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 ``drivers/usb/common/common.c`` and ``drivers/usb/common/debug.c`` you 113will find the USB data types defined in chapter 9 of the USB specification. 114These data types are used throughout USB, and in APIs including this host 115side API, gadget APIs, usb character devices and debugfs interfaces. 116 117.. kernel-doc:: drivers/usb/common/common.c 118 :export: 119 120.. kernel-doc:: drivers/usb/common/debug.c 121 :export: 122 123Host-Side Data Types and Macros 124=============================== 125 126The host side API exposes several layers to drivers, some of which are 127more necessary than others. These support lifecycle models for host side 128drivers and devices, and support passing buffers through usbcore to some 129HCD that performs the I/O for the device driver. 130 131.. kernel-doc:: include/linux/usb.h 132 :internal: 133 134USB Core APIs 135============= 136 137There are two basic I/O models in the USB API. The most elemental one is 138asynchronous: drivers submit requests in the form of an URB, and the 139URB's completion callback handles the next step. All USB transfer types 140support that model, although there are special cases for control URBs 141(which always have setup and status stages, but may not have a data 142stage) and isochronous URBs (which allow large packets and include 143per-packet fault reports). Built on top of that is synchronous API 144support, where a driver calls a routine that allocates one or more URBs, 145submits them, and waits until they complete. There are synchronous 146wrappers for single-buffer control and bulk transfers (which are awkward 147to use in some driver disconnect scenarios), and for scatterlist based 148streaming i/o (bulk or interrupt). 149 150USB drivers need to provide buffers that can be used for DMA, although 151they don't necessarily need to provide the DMA mapping themselves. There 152are APIs to use used when allocating DMA buffers, which can prevent use 153of bounce buffers on some systems. In some cases, drivers may be able to 154rely on 64bit DMA to eliminate another kind of bounce buffer. 155 156.. kernel-doc:: drivers/usb/core/urb.c 157 :export: 158 159.. kernel-doc:: drivers/usb/core/message.c 160 :export: 161 162.. kernel-doc:: drivers/usb/core/file.c 163 :export: 164 165.. kernel-doc:: drivers/usb/core/driver.c 166 :export: 167 168.. kernel-doc:: drivers/usb/core/usb.c 169 :export: 170 171.. kernel-doc:: drivers/usb/core/hub.c 172 :export: 173 174Host Controller APIs 175==================== 176 177These APIs are only for use by host controller drivers, most of which 178implement standard register interfaces such as XHCI, EHCI, OHCI, or UHCI. UHCI 179was one of the first interfaces, designed by Intel and also used by VIA; 180it doesn't do much in hardware. OHCI was designed later, to have the 181hardware do more work (bigger transfers, tracking protocol state, and so 182on). EHCI was designed with USB 2.0; its design has features that 183resemble OHCI (hardware does much more work) as well as UHCI (some parts 184of ISO support, TD list processing). XHCI was designed with USB 3.0. It 185continues to shift support for functionality into hardware. 186 187There are host controllers other than the "big three", although most PCI 188based controllers (and a few non-PCI based ones) use one of those 189interfaces. Not all host controllers use DMA; some use PIO, and there is 190also a simulator and a virtual host controller to pipe USB over the network. 191 192The same basic APIs are available to drivers for all those controllers. 193For historical reasons they are in two layers: :c:type:`struct 194usb_bus <usb_bus>` is a rather thin layer that became available 195in the 2.2 kernels, while :c:type:`struct usb_hcd <usb_hcd>` 196is a more featureful layer 197that lets HCDs share common code, to shrink driver size and 198significantly reduce hcd-specific behaviors. 199 200.. kernel-doc:: drivers/usb/core/hcd.c 201 :export: 202 203.. kernel-doc:: drivers/usb/core/hcd-pci.c 204 :export: 205 206.. kernel-doc:: drivers/usb/core/buffer.c 207 :internal: 208 209The USB character device nodes 210============================== 211 212This chapter presents the Linux character device nodes. You may prefer 213to avoid writing new kernel code for your USB driver. User mode device 214drivers are usually packaged as applications or libraries, and may use 215character devices through some programming library that wraps it. 216Such libraries include: 217 218 - `libusb <http://libusb.sourceforge.net>`__ for C/C++, and 219 - `jUSB <http://jUSB.sourceforge.net>`__ for Java. 220 221Some old information about it can be seen at the "USB Device Filesystem" 222section of the USB Guide. The latest copy of the USB Guide can be found 223at http://www.linux-usb.org/ 224 225.. note:: 226 227 - They were used to be implemented via *usbfs*, but this is not part of 228 the sysfs debug interface. 