hotplug`` 322event handling agent chose that driver to handle the new device. Or 323maybe it's an application that scans all the ``/dev/bus/usb`` device files, 324and ignores most devices. In either case, it should :c:func:`read()` 325all the descriptors from the device file, and check them against what it 326knows how to handle. It might just reject everything except a particular 327vendor and product ID, or need a more complex policy. 328 329Never assume there will only be one such device on the system at a time! 330If your code can't handle more than one device at a time, at least 331detect when there's more than one, and have your users choose which 332device to use. 333 334Once your user mode driver knows what device to use, it interacts with 335it in either of two styles. The simple style is to make only control 336requests; some devices don't need more complex interactions than those. 337(An example might be software using vendor-specific control requests for 338some initialization or configuration tasks, with a kernel driver for the 339rest.) 340 341More likely, you need a more complex style driver: one using non-control 342endpoints, reading or writing data and claiming exclusive use of an 343interface. *Bulk* transfers are easiest to use, but only their sibling 344*interrupt* transfers work with low speed devices. Both interrupt and 345*isochronous* transfers offer service guarantees because their bandwidth 346is reserved. Such "periodic" transfers are awkward to use through usbfs, 347unless you're using the asynchronous calls. However, interrupt transfers 348can also be used in a synchronous "one shot" style. 349 350Your user-mode driver should never need to worry about cleaning up 351request state when the device is disconnected, although it should close 352its open file descriptors as soon as it starts seeing the ENODEV errors. 353 354The ioctl() Requests 355-------------------- 356 357To use these ioctls, you need to include the following headers in your 358userspace program:: 359 360 #include <linux/usb.h> 361 #include <linux/usbdevice_fs.h> 362 #include <asm/byteorder.h> 363 364The standard USB device model requests, from "Chapter 9" of the USB 2.0 365specification, are automatically included from the ``<linux/usb/ch9.h>`` 366header. 367 368Unless noted otherwise, the ioctl requests described here will update 369the modification time on the usbfs file to which they are applied 370(unless they fail). A return of zero indicates success; otherwise, a 371standard USB error code is returned (These are documented in 372:ref:`usb-error-codes`). 373 374Each of these files multiplexes access to several I/O streams, one per 375endpoint. Each device has one control endpoint (endpoint zero) which 376supports a limited RPC style RPC access. Devices are configured by 377hub_wq (in the kernel) setting a device-wide *configuration* that 378affects things like power consumption and basic functionality. The 379endpoints are part of USB *interfaces*, which may have *altsettings* 380affecting things like which endpoints are available. Many devices only 381have a single configuration and interface, so drivers for them will 382ignore configurations and altsettings. 383 384Management/Status Requests 385~~~~~~~~~~~~~~~~~~~~~~~~~~ 386 387A number of usbfs requests don't deal very directly with device I/O. 388They mostly relate to device management and status. These are all 389synchronous requests. 390 391USBDEVFS_CLAIMINTERFACE 392 This is used to force usbfs to claim a specific interface, which has 393 not previously been claimed by usbfs or any other kernel driver. The 394 ioctl parameter is an integer holding the number of the interface 395 (bInterfaceNumber from descriptor). 396 397 Note that if your driver doesn't claim an interface before trying to 398 use one of its endpoints, and no other driver has bound to it, then 399 the interface is automatically claimed by usbfs. 400 401 This claim will be released by a RELEASEINTERFACE ioctl, or by 402 closing the file descriptor. File modification time is not updated 403 by this request. 