Based on kernel version 4.3. Page generated on 2015-11-02 12:48 EST.
1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" 3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" > 4 5 <book id="USB-Gadget-API"> 6 <bookinfo> 7 <title>USB Gadget API for Linux</title> 8 <date>20 August 2004</date> 9 <edition>20 August 2004</edition> 10 11 <legalnotice> 12 <para> 13 This documentation is free software; you can redistribute 14 it and/or modify it under the terms of the GNU General Public 15 License as published by the Free Software Foundation; either 16 version 2 of the License, or (at your option) any later 17 version. 18 </para> 19 20 <para> 21 This program is distributed in the hope that it will be 22 useful, but WITHOUT ANY WARRANTY; without even the implied 23 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 24 See the GNU General Public License for more details. 25 </para> 26 27 <para> 28 You should have received a copy of the GNU General Public 29 License along with this program; if not, write to the Free 30 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, 31 MA 02111-1307 USA 32 </para> 33 34 <para> 35 For more details see the file COPYING in the source 36 distribution of Linux. 37 </para> 38 </legalnotice> 39 <copyright> 40 <year>2003-2004</year> 41 <holder>David Brownell</holder> 42 </copyright> 43 44 <author> 45 <firstname>David</firstname> 46 <surname>Brownell</surname> 47 <affiliation> 48 <address><email>firstname.lastname@example.org</email></address> 49 </affiliation> 50 </author> 51 </bookinfo> 52 53 <toc></toc> 54 55 <chapter id="intro"><title>Introduction</title> 56 57 <para>This document presents a Linux-USB "Gadget" 58 kernel mode 59 API, for use within peripherals and other USB devices 60 that embed Linux. 61 It provides an overview of the API structure, 62 and shows how that fits into a system development project. 63 This is the first such API released on Linux to address 64 a number of important problems, including: </para> 65 66 <itemizedlist> 67 <listitem><para>Supports USB 2.0, for high speed devices which 68 can stream data at several dozen megabytes per second. 69 </para></listitem> 70 <listitem><para>Handles devices with dozens of endpoints just as 71 well as ones with just two fixed-function ones. Gadget drivers 72 can be written so they're easy to port to new hardware. 73 </para></listitem> 74 <listitem><para>Flexible enough to expose more complex USB device 75 capabilities such as multiple configurations, multiple interfaces, 76 composite devices, 77 and alternate interface settings. 78 </para></listitem> 79 <listitem><para>USB "On-The-Go" (OTG) support, in conjunction 80 with updates to the Linux-USB host side. 81 </para></listitem> 82 <listitem><para>Sharing data structures and API models with the 83 Linux-USB host side API. This helps the OTG support, and 84 looks forward to more-symmetric frameworks (where the same 85 I/O model is used by both host and device side drivers). 86 </para></listitem> 87 <listitem><para>Minimalist, so it's easier to support new device 88 controller hardware. I/O processing doesn't imply large 89 demands for memory or CPU resources. 90 </para></listitem> 91 </itemizedlist> 92 93 94 <para>Most Linux developers will not be able to use this API, since they 95 have USB "host" hardware in a PC, workstation, or server. 96 Linux users with embedded systems are more likely to 97 have USB peripheral hardware. 98 To distinguish drivers running inside such hardware from the 99 more familiar Linux "USB device drivers", 100 which are host side proxies for the real USB devices, 101 a different term is used: 102 the drivers inside the peripherals are "USB gadget drivers". 103 In USB protocol interactions, the device driver is the master 104 (or "client driver") 105 and the gadget driver is the slave (or "function driver"). 106 </para> 107 108 <para>The gadget API resembles the host side Linux-USB API in that both 109 use queues of request objects to package I/O buffers, and those requests 110 may be submitted or canceled. 111 They share common definitions for the standard USB 112 <emphasis>Chapter 9</emphasis> messages, structures, and constants. 