229 230 - This particular documentation is incomplete, especially with respect 231 to the asynchronous mode. As of kernel 2.5.66 the code and this 232 (new) documentation need to be cross-reviewed. 233 234What files are in "devtmpfs"? 235----------------------------- 236 237Conventionally mounted at ``/dev/bus/usb/``, usbfs features include: 238 239- ``/dev/bus/usb/BBB/DDD`` ... magic files exposing the each device's 240 configuration descriptors, and supporting a series of ioctls for 241 making device requests, including I/O to devices. (Purely for access 242 by programs.) 243 244Each bus is given a number (``BBB``) based on when it was enumerated; within 245each bus, each device is given a similar number (``DDD``). Those ``BBB/DDD`` 246paths are not "stable" identifiers; expect them to change even if you 247always leave the devices plugged in to the same hub port. *Don't even 248think of saving these in application configuration files.* Stable 249identifiers are available, for user mode applications that want to use 250them. HID and networking devices expose these stable IDs, so that for 251example you can be sure that you told the right UPS to power down its 252second server. Pleast note that it doesn't (yet) expose those IDs. 253 254/dev/bus/usb/BBB/DDD 255-------------------- 256 257Use these files in one of these basic ways: 258 259- *They can be read,* producing first the device descriptor (18 bytes) and 260 then the descriptors for the current configuration. See the USB 2.0 spec 261 for details about those binary data formats. You'll need to convert most 262 multibyte values from little endian format to your native host byte 263 order, although a few of the fields in the device descriptor (both of 264 the BCD-encoded fields, and the vendor and product IDs) will be 265 byteswapped for you. Note that configuration descriptors include 266 descriptors for interfaces, altsettings, endpoints, and maybe additional 267 class descriptors. 268 269- *Perform USB operations* using *ioctl()* requests to make endpoint I/O 270 requests (synchronously or asynchronously) or manage the device. These 271 requests need the ``CAP_SYS_RAWIO`` capability, as well as filesystem 272 access permissions. Only one ioctl request can be made on one of these 273 device files at a time. This means that if you are synchronously reading 274 an endpoint from one thread, you won't be able to write to a different 275 endpoint from another thread until the read completes. This works for 276 *half duplex* protocols, but otherwise you'd use asynchronous i/o 277 requests. 278 279Each connected USB device has one file. The ``BBB`` indicates the bus 280number. The ``DDD`` indicates the device address on that bus. Both 281of these numbers are assigned sequentially, and can be reused, so 282you can't rely on them for stable access to devices. For example, 283it's relatively common for devices to re-enumerate while they are 284still connected (perhaps someone jostled their power supply, hub, 285or USB cable), so a device might be ``002/027`` when you first connect 286it and ``002/048`` sometime later. 287 288These files can be read as binary data. The binary data consists 289of first the device descriptor, then the descriptors for each 290configuration of the device. Multi-byte fields in the device descriptor 291are converted to host endianness by the kernel. The configuration 292descriptors are in bus endian format! The configuration descriptor 293are wTotalLength bytes apart. If a device returns less configuration 294descriptor data than indicated by wTotalLength there will be a hole in 295the file for the missing bytes. This information is also shown 296in text form by the ``/sys/kernel/debug/usb/devices`` file, described later. 297 298These files may also be used to write user-level drivers for the USB 299devices. You would open the ``/dev/bus/usb/BBB/DDD`` file read/write, 300read its descriptors to make sure it's the device you expect, and then 301bind to an interface (or perhaps several) using an ioctl call. You 302would issue more ioctls to the device to communicate to it using 303control, bulk, or other kinds of USB transfers. The IOCTLs are 304listed in the ``<linux/usbdevice_fs.h>`` file, and at this writing the 305source code (``linux/drivers/usb/core/devio.c``) is the primary reference 306for how to access devices through those files. 307 308Note that since by default these ``BBB/DDD`` files are writable only by 309root, only root can write such user mode drivers. You can selectively 310grant read/write permissions to other users by using ``chmod``. Also, 311usbfs mount options such as ``devmode=0666`` may be helpful. 