404 405USBDEVFS_CONNECTINFO 406 Says whether the device is lowspeed. The ioctl parameter points to a 407 structure like this:: 408 409 struct usbdevfs_connectinfo { 410 unsigned int devnum; 411 unsigned char slow; 412 }; 413 414 File modification time is not updated by this request. 415 416 *You can't tell whether a "not slow" device is connected at high 417 speed (480 MBit/sec) or just full speed (12 MBit/sec).* You should 418 know the devnum value already, it's the DDD value of the device file 419 name. 420 421USBDEVFS_GETDRIVER 422 Returns the name of the kernel driver bound to a given interface (a 423 string). Parameter is a pointer to this structure, which is 424 modified:: 425 426 struct usbdevfs_getdriver { 427 unsigned int interface; 428 char driver[USBDEVFS_MAXDRIVERNAME + 1]; 429 }; 430 431 File modification time is not updated by this request. 432 433USBDEVFS_IOCTL 434 Passes a request from userspace through to a kernel driver that has 435 an ioctl entry in the *struct usb_driver* it registered:: 436 437 struct usbdevfs_ioctl { 438 int ifno; 439 int ioctl_code; 440 void *data; 441 }; 442 443 /* user mode call looks like this. 444 * 'request' becomes the driver->ioctl() 'code' parameter. 445 * the size of 'param' is encoded in 'request', and that data 446 * is copied to or from the driver->ioctl() 'buf' parameter. 447 */ 448 static int 449 usbdev_ioctl (int fd, int ifno, unsigned request, void *param) 450 { 451 struct usbdevfs_ioctl wrapper; 452 453 wrapper.ifno = ifno; 454 wrapper.ioctl_code = request; 455 wrapper.data = param; 456 457 return ioctl (fd, USBDEVFS_IOCTL, &wrapper); 458 } 459 460 File modification time is not updated by this request. 461 462 This request lets kernel drivers talk to user mode code through 463 filesystem operations even when they don't create a character or 464 block special device. It's also been used to do things like ask 465 devices what device special file should be used. Two pre-defined 466 ioctls are used to disconnect and reconnect kernel drivers, so that 467 user mode code can completely manage binding and configuration of 468 devices. 469 470USBDEVFS_RELEASEINTERFACE 471 This is used to release the claim usbfs made on interface, either 472 implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the 473 file descriptor is closed. The ioctl parameter is an integer holding 474 the number of the interface (bInterfaceNumber from descriptor); File 475 modification time is not updated by this request. 476 477 .. warning:: 478 479 *No security check is made to ensure that the task which made 480 the claim is the one which is releasing it. This means that user 481 mode driver may interfere other ones.* 482 483USBDEVFS_RESETEP 484 Resets the data toggle value for an endpoint (bulk or interrupt) to 485 DATA0. The ioctl parameter is an integer endpoint number (1 to 15, 486 as identified in the endpoint descriptor), with USB_DIR_IN added 487 if the device's endpoint sends data to the host. 488 489 .. Warning:: 490 491 *Avoid using this request. It should probably be removed.* Using 492 it typically means the device and driver will lose toggle 493 synchronization. If you really lost synchronization, you likely 494 need to completely handshake with the device, using a request 495 like CLEAR_HALT or SET_INTERFACE. 496 497USBDEVFS_DROP_PRIVILEGES 498 This is used to relinquish the ability to do certain operations 499 which are considered to be privileged on a usbfs file descriptor. 500 This includes claiming arbitrary interfaces, resetting a device on 501 which there are currently claimed interfaces from other users, and 502 issuing USBDEVFS_IOCTL calls. The ioctl parameter is a 32 bit mask 503 of interfaces the user is allowed to claim on this file descriptor. 