113 Also, both APIs bind and unbind drivers to devices. 114 The APIs differ in detail, since the host side's current 115 URB framework exposes a number of implementation details 116 and assumptions that are inappropriate for a gadget API. 117 While the model for control transfers and configuration 118 management is necessarily different (one side is a hardware-neutral master, 119 the other is a hardware-aware slave), the endpoint I/0 API used here 120 should also be usable for an overhead-reduced host side API. 121 </para> 122 123 </chapter> 124 125 <chapter id="structure"><title>Structure of Gadget Drivers</title> 126 127 <para>A system running inside a USB peripheral 128 normally has at least three layers inside the kernel to handle 129 USB protocol processing, and may have additional layers in 130 user space code. 131 The "gadget" API is used by the middle layer to interact 132 with the lowest level (which directly handles hardware). 133 </para> 134 135 <para>In Linux, from the bottom up, these layers are: 136 </para> 137 138 <variablelist> 139 140 <varlistentry> 141 <term><emphasis>USB Controller Driver</emphasis></term> 142 143 <listitem> 144 <para>This is the lowest software level. 145 It is the only layer that talks to hardware, 146 through registers, fifos, dma, irqs, and the like. 147 The <filename><linux/usb/gadget.h></filename> API abstracts 148 the peripheral controller endpoint hardware. 149 That hardware is exposed through endpoint objects, which accept 150 streams of IN/OUT buffers, and through callbacks that interact 151 with gadget drivers. 152 Since normal USB devices only have one upstream 153 port, they only have one of these drivers. 154 The controller driver can support any number of different 155 gadget drivers, but only one of them can be used at a time. 156 </para> 157 158 <para>Examples of such controller hardware include 159 the PCI-based NetChip 2280 USB 2.0 high speed controller, 160 the SA-11x0 or PXA-25x UDC (found within many PDAs), 161 and a variety of other products. 162 </para> 163 164 </listitem></varlistentry> 165 166 <varlistentry> 167 <term><emphasis>Gadget Driver</emphasis></term> 168 169 <listitem> 170 <para>The lower boundary of this driver implements hardware-neutral 171 USB functions, using calls to the controller driver. 172 Because such hardware varies widely in capabilities and restrictions, 173 and is used in embedded environments where space is at a premium, 174 the gadget driver is often configured at compile time 175 to work with endpoints supported by one particular controller. 176 Gadget drivers may be portable to several different controllers, 177 using conditional compilation. 178 (Recent kernels substantially simplify the work involved in 179 supporting new hardware, by <emphasis>autoconfiguring</emphasis> 180 endpoints automatically for many bulk-oriented drivers.) 181 Gadget driver responsibilities include: 182 </para> 183 <itemizedlist> 184 <listitem><para>handling setup requests (ep0 protocol responses) 185 possibly including class-specific functionality 186 </para></listitem> 187 <listitem><para>returning configuration and string descriptors 188 </para></listitem> 189 <listitem><para>(re)setting configurations and interface 190 altsettings, including enabling and configuring endpoints 191 </para></listitem> 192 <listitem><para>handling life cycle events, such as managing 193 bindings to hardware, 194 USB suspend/resume, remote wakeup, 195 and disconnection from the USB host. 196 </para></listitem> 197 <listitem><para>managing IN and OUT transfers on all currently 198 enabled endpoints 199 </para></listitem> 200 </itemizedlist> 201 202 <para> 203 Such drivers may be modules of proprietary code, although 204 that approach is discouraged in the Linux community. 205 </para> 206 </listitem></varlistentry> 207 208 <varlistentry> 209 <term><emphasis>Upper Level</emphasis></term> 210 211 <listitem> 212 <para>Most gadget drivers have an upper boundary that connects 213 to some Linux driver or framework in Linux. 