312 313 314Life Cycle of User Mode Drivers 315------------------------------- 316 317Such a driver first needs to find a device file for a device it knows 318how to handle. Maybe it was told about it because a ``/sbin/hotplug`` 319event handling agent chose that driver to handle the new device. Or 320maybe it's an application that scans all the ``/dev/bus/usb`` device files, 321and ignores most devices. In either case, it should :c:func:`read()` 322all the descriptors from the device file, and check them against what it 323knows how to handle. It might just reject everything except a particular 324vendor and product ID, or need a more complex policy. 325 326Never assume there will only be one such device on the system at a time! 327If your code can't handle more than one device at a time, at least 328detect when there's more than one, and have your users choose which 329device to use. 330 331Once your user mode driver knows what device to use, it interacts with 332it in either of two styles. The simple style is to make only control 333requests; some devices don't need more complex interactions than those. 334(An example might be software using vendor-specific control requests for 335some initialization or configuration tasks, with a kernel driver for the 336rest.) 337 338More likely, you need a more complex style driver: one using non-control 339endpoints, reading or writing data and claiming exclusive use of an 340interface. *Bulk* transfers are easiest to use, but only their sibling 341*interrupt* transfers work with low speed devices. Both interrupt and 342*isochronous* transfers offer service guarantees because their bandwidth 343is reserved. Such "periodic" transfers are awkward to use through usbfs, 344unless you're using the asynchronous calls. However, interrupt transfers 345can also be used in a synchronous "one shot" style. 346 347Your user-mode driver should never need to worry about cleaning up 348request state when the device is disconnected, although it should close 349its open file descriptors as soon as it starts seeing the ENODEV errors. 350 351The ioctl() Requests 352-------------------- 353 354To use these ioctls, you need to include the following headers in your 355userspace program:: 356 357 #include <linux/usb.h> 358 #include <linux/usbdevice_fs.h> 359 #include <asm/byteorder.h> 360 361The standard USB device model requests, from "Chapter 9" of the USB 2.0 362specification, are automatically included from the ``<linux/usb/ch9.h>`` 363header. 364 365Unless noted otherwise, the ioctl requests described here will update 366the modification time on the usbfs file to which they are applied 367(unless they fail). A return of zero indicates success; otherwise, a 368standard USB error code is returned (These are documented in 369:ref:`usb-error-codes`). 370 371Each of these files multiplexes access to several I/O streams, one per 372endpoint. Each device has one control endpoint (endpoint zero) which 373supports a limited RPC style RPC access. Devices are configured by 374hub_wq (in the kernel) setting a device-wide *configuration* that 375affects things like power consumption and basic functionality. The 376endpoints are part of USB *interfaces*, which may have *altsettings* 377affecting things like which endpoints are available. Many devices only 378have a single configuration and interface, so drivers for them will 379ignore configurations and altsettings. 380 381Management/Status Requests 382~~~~~~~~~~~~~~~~~~~~~~~~~~ 383 384A number of usbfs requests don't deal very directly with device I/O. 385They mostly relate to device management and status. These are all 386synchronous requests. 387 388USBDEVFS_CLAIMINTERFACE 389 This is used to force usbfs to claim a specific interface, which has 390 not previously been claimed by usbfs or any other kernel driver. The 391 ioctl parameter is an integer holding the number of the interface 392 (bInterfaceNumber from descriptor). 393 394 Note that if your driver doesn't claim an interface before trying to 395 use one of its endpoints, and no other driver has bound to it, then 396 the interface is automatically claimed by usbfs. 397 398 This claim will be released by a RELEASEINTERFACE ioctl, or by 399 closing the file descriptor. File modification time is not updated 400 by this request. 401 402USBDEVFS_CONNECTINFO 403 Says whether the device is lowspeed. The ioctl parameter points to a 404 structure like this:: 405 406 struct usbdevfs_connectinfo { 407 unsigned int devnum; 408 unsigned char slow; 409 }; 410 411 File modification time is not updated by this request. 412 413 *You can't tell whether a "not slow" device is connected at high 414 speed (480 MBit/sec) or just full speed (12 MBit/sec).