504 You may issue this ioctl more than one time to narrow said mask. 505 506Synchronous I/O Support 507~~~~~~~~~~~~~~~~~~~~~~~ 508 509Synchronous requests involve the kernel blocking until the user mode 510request completes, either by finishing successfully or by reporting an 511error. In most cases this is the simplest way to use usbfs, although as 512noted above it does prevent performing I/O to more than one endpoint at 513a time. 514 515USBDEVFS_BULK 516 Issues a bulk read or write request to the device. The ioctl 517 parameter is a pointer to this structure:: 518 519 struct usbdevfs_bulktransfer { 520 unsigned int ep; 521 unsigned int len; 522 unsigned int timeout; /* in milliseconds */ 523 void *data; 524 }; 525 526 The ``ep`` value identifies a bulk endpoint number (1 to 15, as 527 identified in an endpoint descriptor), masked with USB_DIR_IN when 528 referring to an endpoint which sends data to the host from the 529 device. The length of the data buffer is identified by ``len``; Recent 530 kernels support requests up to about 128KBytes. *FIXME say how read 531 length is returned, and how short reads are handled.*. 532 533USBDEVFS_CLEAR_HALT 534 Clears endpoint halt (stall) and resets the endpoint toggle. This is 535 only meaningful for bulk or interrupt endpoints. The ioctl parameter 536 is an integer endpoint number (1 to 15, as identified in an endpoint 537 descriptor), masked with USB_DIR_IN when referring to an endpoint 538 which sends data to the host from the device. 539 540 Use this on bulk or interrupt endpoints which have stalled, 541 returning ``-EPIPE`` status to a data transfer request. Do not issue 542 the control request directly, since that could invalidate the host's 543 record of the data toggle. 544 545USBDEVFS_CONTROL 546 Issues a control request to the device. The ioctl parameter points 547 to a structure like this:: 548 549 struct usbdevfs_ctrltransfer { 550 __u8 bRequestType; 551 __u8 bRequest; 552 __u16 wValue; 553 __u16 wIndex; 554 __u16 wLength; 555 __u32 timeout; /* in milliseconds */ 556 void *data; 557 }; 558 559 The first eight bytes of this structure are the contents of the 560 SETUP packet to be sent to the device; see the USB 2.0 specification 561 for details. The bRequestType value is composed by combining a 562 ``USB_TYPE_*`` value, a ``USB_DIR_*`` value, and a ``USB_RECIP_*`` 563 value (from ``linux/usb.h``). If wLength is nonzero, it describes 564 the length of the data buffer, which is either written to the device 565 (USB_DIR_OUT) or read from the device (USB_DIR_IN). 566 567 At this writing, you can't transfer more than 4 KBytes of data to or 568 from a device; usbfs has a limit, and some host controller drivers 569 have a limit. (That's not usually a problem.) *Also* there's no way 570 to say it's not OK to get a short read back from the device. 571 572USBDEVFS_RESET 573 Does a USB level device reset. The ioctl parameter is ignored. After 574 the reset, this rebinds all device interfaces. File modification 575 time is not updated by this request. 576 577.. warning:: 578 579 *Avoid using this call* until some usbcore bugs get fixed, since 580 it does not fully synchronize device, interface, and driver (not 581 just usbfs) state. 582 583USBDEVFS_SETINTERFACE 584 Sets the alternate setting for an interface. The ioctl parameter is 585 a pointer to a structure like this:: 586 587 struct usbdevfs_setinterface { 588 unsigned int interface; 589 unsigned int altsetting; 590 }; 591 592 File modification time is not updated by this request. 593 594 Those struct members are from some interface descriptor applying to 595 the current configuration. The interface number is the 596 bInterfaceNumber value, and the altsetting number is the 597 bAlternateSetting value. (This resets each endpoint in the 598 interface.) 599 600USBDEVFS_SETCONFIGURATION 601 Issues the :c:func:`usb_set_configuration()` call for the 602 device. The parameter is an integer holding the number of a 603 configuration (bConfigurationValue from descriptor). File 604 modification time is not updated by this request. 605 606.. warning:: 607 608 *Avoid using this call* until some usbcore bugs get fixed, since 609 it does not fully synchronize device, interface, and driver (not 610 just usbfs) state. 