214 Through that boundary flows the data which the gadget driver 215 produces and/or consumes through protocol transfers over USB. 216 Examples include: 217 </para> 218 <itemizedlist> 219 <listitem><para>user mode code, using generic (gadgetfs) 220 or application specific files in 221 <filename>/dev</filename> 222 </para></listitem> 223 <listitem><para>networking subsystem (for network gadgets, 224 like the CDC Ethernet Model gadget driver) 225 </para></listitem> 226 <listitem><para>data capture drivers, perhaps video4Linux or 227 a scanner driver; or test and measurement hardware. 228 </para></listitem> 229 <listitem><para>input subsystem (for HID gadgets) 230 </para></listitem> 231 <listitem><para>sound subsystem (for audio gadgets) 232 </para></listitem> 233 <listitem><para>file system (for PTP gadgets) 234 </para></listitem> 235 <listitem><para>block i/o subsystem (for usb-storage gadgets) 236 </para></listitem> 237 <listitem><para>... and more </para></listitem> 238 </itemizedlist> 239 </listitem></varlistentry> 240 241 <varlistentry> 242 <term><emphasis>Additional Layers</emphasis></term> 243 244 <listitem> 245 <para>Other layers may exist. 246 These could include kernel layers, such as network protocol stacks, 247 as well as user mode applications building on standard POSIX 248 system call APIs such as 249 <emphasis>open()</emphasis>, <emphasis>close()</emphasis>, 250 <emphasis>read()</emphasis> and <emphasis>write()</emphasis>. 251 On newer systems, POSIX Async I/O calls may be an option. 252 Such user mode code will not necessarily be subject to 253 the GNU General Public License (GPL). 254 </para> 255 </listitem></varlistentry> 256 257 258 </variablelist> 259 260 <para>OTG-capable systems will also need to include a standard Linux-USB 261 host side stack, 262 with <emphasis>usbcore</emphasis>, 263 one or more <emphasis>Host Controller Drivers</emphasis> (HCDs), 264 <emphasis>USB Device Drivers</emphasis> to support 265 the OTG "Targeted Peripheral List", 266 and so forth. 267 There will also be an <emphasis>OTG Controller Driver</emphasis>, 268 which is visible to gadget and device driver developers only indirectly. 269 That helps the host and device side USB controllers implement the 270 two new OTG protocols (HNP and SRP). 271 Roles switch (host to peripheral, or vice versa) using HNP 272 during USB suspend processing, and SRP can be viewed as a 273 more battery-friendly kind of device wakeup protocol. 274 </para> 275 276 <para>Over time, reusable utilities are evolving to help make some 277 gadget driver tasks simpler. 278 For example, building configuration descriptors from vectors of 279 descriptors for the configurations interfaces and endpoints is 280 now automated, and many drivers now use autoconfiguration to 281 choose hardware endpoints and initialize their descriptors. 282 283 A potential example of particular interest 284 is code implementing standard USB-IF protocols for 285 HID, networking, storage, or audio classes. 286 Some developers are interested in KDB or KGDB hooks, to let 287 target hardware be remotely debugged. 288 Most such USB protocol code doesn't need to be hardware-specific, 289 any more than network protocols like X11, HTTP, or NFS are. 290 Such gadget-side interface drivers should eventually be combined, 291 to implement composite devices. 292 </para> 293 294 </chapter> 295 296 297 <chapter id="api"><title>Kernel Mode Gadget API</title> 298 299 <para>Gadget drivers declare themselves through a 300 <emphasis>struct usb_gadget_driver</emphasis>, which is responsible for 301 most parts of enumeration for a <emphasis>struct usb_gadget</emphasis>. 302 The response to a set_configuration usually involves 303 enabling one or more of the <emphasis>struct usb_ep</emphasis> objects 304 exposed by the gadget, and submitting one or more 305 <emphasis>struct usb_request</emphasis> buffers to transfer data. 306 Understand those four data types, and their operations, and 307 you will understand how this API works. 