* You should 415 know the devnum value already, it's the DDD value of the device file 416 name. 417 418USBDEVFS_GETDRIVER 419 Returns the name of the kernel driver bound to a given interface (a 420 string). Parameter is a pointer to this structure, which is 421 modified:: 422 423 struct usbdevfs_getdriver { 424 unsigned int interface; 425 char driver[USBDEVFS_MAXDRIVERNAME + 1]; 426 }; 427 428 File modification time is not updated by this request. 429 430USBDEVFS_IOCTL 431 Passes a request from userspace through to a kernel driver that has 432 an ioctl entry in the *struct usb_driver* it registered:: 433 434 struct usbdevfs_ioctl { 435 int ifno; 436 int ioctl_code; 437 void *data; 438 }; 439 440 /* user mode call looks like this. 441 * 'request' becomes the driver->ioctl() 'code' parameter. 442 * the size of 'param' is encoded in 'request', and that data 443 * is copied to or from the driver->ioctl() 'buf' parameter. 444 */ 445 static int 446 usbdev_ioctl (int fd, int ifno, unsigned request, void *param) 447 { 448 struct usbdevfs_ioctl wrapper; 449 450 wrapper.ifno = ifno; 451 wrapper.ioctl_code = request; 452 wrapper.data = param; 453 454 return ioctl (fd, USBDEVFS_IOCTL, &wrapper); 455 } 456 457 File modification time is not updated by this request. 458 459 This request lets kernel drivers talk to user mode code through 460 filesystem operations even when they don't create a character or 461 block special device. It's also been used to do things like ask 462 devices what device special file should be used. Two pre-defined 463 ioctls are used to disconnect and reconnect kernel drivers, so that 464 user mode code can completely manage binding and configuration of 465 devices. 466 467USBDEVFS_RELEASEINTERFACE 468 This is used to release the claim usbfs made on interface, either 469 implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the 470 file descriptor is closed. The ioctl parameter is an integer holding 471 the number of the interface (bInterfaceNumber from descriptor); File 472 modification time is not updated by this request. 473 474 .. warning:: 475 476 *No security check is made to ensure that the task which made 477 the claim is the one which is releasing it. This means that user 478 mode driver may interfere other ones.* 479 480USBDEVFS_RESETEP 481 Resets the data toggle value for an endpoint (bulk or interrupt) to 482 DATA0. The ioctl parameter is an integer endpoint number (1 to 15, 483 as identified in the endpoint descriptor), with USB_DIR_IN added 484 if the device's endpoint sends data to the host. 485 486 .. Warning:: 487 488 *Avoid using this request. It should probably be removed.* Using 489 it typically means the device and driver will lose toggle 490 synchronization. If you really lost synchronization, you likely 491 need to completely handshake with the device, using a request 492 like CLEAR_HALT or SET_INTERFACE. 493 494USBDEVFS_DROP_PRIVILEGES 495 This is used to relinquish the ability to do certain operations 496 which are considered to be privileged on a usbfs file descriptor. 497 This includes claiming arbitrary interfaces, resetting a device on 498 which there are currently claimed interfaces from other users, and 499 issuing USBDEVFS_IOCTL calls. The ioctl parameter is a 32 bit mask 500 of interfaces the user is allowed to claim on this file descriptor. 501 You may issue this ioctl more than one time to narrow said mask. 502 503Synchronous I/O Support 504~~~~~~~~~~~~~~~~~~~~~~~ 505 506Synchronous requests involve the kernel blocking until the user mode 507request completes, either by finishing successfully or by reporting an 508error. In most cases this is the simplest way to use usbfs, although as 509noted above it does prevent performing I/O to more than one endpoint at 510a time. 511 512USBDEVFS_BULK 513 Issues a bulk read or write request to the device. The ioctl 514 parameter is a pointer to this structure:: 515 516 struct usbdevfs_bulktransfer { 517 unsigned int ep; 518 unsigned int len; 519 unsigned int timeout; /* in milliseconds */ 520 void *data; 521 }; 522 523 The ``ep`` value identifies a bulk endpoint number (1 to 15, as 524 identified in an endpoint descriptor), masked with USB_DIR_IN when 525 referring to an endpoint which sends data to the host from the 526 device. The length of the data buffer is identified by ``len``; Recent 527 kernels support requests up to about 128KBytes. *FIXME say how read 528 length is returned, and how short reads are handled.*. 529 530USBDEVFS_CLEAR_HALT 531 Clears endpoint halt (stall) and resets the endpoint toggle. This is 532 only meaningful for bulk or interrupt endpoints. The ioctl parameter 533 is an integer endpoint number (1 to 15, as identified in an endpoint 534 descriptor), masked with USB_DIR_IN when referring to an endpoint 535 which sends data to the host from the device. 