611 612Asynchronous I/O Support 613~~~~~~~~~~~~~~~~~~~~~~~~ 614 615As mentioned above, there are situations where it may be important to 616initiate concurrent operations from user mode code. This is particularly 617important for periodic transfers (interrupt and isochronous), but it can 618be used for other kinds of USB requests too. In such cases, the 619asynchronous requests described here are essential. Rather than 620submitting one request and having the kernel block until it completes, 621the blocking is separate. 622 623These requests are packaged into a structure that resembles the URB used 624by kernel device drivers. (No POSIX Async I/O support here, sorry.) It 625identifies the endpoint type (``USBDEVFS_URB_TYPE_*``), endpoint 626(number, masked with USB_DIR_IN as appropriate), buffer and length, 627and a user "context" value serving to uniquely identify each request. 628(It's usually a pointer to per-request data.) Flags can modify requests 629(not as many as supported for kernel drivers). 630 631Each request can specify a realtime signal number (between SIGRTMIN and 632SIGRTMAX, inclusive) to request a signal be sent when the request 633completes. 634 635When usbfs returns these urbs, the status value is updated, and the 636buffer may have been modified. Except for isochronous transfers, the 637actual_length is updated to say how many bytes were transferred; if the 638USBDEVFS_URB_DISABLE_SPD flag is set ("short packets are not OK"), if 639fewer bytes were read than were requested then you get an error report:: 640 641 struct usbdevfs_iso_packet_desc { 642 unsigned int length; 643 unsigned int actual_length; 644 unsigned int status; 645 }; 646 647 struct usbdevfs_urb { 648 unsigned char type; 649 unsigned char endpoint; 650 int status; 651 unsigned int flags; 652 void *buffer; 653 int buffer_length; 654 int actual_length; 655 int start_frame; 656 int number_of_packets; 657 int error_count; 658 unsigned int signr; 659 void *usercontext; 660 struct usbdevfs_iso_packet_desc iso_frame_desc[]; 661 }; 662 663For these asynchronous requests, the file modification time reflects 664when the request was initiated. This contrasts with their use with the 665synchronous requests, where it reflects when requests complete. 666 667USBDEVFS_DISCARDURB 668 *TBS* File modification time is not updated by this request. 669 670USBDEVFS_DISCSIGNAL 671 *TBS* File modification time is not updated by this request. 672 673USBDEVFS_REAPURB 674 *TBS* File modification time is not updated by this request. 675 676USBDEVFS_REAPURBNDELAY 677 *TBS* File modification time is not updated by this request. 678 679USBDEVFS_SUBMITURB 680 *TBS* 681 682The USB devices 683=============== 684 685The USB devices are now exported via debugfs: 686 687- ``/sys/kernel/debug/usb/devices`` ... a text file showing each of the USB 688 devices on known to the kernel, and their configuration descriptors. 689 You can also poll() this to learn about new devices. 690 691/sys/kernel/debug/usb/devices 692----------------------------- 693 694This file is handy for status viewing tools in user mode, which can scan 695the text format and ignore most of it. More detailed device status 696(including class and vendor status) is available from device-specific 697files. For information about the current format of this file, see below. 698 699This file, in combination with the poll() system call, can also be used 700to detect when devices are added or removed:: 701 702 int fd; 703 struct pollfd pfd; 704 705 fd = open("/sys/kernel/debug/usb/devices", O_RDONLY); 706 pfd = { fd, POLLIN, 0 }; 707 for (;;) { 708 /* The first time through, this call will return immediately. */ 709 poll(&pfd, 1, -1); 710 711 /* To see what's changed, compare the file's previous and current 712 contents or scan the filesystem. (Scanning is more precise.) */ 713 } 714 715Note that this behavior is intended to be used for informational and 716debug purposes. It would be more appropriate to use programs such as 717udev or HAL to initialize a device or start a user-mode helper program, 718for instance. 719 720In this file, each device's output has multiple lines of ASCII output. 