308 </para> 309 310 <note><title>Incomplete Data Type Descriptions</title> 311 312 <para>This documentation was prepared using the standard Linux 313 kernel <filename>docproc</filename> tool, which turns text 314 and in-code comments into SGML DocBook and then into usable 315 formats such as HTML or PDF. 316 Other than the "Chapter 9" data types, most of the significant 317 data types and functions are described here. 318 </para> 319 320 <para>However, docproc does not understand all the C constructs 321 that are used, so some relevant information is likely omitted from 322 what you are reading. 323 One example of such information is endpoint autoconfiguration. 324 You'll have to read the header file, and use example source 325 code (such as that for "Gadget Zero"), to fully understand the API. 326 </para> 327 328 <para>The part of the API implementing some basic 329 driver capabilities is specific to the version of the 330 Linux kernel that's in use. 331 The 2.6 kernel includes a <emphasis>driver model</emphasis> 332 framework that has no analogue on earlier kernels; 333 so those parts of the gadget API are not fully portable. 334 (They are implemented on 2.4 kernels, but in a different way.) 335 The driver model state is another part of this API that is 336 ignored by the kerneldoc tools. 337 </para> 338 </note> 339 340 <para>The core API does not expose 341 every possible hardware feature, only the most widely available ones. 342 There are significant hardware features, such as device-to-device DMA 343 (without temporary storage in a memory buffer) 344 that would be added using hardware-specific APIs. 345 </para> 346 347 <para>This API allows drivers to use conditional compilation to handle 348 endpoint capabilities of different hardware, but doesn't require that. 349 Hardware tends to have arbitrary restrictions, relating to 350 transfer types, addressing, packet sizes, buffering, and availability. 351 As a rule, such differences only matter for "endpoint zero" logic 352 that handles device configuration and management. 353 The API supports limited run-time 354 detection of capabilities, through naming conventions for endpoints. 355 Many drivers will be able to at least partially autoconfigure 356 themselves. 357 In particular, driver init sections will often have endpoint 358 autoconfiguration logic that scans the hardware's list of endpoints 359 to find ones matching the driver requirements 360 (relying on those conventions), to eliminate some of the most 361 common reasons for conditional compilation. 362 </para> 363 364 <para>Like the Linux-USB host side API, this API exposes 365 the "chunky" nature of USB messages: I/O requests are in terms 366 of one or more "packets", and packet boundaries are visible to drivers. 367 Compared to RS-232 serial protocols, USB resembles 368 synchronous protocols like HDLC 369 (N bytes per frame, multipoint addressing, host as the primary 370 station and devices as secondary stations) 371 more than asynchronous ones 372 (tty style: 8 data bits per frame, no parity, one stop bit). 373 So for example the controller drivers won't buffer 374 two single byte writes into a single two-byte USB IN packet, 375 although gadget drivers may do so when they implement 376 protocols where packet boundaries (and "short packets") 377 are not significant. 378 </para> 379 380 <sect1 id="lifecycle"><title>Driver Life Cycle</title> 381 382 <para>Gadget drivers make endpoint I/O requests to hardware without 383 needing to know many details of the hardware, but driver 384 setup/configuration code needs to handle some differences. 385 Use the API like this: 386 </para> 387 388 <orderedlist numeration='arabic'> 389 390 <listitem><para>Register a driver for the particular device side 391 usb controller hardware, 392 such as the net2280 on PCI (USB 2.0), 393 sa11x0 or pxa25x as found in Linux PDAs, 394 and so on. 395 At this point the device is logically in the USB ch9 initial state 396 ("attached"), drawing no power and not usable 397 (since it does not yet support enumeration). 398 Any host should not see the device, since it's not 399 activated the data line pullup used by the host to 400 detect a device, even if VBUS power is available. 