536 537 Use this on bulk or interrupt endpoints which have stalled, 538 returning ``-EPIPE`` status to a data transfer request. Do not issue 539 the control request directly, since that could invalidate the host's 540 record of the data toggle. 541 542USBDEVFS_CONTROL 543 Issues a control request to the device. The ioctl parameter points 544 to a structure like this:: 545 546 struct usbdevfs_ctrltransfer { 547 __u8 bRequestType; 548 __u8 bRequest; 549 __u16 wValue; 550 __u16 wIndex; 551 __u16 wLength; 552 __u32 timeout; /* in milliseconds */ 553 void *data; 554 }; 555 556 The first eight bytes of this structure are the contents of the 557 SETUP packet to be sent to the device; see the USB 2.0 specification 558 for details. The bRequestType value is composed by combining a 559 ``USB_TYPE_*`` value, a ``USB_DIR_*`` value, and a ``USB_RECIP_*`` 560 value (from ``linux/usb.h``). If wLength is nonzero, it describes 561 the length of the data buffer, which is either written to the device 562 (USB_DIR_OUT) or read from the device (USB_DIR_IN). 563 564 At this writing, you can't transfer more than 4 KBytes of data to or 565 from a device; usbfs has a limit, and some host controller drivers 566 have a limit. (That's not usually a problem.) *Also* there's no way 567 to say it's not OK to get a short read back from the device. 568 569USBDEVFS_RESET 570 Does a USB level device reset. The ioctl parameter is ignored. After 571 the reset, this rebinds all device interfaces. File modification 572 time is not updated by this request. 573 574.. warning:: 575 576 *Avoid using this call* until some usbcore bugs get fixed, since 577 it does not fully synchronize device, interface, and driver (not 578 just usbfs) state. 579 580USBDEVFS_SETINTERFACE 581 Sets the alternate setting for an interface. The ioctl parameter is 582 a pointer to a structure like this:: 583 584 struct usbdevfs_setinterface { 585 unsigned int interface; 586 unsigned int altsetting; 587 }; 588 589 File modification time is not updated by this request. 590 591 Those struct members are from some interface descriptor applying to 592 the current configuration. The interface number is the 593 bInterfaceNumber value, and the altsetting number is the 594 bAlternateSetting value. (This resets each endpoint in the 595 interface.) 596 597USBDEVFS_SETCONFIGURATION 598 Issues the :c:func:`usb_set_configuration()` call for the 599 device. The parameter is an integer holding the number of a 600 configuration (bConfigurationValue from descriptor). File 601 modification time is not updated by this request. 602 603.. warning:: 604 605 *Avoid using this call* until some usbcore bugs get fixed, since 606 it does not fully synchronize device, interface, and driver (not 607 just usbfs) state. 608 609Asynchronous I/O Support 610~~~~~~~~~~~~~~~~~~~~~~~~ 611 612As mentioned above, there are situations where it may be important to 613initiate concurrent operations from user mode code. This is particularly 614important for periodic transfers (interrupt and isochronous), but it can 615be used for other kinds of USB requests too. In such cases, the 616asynchronous requests described here are essential. Rather than 617submitting one request and having the kernel block until it completes, 618the blocking is separate. 619 620These requests are packaged into a structure that resembles the URB used 621by kernel device drivers. (No POSIX Async I/O support here, sorry.) It 622identifies the endpoint type (``USBDEVFS_URB_TYPE_*``), endpoint 623(number, masked with USB_DIR_IN as appropriate), buffer and length, 624and a user "context" value serving to uniquely identify each request. 625(It's usually a pointer to per-request data.) Flags can modify requests 626(not as many as supported for kernel drivers). 627 628Each request can specify a realtime signal number (between SIGRTMIN and 629SIGRTMAX, inclusive) to request a signal be sent when the request 630completes. 631 632When usbfs returns these urbs, the status value is updated, and the 633buffer may have been modified. Except for isochronous transfers, the 634actual_length is updated to say how many bytes were transferred; if the 635USBDEVFS_URB_DISABLE_SPD flag is set ("short packets are not OK"), if 636fewer bytes were read than were requested then you get an error report:: 637 638 struct usbdevfs_iso_packet_desc { 639 unsigned int length; 640 unsigned int actual_length; 641 unsigned int status; 642 }; 643 644 struct usbdevfs_urb { 645 unsigned char type; 646 unsigned char endpoint; 647 int status; 648 unsigned int flags; 649 void *buffer; 650 int buffer_length; 651 int actual_length; 652 int start_frame; 653 int number_of_packets; 654 int error_count; 655 unsigned int signr; 656 void *usercontext; 657 struct usbdevfs_iso_packet_desc iso_frame_desc[]; 658 }; 659 660For these asynchronous requests, the file modification time reflects 661when the request was initiated. This contrasts with their use with the 662synchronous requests, where it reflects when requests complete. 