721 722I made it ASCII instead of binary on purpose, so that someone 723can obtain some useful data from it without the use of an 724auxiliary program. However, with an auxiliary program, the numbers 725in the first 4 columns of each ``T:`` line (topology info: 726Lev, Prnt, Port, Cnt) can be used to build a USB topology diagram. 727 728Each line is tagged with a one-character ID for that line:: 729 730 T = Topology (etc.) 731 B = Bandwidth (applies only to USB host controllers, which are 732 virtualized as root hubs) 733 D = Device descriptor info. 734 P = Product ID info. (from Device descriptor, but they won't fit 735 together on one line) 736 S = String descriptors. 737 C = Configuration descriptor info. (* = active configuration) 738 I = Interface descriptor info. 739 E = Endpoint descriptor info. 740 741/sys/kernel/debug/usb/devices output format 742~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 743 744Legend:: 745 d = decimal number (may have leading spaces or 0's) 746 x = hexadecimal number (may have leading spaces or 0's) 747 s = string 748 749 750 751Topology info 752^^^^^^^^^^^^^ 753 754:: 755 756 T: Bus=dd Lev=dd Prnt=dd Port=dd Cnt=dd Dev#=ddd Spd=dddd MxCh=dd 757 | | | | | | | | |__MaxChildren 758 | | | | | | | |__Device Speed in Mbps 759 | | | | | | |__DeviceNumber 760 | | | | | |__Count of devices at this level 761 | | | | |__Connector/Port on Parent for this device 762 | | | |__Parent DeviceNumber 763 | | |__Level in topology for this bus 764 | |__Bus number 765 |__Topology info tag 766 767Speed may be: 768 769 ======= ====================================================== 770 1.5 Mbit/s for low speed USB 771 12 Mbit/s for full speed USB 772 480 Mbit/s for high speed USB (added for USB 2.0); 773 also used for Wireless USB, which has no fixed speed 774 5000 Mbit/s for SuperSpeed USB (added for USB 3.0) 775 ======= ====================================================== 776 777For reasons lost in the mists of time, the Port number is always 778too low by 1. For example, a device plugged into port 4 will 779show up with ``Port=03``. 780 781Bandwidth info 782^^^^^^^^^^^^^^ 783 784:: 785 786 B: Alloc=ddd/ddd us (xx%), #Int=ddd, #Iso=ddd 787 | | | |__Number of isochronous requests 788 | | |__Number of interrupt requests 789 | |__Total Bandwidth allocated to this bus 790 |__Bandwidth info tag 791 792Bandwidth allocation is an approximation of how much of one frame 793(millisecond) is in use. It reflects only periodic transfers, which 794are the only transfers that reserve bandwidth. Control and bulk 795transfers use all other bandwidth, including reserved bandwidth that 796is not used for transfers (such as for short packets). 797 798The percentage is how much of the "reserved" bandwidth is scheduled by 799those transfers. For a low or full speed bus (loosely, "USB 1.1"), 80090% of the bus bandwidth is reserved. For a high speed bus (loosely, 801"USB 2.0") 80% is reserved. 802 803 804Device descriptor info & Product ID info 805^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 806 807:: 808 809 D: Ver=x.xx Cls=xx(s) Sub=xx Prot=xx MxPS=dd #Cfgs=dd 810 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx 811 812where:: 813 814 D: Ver=x.xx Cls=xx(sssss) Sub=xx Prot=xx MxPS=dd #Cfgs=dd 815 | | | | | | |__NumberConfigurations 816 | | | | | |__MaxPacketSize of Default Endpoint 817 | | | | |__DeviceProtocol 818 | | | |__DeviceSubClass 819 | | |__DeviceClass 820 | |__Device USB version 821 |__Device info tag #1 822 823where:: 824 825 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx 826 | | | |__Product revision number 827 | | |__Product ID code 828 | |__Vendor ID code 829 |__Device info tag #2 830 831 832String descriptor info 833^^^^^^^^^^^^^^^^^^^^^^ 834:: 835 836 S: Manufacturer=ssss 837 | |__Manufacturer of this device as read from the device. 838 | For USB host controller drivers (virtual root hubs) this may 839 | be omitted, or (for newer drivers) will identify the kernel 840 | version and the driver which provides this hub emulation. 