401 </para></listitem> 402 403 <listitem><para>Register a gadget driver that implements some higher level 404 device function. That will then bind() to a usb_gadget, which 405 activates the data line pullup sometime after detecting VBUS. 406 </para></listitem> 407 408 <listitem><para>The hardware driver can now start enumerating. 409 The steps it handles are to accept USB power and set_address requests. 410 Other steps are handled by the gadget driver. 411 If the gadget driver module is unloaded before the host starts to 412 enumerate, steps before step 7 are skipped. 413 </para></listitem> 414 415 <listitem><para>The gadget driver's setup() call returns usb descriptors, 416 based both on what the bus interface hardware provides and on the 417 functionality being implemented. 418 That can involve alternate settings or configurations, 419 unless the hardware prevents such operation. 420 For OTG devices, each configuration descriptor includes 421 an OTG descriptor. 422 </para></listitem> 423 424 <listitem><para>The gadget driver handles the last step of enumeration, 425 when the USB host issues a set_configuration call. 426 It enables all endpoints used in that configuration, 427 with all interfaces in their default settings. 428 That involves using a list of the hardware's endpoints, enabling each 429 endpoint according to its descriptor. 430 It may also involve using <function>usb_gadget_vbus_draw</function> 431 to let more power be drawn from VBUS, as allowed by that configuration. 432 For OTG devices, setting a configuration may also involve reporting 433 HNP capabilities through a user interface. 434 </para></listitem> 435 436 <listitem><para>Do real work and perform data transfers, possibly involving 437 changes to interface settings or switching to new configurations, until the 438 device is disconnect()ed from the host. 439 Queue any number of transfer requests to each endpoint. 440 It may be suspended and resumed several times before being disconnected. 441 On disconnect, the drivers go back to step 3 (above). 442 </para></listitem> 443 444 <listitem><para>When the gadget driver module is being unloaded, 445 the driver unbind() callback is issued. That lets the controller 446 driver be unloaded. 447 </para></listitem> 448 449 </orderedlist> 450 451 <para>Drivers will normally be arranged so that just loading the 452 gadget driver module (or statically linking it into a Linux kernel) 453 allows the peripheral device to be enumerated, but some drivers 454 will defer enumeration until some higher level component (like 455 a user mode daemon) enables it. 456 Note that at this lowest level there are no policies about how 457 ep0 configuration logic is implemented, 458 except that it should obey USB specifications. 459 Such issues are in the domain of gadget drivers, 460 including knowing about implementation constraints 461 imposed by some USB controllers 462 or understanding that composite devices might happen to 463 be built by integrating reusable components. 464 </para> 465 466 <para>Note that the lifecycle above can be slightly different 467 for OTG devices. 468 Other than providing an additional OTG descriptor in each 469 configuration, only the HNP-related differences are particularly 470 visible to driver code. 471 They involve reporting requirements during the SET_CONFIGURATION 472 request, and the option to invoke HNP during some suspend callbacks. 473 Also, SRP changes the semantics of 474 <function>usb_gadget_wakeup</function> 475 slightly. 476 </para> 477 478 </sect1> 479 480 <sect1 id="ch9"><title>USB 2.0 Chapter 9 Types and Constants</title> 481 482 <para>Gadget drivers 483 rely on common USB structures and constants 484 defined in the 485 <filename><linux/usb/ch9.h></filename> 486 header file, which is standard in Linux 2.6 kernels. 487 These are the same types and constants used by host 488 side drivers (and usbcore). 489 </para> 490 491 !Iinclude/linux/usb/ch9.h 492 </sect1> 493 494 <sect1 id="core"><title>Core Objects and Methods</title> 495 496 <para>These are declared in 497 <filename><linux/usb/gadget.h></filename>, 498 and are used by gadget drivers to interact with 499 USB peripheral controller drivers. 