663 664USBDEVFS_DISCARDURB 665 *TBS* File modification time is not updated by this request. 666 667USBDEVFS_DISCSIGNAL 668 *TBS* File modification time is not updated by this request. 669 670USBDEVFS_REAPURB 671 *TBS* File modification time is not updated by this request. 672 673USBDEVFS_REAPURBNDELAY 674 *TBS* File modification time is not updated by this request. 675 676USBDEVFS_SUBMITURB 677 *TBS* 678 679The USB devices 680=============== 681 682The USB devices are now exported via debugfs: 683 684- ``/sys/kernel/debug/usb/devices`` ... a text file showing each of the USB 685 devices on known to the kernel, and their configuration descriptors. 686 You can also poll() this to learn about new devices. 687 688/sys/kernel/debug/usb/devices 689----------------------------- 690 691This file is handy for status viewing tools in user mode, which can scan 692the text format and ignore most of it. More detailed device status 693(including class and vendor status) is available from device-specific 694files. For information about the current format of this file, see below. 695 696This file, in combination with the poll() system call, can also be used 697to detect when devices are added or removed:: 698 699 int fd; 700 struct pollfd pfd; 701 702 fd = open("/sys/kernel/debug/usb/devices", O_RDONLY); 703 pfd = { fd, POLLIN, 0 }; 704 for (;;) { 705 /* The first time through, this call will return immediately. */ 706 poll(&pfd, 1, -1); 707 708 /* To see what's changed, compare the file's previous and current 709 contents or scan the filesystem. (Scanning is more precise.) */ 710 } 711 712Note that this behavior is intended to be used for informational and 713debug purposes. It would be more appropriate to use programs such as 714udev or HAL to initialize a device or start a user-mode helper program, 715for instance. 716 717In this file, each device's output has multiple lines of ASCII output. 718 719I made it ASCII instead of binary on purpose, so that someone 720can obtain some useful data from it without the use of an 721auxiliary program. However, with an auxiliary program, the numbers 722in the first 4 columns of each ``T:`` line (topology info: 723Lev, Prnt, Port, Cnt) can be used to build a USB topology diagram. 724 725Each line is tagged with a one-character ID for that line:: 726 727 T = Topology (etc.) 728 B = Bandwidth (applies only to USB host controllers, which are 729 virtualized as root hubs) 730 D = Device descriptor info. 731 P = Product ID info. (from Device descriptor, but they won't fit 732 together on one line) 733 S = String descriptors. 734 C = Configuration descriptor info. (* = active configuration) 735 I = Interface descriptor info. 736 E = Endpoint descriptor info. 737 738/sys/kernel/debug/usb/devices output format 739~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 740 741Legend:: 742 d = decimal number (may have leading spaces or 0's) 743 x = hexadecimal number (may have leading spaces or 0's) 744 s = string 745 746 747 748Topology info 749^^^^^^^^^^^^^ 750 751:: 752 753 T: Bus=dd Lev=dd Prnt=dd Port=dd Cnt=dd Dev#=ddd Spd=dddd MxCh=dd 754 | | | | | | | | |__MaxChildren 755 | | | | | | | |__Device Speed in Mbps 756 | | | | | | |__DeviceNumber 757 | | | | | |__Count of devices at this level 758 | | | | |__Connector/Port on Parent for this device 759 | | | |__Parent DeviceNumber 760 | | |__Level in topology for this bus 761 | |__Bus number 762 |__Topology info tag 763 764Speed may be: 765 766 ======= ====================================================== 767 1.5 Mbit/s for low speed USB 768 12 Mbit/s for full speed USB 769 480 Mbit/s for high speed USB (added for USB 2.0); 770 also used for Wireless USB, which has no fixed speed 771 5000 Mbit/s for SuperSpeed USB (added for USB 3.0) 772 ======= ====================================================== 773 774For reasons lost in the mists of time, the Port number is always 775too low by 1. For example, a device plugged into port 4 will 776show up with ``Port=03``. 777 778Bandwidth info 779^^^^^^^^^^^^^^ 780 781:: 782 783 B: Alloc=ddd/ddd us (xx%), #Int=ddd, #Iso=ddd 784 | | | |__Number of isochronous requests 785 | | |__Number of interrupt requests 786 | |__Total Bandwidth allocated to this bus 787 |__Bandwidth info tag 788 789Bandwidth allocation is an approximation of how much of one frame 790(millisecond) is in use. It reflects only periodic transfers, which 791are the only transfers that reserve bandwidth. Control and bulk 792transfers use all other bandwidth, including reserved bandwidth that 793is not used for transfers (such as for short packets). 