841 |__String info tag 842 843 S: Product=ssss 844 | |__Product description of this device as read from the device. 845 | For older USB host controller drivers (virtual root hubs) this 846 | indicates the driver; for newer ones, it's a product (and vendor) 847 | description that often comes from the kernel's PCI ID database. 848 |__String info tag 849 850 S: SerialNumber=ssss 851 | |__Serial Number of this device as read from the device. 852 | For USB host controller drivers (virtual root hubs) this is 853 | some unique ID, normally a bus ID (address or slot name) that 854 | can't be shared with any other device. 855 |__String info tag 856 857 858 859Configuration descriptor info 860^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 861:: 862 863 C:* #Ifs=dd Cfg#=dd Atr=xx MPwr=dddmA 864 | | | | | |__MaxPower in mA 865 | | | | |__Attributes 866 | | | |__ConfiguratioNumber 867 | | |__NumberOfInterfaces 868 | |__ "*" indicates the active configuration (others are " ") 869 |__Config info tag 870 871USB devices may have multiple configurations, each of which act 872rather differently. For example, a bus-powered configuration 873might be much less capable than one that is self-powered. Only 874one device configuration can be active at a time; most devices 875have only one configuration. 876 877Each configuration consists of one or more interfaces. Each 878interface serves a distinct "function", which is typically bound 879to a different USB device driver. One common example is a USB 880speaker with an audio interface for playback, and a HID interface 881for use with software volume control. 882 883Interface descriptor info (can be multiple per Config) 884^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 885:: 886 887 I:* If#=dd Alt=dd #EPs=dd Cls=xx(sssss) Sub=xx Prot=xx Driver=ssss 888 | | | | | | | | |__Driver name 889 | | | | | | | | or "(none)" 890 | | | | | | | |__InterfaceProtocol 891 | | | | | | |__InterfaceSubClass 892 | | | | | |__InterfaceClass 893 | | | | |__NumberOfEndpoints 894 | | | |__AlternateSettingNumber 895 | | |__InterfaceNumber 896 | |__ "*" indicates the active altsetting (others are " ") 897 |__Interface info tag 898 899A given interface may have one or more "alternate" settings. 900For example, default settings may not use more than a small 901amount of periodic bandwidth. To use significant fractions 902of bus bandwidth, drivers must select a non-default altsetting. 903 904Only one setting for an interface may be active at a time, and 905only one driver may bind to an interface at a time. Most devices 906have only one alternate setting per interface. 907 908 909Endpoint descriptor info (can be multiple per Interface) 910^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 911 912:: 913 914 E: Ad=xx(s) Atr=xx(ssss) MxPS=dddd Ivl=dddss 915 | | | | |__Interval (max) between transfers 916 | | | |__EndpointMaxPacketSize 917 | | |__Attributes(EndpointType) 918 | |__EndpointAddress(I=In,O=Out) 919 |__Endpoint info tag 920 921The interval is nonzero for all periodic (interrupt or isochronous) 922endpoints. For high speed endpoints the transfer interval may be 923measured in microseconds rather than milliseconds. 924 925For high speed periodic endpoints, the ``EndpointMaxPacketSize`` reflects 926the per-microframe data transfer size. For "high bandwidth" 927endpoints, that can reflect two or three packets (for up to 9283KBytes every 125 usec) per endpoint. 929 930With the Linux-USB stack, periodic bandwidth reservations use the 931transfer intervals and sizes provided by URBs, which can be less 932than those found in endpoint descriptor. 933 934Usage examples 935~~~~~~~~~~~~~~ 936 937If a user or script is interested only in Topology info, for 938example, use something like ``grep ^T: /sys/kernel/debug/usb/devices`` 939for only the Topology lines. A command like 940``grep -i ^[tdp]: /sys/kernel/debug/usb/devices`` can be used to list 941only the lines that begin with the characters in square brackets, 942where the valid characters are TDPCIE. With a slightly more able 943script, it can display any selected lines (for example, only T, D, 944and P lines) and change their output format. (The ``procusb`` 945Perl script is the beginning of this idea. It will list only 946selected lines [selected from TBDPSCIE] or "All" lines from 947``/sys/kernel/debug/usb/devices``.) 