500 </para> 501 502 <!-- yeech, this is ugly in nsgmls PDF output. 503 504 the PDF bookmark and refentry output nesting is wrong, 505 and the member/argument documentation indents ugly. 506 507 plus something (docproc?) adds whitespace before the 508 descriptive paragraph text, so it can't line up right 509 unless the explanations are trivial. 510 --> 511 512 !Iinclude/linux/usb/gadget.h 513 </sect1> 514 515 <sect1 id="utils"><title>Optional Utilities</title> 516 517 <para>The core API is sufficient for writing a USB Gadget Driver, 518 but some optional utilities are provided to simplify common tasks. 519 These utilities include endpoint autoconfiguration. 520 </para> 521 522 !Edrivers/usb/gadget/usbstring.c 523 !Edrivers/usb/gadget/config.c 524 <!-- !Edrivers/usb/gadget/epautoconf.c --> 525 </sect1> 526 527 <sect1 id="composite"><title>Composite Device Framework</title> 528 529 <para>The core API is sufficient for writing drivers for composite 530 USB devices (with more than one function in a given configuration), 531 and also multi-configuration devices (also more than one function, 532 but not necessarily sharing a given configuration). 533 There is however an optional framework which makes it easier to 534 reuse and combine functions. 535 </para> 536 537 <para>Devices using this framework provide a <emphasis>struct 538 usb_composite_driver</emphasis>, which in turn provides one or 539 more <emphasis>struct usb_configuration</emphasis> instances. 540 Each such configuration includes at least one 541 <emphasis>struct usb_function</emphasis>, which packages a user 542 visible role such as "network link" or "mass storage device". 543 Management functions may also exist, such as "Device Firmware 544 Upgrade". 545 </para> 546 547 !Iinclude/linux/usb/composite.h 548 !Edrivers/usb/gadget/composite.c 549 550 </sect1> 551 552 <sect1 id="functions"><title>Composite Device Functions</title> 553 554 <para>At this writing, a few of the current gadget drivers have 555 been converted to this framework. 556 Near-term plans include converting all of them, except for "gadgetfs". 557 </para> 558 559 !Edrivers/usb/gadget/function/f_acm.c 560 !Edrivers/usb/gadget/function/f_ecm.c 561 !Edrivers/usb/gadget/function/f_subset.c 562 !Edrivers/usb/gadget/function/f_obex.c 563 !Edrivers/usb/gadget/function/f_serial.c 564 565 </sect1> 566 567 568 </chapter> 569 570 <chapter id="controllers"><title>Peripheral Controller Drivers</title> 571 572 <para>The first hardware supporting this API was the NetChip 2280 573 controller, which supports USB 2.0 high speed and is based on PCI. 574 This is the <filename>net2280</filename> driver module. 575 The driver supports Linux kernel versions 2.4 and 2.6; 576 contact NetChip Technologies for development boards and product 577 information. 578 </para> 579 580 <para>Other hardware working in the "gadget" framework includes: 581 Intel's PXA 25x and IXP42x series processors 582 (<filename>pxa2xx_udc</filename>), 583 Toshiba TC86c001 "Goku-S" (<filename>goku_udc</filename>), 584 Renesas SH7705/7727 (<filename>sh_udc</filename>), 585 MediaQ 11xx (<filename>mq11xx_udc</filename>), 586 Hynix HMS30C7202 (<filename>h7202_udc</filename>), 587 National 9303/4 (<filename>n9604_udc</filename>), 588 Texas Instruments OMAP (<filename>omap_udc</filename>), 589 Sharp LH7A40x (<filename>lh7a40x_udc</filename>), 590 and more. 591 Most of those are full speed controllers. 592 </para> 593 594 <para>At this writing, there are people at work on drivers in 595 this framework for several other USB device controllers, 596 with plans to make many of them be widely available. 597 </para> 598 599 <!-- !Edrivers/usb/gadget/net2280.c --> 600 601 <para>A partial USB simulator, 602 the <filename>dummy_hcd</filename> driver, is available. 603 It can act like a net2280, a pxa25x, or an sa11x0 in terms 604 of available endpoints and device speeds; and it simulates 605 control, bulk, and to some extent interrupt transfers. 606 That lets you develop some parts of a gadget driver on a normal PC, 607 without any special hardware, and perhaps with the assistance 608 of tools such as GDB running with User Mode Linux. 