794 795The percentage is how much of the "reserved" bandwidth is scheduled by 796those transfers. For a low or full speed bus (loosely, "USB 1.1"), 79790% of the bus bandwidth is reserved. For a high speed bus (loosely, 798"USB 2.0") 80% is reserved. 799 800 801Device descriptor info & Product ID info 802^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 803 804:: 805 806 D: Ver=x.xx Cls=xx(s) Sub=xx Prot=xx MxPS=dd #Cfgs=dd 807 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx 808 809where:: 810 811 D: Ver=x.xx Cls=xx(sssss) Sub=xx Prot=xx MxPS=dd #Cfgs=dd 812 | | | | | | |__NumberConfigurations 813 | | | | | |__MaxPacketSize of Default Endpoint 814 | | | | |__DeviceProtocol 815 | | | |__DeviceSubClass 816 | | |__DeviceClass 817 | |__Device USB version 818 |__Device info tag #1 819 820where:: 821 822 P: Vendor=xxxx ProdID=xxxx Rev=xx.xx 823 | | | |__Product revision number 824 | | |__Product ID code 825 | |__Vendor ID code 826 |__Device info tag #2 827 828 829String descriptor info 830^^^^^^^^^^^^^^^^^^^^^^ 831:: 832 833 S: Manufacturer=ssss 834 | |__Manufacturer of this device as read from the device. 835 | For USB host controller drivers (virtual root hubs) this may 836 | be omitted, or (for newer drivers) will identify the kernel 837 | version and the driver which provides this hub emulation. 838 |__String info tag 839 840 S: Product=ssss 841 | |__Product description of this device as read from the device. 842 | For older USB host controller drivers (virtual root hubs) this 843 | indicates the driver; for newer ones, it's a product (and vendor) 844 | description that often comes from the kernel's PCI ID database. 845 |__String info tag 846 847 S: SerialNumber=ssss 848 | |__Serial Number of this device as read from the device. 849 | For USB host controller drivers (virtual root hubs) this is 850 | some unique ID, normally a bus ID (address or slot name) that 851 | can't be shared with any other device. 852 |__String info tag 853 854 855 856Configuration descriptor info 857^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 858:: 859 860 C:* #Ifs=dd Cfg#=dd Atr=xx MPwr=dddmA 861 | | | | | |__MaxPower in mA 862 | | | | |__Attributes 863 | | | |__ConfiguratioNumber 864 | | |__NumberOfInterfaces 865 | |__ "*" indicates the active configuration (others are " ") 866 |__Config info tag 867 868USB devices may have multiple configurations, each of which act 869rather differently. For example, a bus-powered configuration 870might be much less capable than one that is self-powered. Only 871one device configuration can be active at a time; most devices 872have only one configuration. 873 874Each configuration consists of one or more interfaces. Each 875interface serves a distinct "function", which is typically bound 876to a different USB device driver. One common example is a USB 877speaker with an audio interface for playback, and a HID interface 878for use with software volume control. 879 880Interface descriptor info (can be multiple per Config) 881^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 882:: 883 884 I:* If#=dd Alt=dd #EPs=dd Cls=xx(sssss) Sub=xx Prot=xx Driver=ssss 885 | | | | | | | | |__Driver name 886 | | | | | | | | or "(none)" 887 | | | | | | | |__InterfaceProtocol 888 | | | | | | |__InterfaceSubClass 889 | | | | | |__InterfaceClass 890 | | | | |__NumberOfEndpoints 891 | | | |__AlternateSettingNumber 892 | | |__InterfaceNumber 893 | |__ "*" indicates the active altsetting (others are " ") 894 |__Interface info tag 895 896A given interface may have one or more "alternate" settings. 897For example, default settings may not use more than a small 898amount of periodic bandwidth. To use significant fractions 899of bus bandwidth, drivers must select a non-default altsetting. 900 901Only one setting for an interface may be active at a time, and 902only one driver may bind to an interface at a time. Most devices 903have only one alternate setting per interface. 904 905 906Endpoint descriptor info (can be multiple per Interface) 907^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 908 909:: 910 911 E: Ad=xx(s) Atr=xx(ssss) MxPS=dddd Ivl=dddss 912 | | | | |__Interval (max) between transfers 913 | | | |__EndpointMaxPacketSize 914 | | |__Attributes(EndpointType) 915 | |__EndpointAddress(I=In,O=Out) 916 |__Endpoint info tag 917 918The interval is nonzero for all periodic (interrupt or isochronous) 919endpoints. For high speed endpoints the transfer interval may be 920measured in microseconds rather than milliseconds. 921 922For high speed periodic endpoints, the ``EndpointMaxPacketSize`` reflects 923the per-microframe data transfer size. For "high bandwidth" 924endpoints, that can reflect two or three packets (for up to 9253KBytes every 125 usec) per endpoint. 