948 949The Topology lines can be used to generate a graphic/pictorial 950of the USB devices on a system's root hub. (See more below 951on how to do this.) 952 953The Interface lines can be used to determine what driver is 954being used for each device, and which altsetting it activated. 955 956The Configuration lines could be used to list maximum power 957(in milliamps) that a system's USB devices are using. 958For example, ``grep ^C: /sys/kernel/debug/usb/devices``. 959 960 961Here's an example, from a system which has a UHCI root hub, 962an external hub connected to the root hub, and a mouse and 963a serial converter connected to the external hub. 964 965:: 966 967 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2 968 B: Alloc= 28/900 us ( 3%), #Int= 2, #Iso= 0 969 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 970 P: Vendor=0000 ProdID=0000 Rev= 0.00 971 S: Product=USB UHCI Root Hub 972 S: SerialNumber=dce0 973 C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr= 0mA 974 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 975 E: Ad=81(I) Atr=03(Int.) MxPS= 8 Ivl=255ms 976 977 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4 978 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 979 P: Vendor=0451 ProdID=1446 Rev= 1.00 980 C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=100mA 981 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 982 E: Ad=81(I) Atr=03(Int.) MxPS= 1 Ivl=255ms 983 984 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0 985 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 986 P: Vendor=04b4 ProdID=0001 Rev= 0.00 987 C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=100mA 988 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse 989 E: Ad=81(I) Atr=03(Int.) MxPS= 3 Ivl= 10ms 990 991 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0 992 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 993 P: Vendor=0565 ProdID=0001 Rev= 1.08 994 S: Manufacturer=Peracom Networks, Inc. 995 S: Product=Peracom USB to Serial Converter 996 C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=100mA 997 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial 998 E: Ad=81(I) Atr=02(Bulk) MxPS= 64 Ivl= 16ms 999 E: Ad=01(O) Atr=02(Bulk) MxPS= 16 Ivl= 16ms 1000 E: Ad=82(I) Atr=03(Int.) MxPS= 8 Ivl= 8ms 1001 1002 1003Selecting only the ``T:`` and ``I:`` lines from this (for example, by using 1004``procusb ti``), we have 1005 1006:: 1007 1008 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2 1009 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4 1010 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 1011 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0 1012 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse 1013 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0 1014 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial 1015 1016 1017Physically this looks like (or could be converted to):: 1018 1019 +------------------+ 1020 | PC/root_hub (12)| Dev# = 1 1021 +------------------+ (nn) is Mbps. 1022 Level 0 | CN.0 | CN.1 | [CN = connector/port #] 1023 +------------------+ 1024 / 1025 / 1026 +-----------------------+ 1027 Level 1 | Dev#2: 4-port hub (12)| 1028 +-----------------------+ 1029 |CN.0 |CN.1 |CN.2 |CN.3 | 1030 +-----------------------+ 1031 \ \____________________ 1032 \_____ \ 1033 \ \ 1034 +--------------------+ +--------------------+ 1035 Level 2 | Dev# 3: mouse (1.5)| | Dev# 4: serial (12)| 1036 +--------------------+ +--------------------+ 1037 1038 1039 1040Or, in a more tree-like structure (ports [Connectors] without 1041connections could be omitted):: 1042 1043 PC: Dev# 1, root hub, 2 ports, 12 Mbps 1044 |_ CN.0: Dev# 2, hub, 4 ports, 12 Mbps 1045 |_ CN.0: Dev #3, mouse, 1.5 Mbps 1046 |_ CN.1: 1047 |_ CN.2: Dev #4, serial, 12 Mbps 1048 |_ CN.3: 1049 |_ CN.1: 1050