609 At least one person has expressed interest in adapting that 610 approach, hooking it up to a simulator for a microcontroller. 611 Such simulators can help debug subsystems where the runtime hardware 612 is unfriendly to software development, or is not yet available. 613 </para> 614 615 <para>Support for other controllers is expected to be developed 616 and contributed 617 over time, as this driver framework evolves. 618 </para> 619 620 </chapter> 621 622 <chapter id="gadget"><title>Gadget Drivers</title> 623 624 <para>In addition to <emphasis>Gadget Zero</emphasis> 625 (used primarily for testing and development with drivers 626 for usb controller hardware), other gadget drivers exist. 627 </para> 628 629 <para>There's an <emphasis>ethernet</emphasis> gadget 630 driver, which implements one of the most useful 631 <emphasis>Communications Device Class</emphasis> (CDC) models. 632 One of the standards for cable modem interoperability even 633 specifies the use of this ethernet model as one of two 634 mandatory options. 635 Gadgets using this code look to a USB host as if they're 636 an Ethernet adapter. 637 It provides access to a network where the gadget's CPU is one host, 638 which could easily be bridging, routing, or firewalling 639 access to other networks. 640 Since some hardware can't fully implement the CDC Ethernet 641 requirements, this driver also implements a "good parts only" 642 subset of CDC Ethernet. 643 (That subset doesn't advertise itself as CDC Ethernet, 644 to avoid creating problems.) 645 </para> 646 647 <para>Support for Microsoft's <emphasis>RNDIS</emphasis> 648 protocol has been contributed by Pengutronix and Auerswald GmbH. 649 This is like CDC Ethernet, but it runs on more slightly USB hardware 650 (but less than the CDC subset). 651 However, its main claim to fame is being able to connect directly to 652 recent versions of Windows, using drivers that Microsoft bundles 653 and supports, making it much simpler to network with Windows. 654 </para> 655 656 <para>There is also support for user mode gadget drivers, 657 using <emphasis>gadgetfs</emphasis>. 658 This provides a <emphasis>User Mode API</emphasis> that presents 659 each endpoint as a single file descriptor. I/O is done using 660 normal <emphasis>read()</emphasis> and <emphasis>read()</emphasis> calls. 661 Familiar tools like GDB and pthreads can be used to 662 develop and debug user mode drivers, so that once a robust 663 controller driver is available many applications for it 664 won't require new kernel mode software. 665 Linux 2.6 <emphasis>Async I/O (AIO)</emphasis> 666 support is available, so that user mode software 667 can stream data with only slightly more overhead 668 than a kernel driver. 669 </para> 670 671 <para>There's a USB Mass Storage class driver, which provides 672 a different solution for interoperability with systems such 673 as MS-Windows and MacOS. 674 That <emphasis>Mass Storage</emphasis> driver uses a 675 file or block device as backing store for a drive, 676 like the <filename>loop</filename> driver. 677 The USB host uses the BBB, CB, or CBI versions of the mass 678 storage class specification, using transparent SCSI commands 679 to access the data from the backing store. 680 </para> 681 682 <para>There's a "serial line" driver, useful for TTY style 683 operation over USB. 684 The latest version of that driver supports CDC ACM style 685 operation, like a USB modem, and so on most hardware it can 686 interoperate easily with MS-Windows. 687 One interesting use of that driver is in boot firmware (like a BIOS), 688 which can sometimes use that model with very small systems without 689 real serial lines. 690 </para> 691 692 <para>Support for other kinds of gadget is expected to 693 be developed and contributed 694 over time, as this driver framework evolves. 695 </para> 696 697 </chapter> 698 699 <chapter id="otg"><title>USB On-The-GO (OTG)</title> 700 701 <para>USB OTG support on Linux 2.6 was initially developed 702 by Texas Instruments for 703 <ulink url="http://www.omap.com">OMAP</ulink> 16xx and 17xx 704 series processors. 