926 927With the Linux-USB stack, periodic bandwidth reservations use the 928transfer intervals and sizes provided by URBs, which can be less 929than those found in endpoint descriptor. 930 931Usage examples 932~~~~~~~~~~~~~~ 933 934If a user or script is interested only in Topology info, for 935example, use something like ``grep ^T: /sys/kernel/debug/usb/devices`` 936for only the Topology lines. A command like 937``grep -i ^[tdp]: /sys/kernel/debug/usb/devices`` can be used to list 938only the lines that begin with the characters in square brackets, 939where the valid characters are TDPCIE. With a slightly more able 940script, it can display any selected lines (for example, only T, D, 941and P lines) and change their output format. (The ``procusb`` 942Perl script is the beginning of this idea. It will list only 943selected lines [selected from TBDPSCIE] or "All" lines from 944``/sys/kernel/debug/usb/devices``.) 945 946The Topology lines can be used to generate a graphic/pictorial 947of the USB devices on a system's root hub. (See more below 948on how to do this.) 949 950The Interface lines can be used to determine what driver is 951being used for each device, and which altsetting it activated. 952 953The Configuration lines could be used to list maximum power 954(in milliamps) that a system's USB devices are using. 955For example, ``grep ^C: /sys/kernel/debug/usb/devices``. 956 957 958Here's an example, from a system which has a UHCI root hub, 959an external hub connected to the root hub, and a mouse and 960a serial converter connected to the external hub. 961 962:: 963 964 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2 965 B: Alloc= 28/900 us ( 3%), #Int= 2, #Iso= 0 966 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 967 P: Vendor=0000 ProdID=0000 Rev= 0.00 968 S: Product=USB UHCI Root Hub 969 S: SerialNumber=dce0 970 C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr= 0mA 971 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 972 E: Ad=81(I) Atr=03(Int.) MxPS= 8 Ivl=255ms 973 974 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4 975 D: Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 976 P: Vendor=0451 ProdID=1446 Rev= 1.00 977 C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=100mA 978 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 979 E: Ad=81(I) Atr=03(Int.) MxPS= 1 Ivl=255ms 980 981 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0 982 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 983 P: Vendor=04b4 ProdID=0001 Rev= 0.00 984 C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=100mA 985 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse 986 E: Ad=81(I) Atr=03(Int.) MxPS= 3 Ivl= 10ms 987 988 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0 989 D: Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1 990 P: Vendor=0565 ProdID=0001 Rev= 1.08 991 S: Manufacturer=Peracom Networks, Inc. 992 S: Product=Peracom USB to Serial Converter 993 C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=100mA 994 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial 995 E: Ad=81(I) Atr=02(Bulk) MxPS= 64 Ivl= 16ms 996 E: Ad=01(O) Atr=02(Bulk) MxPS= 16 Ivl= 16ms 997 E: Ad=82(I) Atr=03(Int.) MxPS= 8 Ivl= 8ms 998 999 1000Selecting only the ``T:`` and ``I:`` lines from this (for example, by using 1001``procusb ti``), we have 1002 1003:: 1004 1005 T: Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2 1006 T: Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 4 1007 I: If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub 1008 T: Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#= 3 Spd=1.5 MxCh= 0 1009 I: If#= 0 Alt= 0 #EPs= 1 Cls=03(HID ) Sub=01 Prot=02 Driver=mouse 1010 T: Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#= 4 Spd=12 MxCh= 0 1011 I: If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial 1012 1013 1014Physically this looks like (or could be converted to):: 1015 1016 +------------------+ 1017 | PC/root_hub (12)| Dev# = 1 1018 +------------------+ (nn) is Mbps. 1019 Level 0 | CN.0 | CN.1 | [CN = connector/port #] 1020 +------------------+ 1021 / 1022 / 1023 +-----------------------+ 1024 Level 1 | Dev#2: 4-port hub (12)| 1025 +-----------------------+ 1026 |CN.0 |CN.1 |CN.2 |CN.3 | 1027 +-----------------------+ 1028 \ \____________________ 1029 \_____ \ 1030 \ \ 1031 +--------------------+ +--------------------+ 1032 Level 2 | Dev# 3: mouse (1.5)| | Dev# 4: serial (12)| 1033 +--------------------+ +--------------------+ 1034 1035 1036 1037Or, in a more tree-like structure (ports [Connectors] without 1038connections could be omitted):: 1039 1040 PC: Dev# 1, root hub, 2 ports, 12 Mbps 1041 |_ CN.0: Dev# 2, hub, 4 ports, 12 Mbps 1042 |_ CN.0: Dev #3, mouse, 1.5 Mbps 1043 |_ CN.1: 1044 |_ CN.2: Dev #4, serial, 12 Mbps 1045 |_ CN.3: 1046 |_ CN.1: 1047