705 Other OTG systems should work in similar ways, but the 706 hardware level details could be very different. 707 </para> 708 709 <para>Systems need specialized hardware support to implement OTG, 710 notably including a special <emphasis>Mini-AB</emphasis> jack 711 and associated transceiver to support <emphasis>Dual-Role</emphasis> 712 operation: 713 they can act either as a host, using the standard 714 Linux-USB host side driver stack, 715 or as a peripheral, using this "gadget" framework. 716 To do that, the system software relies on small additions 717 to those programming interfaces, 718 and on a new internal component (here called an "OTG Controller") 719 affecting which driver stack connects to the OTG port. 720 In each role, the system can re-use the existing pool of 721 hardware-neutral drivers, layered on top of the controller 722 driver interfaces (<emphasis>usb_bus</emphasis> or 723 <emphasis>usb_gadget</emphasis>). 724 Such drivers need at most minor changes, and most of the calls 725 added to support OTG can also benefit non-OTG products. 726 </para> 727 728 <itemizedlist> 729 <listitem><para>Gadget drivers test the <emphasis>is_otg</emphasis> 730 flag, and use it to determine whether or not to include 731 an OTG descriptor in each of their configurations. 732 </para></listitem> 733 <listitem><para>Gadget drivers may need changes to support the 734 two new OTG protocols, exposed in new gadget attributes 735 such as <emphasis>b_hnp_enable</emphasis> flag. 736 HNP support should be reported through a user interface 737 (two LEDs could suffice), and is triggered in some cases 738 when the host suspends the peripheral. 739 SRP support can be user-initiated just like remote wakeup, 740 probably by pressing the same button. 741 </para></listitem> 742 <listitem><para>On the host side, USB device drivers need 743 to be taught to trigger HNP at appropriate moments, using 744 <function>usb_suspend_device()</function>. 745 That also conserves battery power, which is useful even 746 for non-OTG configurations. 747 </para></listitem> 748 <listitem><para>Also on the host side, a driver must support the 749 OTG "Targeted Peripheral List". That's just a whitelist, 750 used to reject peripherals not supported with a given 751 Linux OTG host. 752 <emphasis>This whitelist is product-specific; 753 each product must modify <filename>otg_whitelist.h</filename> 754 to match its interoperability specification. 755 </emphasis> 756 </para> 757 <para>Non-OTG Linux hosts, like PCs and workstations, 758 normally have some solution for adding drivers, so that 759 peripherals that aren't recognized can eventually be supported. 760 That approach is unreasonable for consumer products that may 761 never have their firmware upgraded, and where it's usually 762 unrealistic to expect traditional PC/workstation/server kinds 763 of support model to work. 764 For example, it's often impractical to change device firmware 765 once the product has been distributed, so driver bugs can't 766 normally be fixed if they're found after shipment. 767 </para></listitem> 768 </itemizedlist> 769 770 <para> 771 Additional changes are needed below those hardware-neutral 772 <emphasis>usb_bus</emphasis> and <emphasis>usb_gadget</emphasis> 773 driver interfaces; those aren't discussed here in any detail. 774 Those affect the hardware-specific code for each USB Host or Peripheral 775 controller, and how the HCD initializes (since OTG can be active only 776 on a single port). 777 They also involve what may be called an <emphasis>OTG Controller 778 Driver</emphasis>, managing the OTG transceiver and the OTG state 779 machine logic as well as much of the root hub behavior for the 780 OTG port. 781 The OTG controller driver needs to activate and deactivate USB 782 controllers depending on the relevant device role. 783 Some related changes were needed inside usbcore, so that it 784 can identify OTG-capable devices and respond appropriately 785 to HNP or SRP protocols. 786 </para> 787 788 </chapter> 789 790 </book> 791 <!-- 792 vim:syntax=sgml:sw=4 793 -->