Documentation/usb/gadget-testing.txt

   1 This file summarizes information on basic testing of USB functions
   2 provided by gadgets.
   3
   4 1. ACM function
   5 2. ECM function
   6 3. ECM subset function
   7 4. EEM function
   8 5. FFS function
   9 6. HID function
  10 7. LOOPBACK function
  11 8. MASS STORAGE function
  12 9. MIDI function
  13 10. NCM function
  14 11. OBEX function
  15 12. PHONET function
  16 13. RNDIS function
  17 14. SERIAL function
  18 15. SOURCESINK function
  19 16. UAC1 function
  20 17. UAC2 function
  21 18. UVC function
  22 19. PRINTER function
  23
  24
  25 1. ACM function
  26 ===============
  27
  28 The function is provided by usb_f_acm.ko module.
  29
  30 Function-specific configfs interface
  31 ------------------------------------
  32
  33 The function name to use when creating the function directory is "acm".
  34 The ACM function provides just one attribute in its function directory:
  35
  36         port_num
  37
  38 The attribute is read-only.
  39
  40 There can be at most 4 ACM/generic serial/OBEX ports in the system.
  41
  42
  43 Testing the ACM function
  44 ------------------------
  45
  46 On the host: cat > /dev/ttyACM<X>
  47 On the device : cat /dev/ttyGS<Y>
  48
  49 then the other way round
  50
  51 On the device: cat > /dev/ttyGS<Y>
  52 On the host: cat /dev/ttyACM<X>
  53
  54 2. ECM function
  55 ===============
  56
  57 The function is provided by usb_f_ecm.ko module.
  58
  59 Function-specific configfs interface
  60 ------------------------------------
  61
  62 The function name to use when creating the function directory is "ecm".
  63 The ECM function provides these attributes in its function directory:
  64
  65         ifname          - network device interface name associated with this
  66                         function instance
  67         qmult           - queue length multiplier for high and super speed
  68         host_addr       - MAC address of host's end of this
  69                         Ethernet over USB link
  70         dev_addr        - MAC address of device's end of this
  71                         Ethernet over USB link
  72
  73 and after creating the functions/ecm.<instance name> they contain default
  74 values: qmult is 5, dev_addr and host_addr are randomly selected.
  75 Except for ifname they can be written to until the function is linked to a
  76 configuration. The ifname is read-only and contains the name of the interface
  77 which was assigned by the net core, e. g. usb0.
  78
  79 Testing the ECM function
  80 ------------------------
  81
  82 Configure IP addresses of the device and the host. Then:
  83
  84 On the device: ping <host's IP>
  85 On the host: ping <device's IP>
  86
  87 3. ECM subset function
  88 ======================
  89
  90 The function is provided by usb_f_ecm_subset.ko module.
  91
  92 Function-specific configfs interface
  93 ------------------------------------
  94
  95 The function name to use when creating the function directory is "geth".
  96 The ECM subset function provides these attributes in its function directory:
  97
  98         ifname          - network device interface name associated with this
  99                         function instance
 100         qmult           - queue length multiplier for high and super speed
 101         host_addr       - MAC address of host's end of this
 102                         Ethernet over USB link
 103         dev_addr        - MAC address of device's end of this
 104                         Ethernet over USB link
 105
 106 and after creating the functions/ecm.<instance name> they contain default
 107 values: qmult is 5, dev_addr and host_addr are randomly selected.
 108 Except for ifname they can be written to until the function is linked to a
 109 configuration. The ifname is read-only and contains the name of the interface
 110 which was assigned by the net core, e. g. usb0.
 111
 112 Testing the ECM subset function
 113 -------------------------------
 114
 115 Configure IP addresses of the device and the host. Then:
 116
 117 On the device: ping <host's IP>
 118 On the host: ping <device's IP>
 119
 120 4. EEM function
 121 ===============
 122
 123 The function is provided by usb_f_eem.ko module.
 124
 125 Function-specific configfs interface
 126 ------------------------------------
 127
 128 The function name to use when creating the function directory is "eem".
 129 The EEM function provides these attributes in its function directory:
 130
 131         ifname          - network device interface name associated with this
 132                         function instance
 133         qmult           - queue length multiplier for high and super speed
 134         host_addr       - MAC address of host's end of this
 135                         Ethernet over USB link
 136         dev_addr        - MAC address of device's end of this
 137                         Ethernet over USB link
 138
 139 and after creating the functions/eem.<instance name> they contain default
 140 values: qmult is 5, dev_addr and host_addr are randomly selected.
 141 Except for ifname they can be written to until the function is linked to a
 142 configuration. The ifname is read-only and contains the name of the interface
 143 which was assigned by the net core, e. g. usb0.
 144
 145 Testing the EEM function
 146 ------------------------
 147
 148 Configure IP addresses of the device and the host. Then:
 149
 150 On the device: ping <host's IP>
 151 On the host: ping <device's IP>
 152
 153 5. FFS function
 154 ===============
 155
 156 The function is provided by usb_f_fs.ko module.
 157
 158 Function-specific configfs interface
 159 ------------------------------------
 160
 161 The function name to use when creating the function directory is "ffs".
 162 The function directory is intentionally empty and not modifiable.
 163
 164 After creating the directory there is a new instance (a "device") of FunctionFS
 165 available in the system. Once a "device" is available, the user should follow
 166 the standard procedure for using FunctionFS (mount it, run the userspace
 167 process which implements the function proper). The gadget should be enabled
 168 by writing a suitable string to usb_gadget/<gadget>/UDC.
 169
 170 Testing the FFS function
 171 ------------------------
 172
 173 On the device: start the function's userspace daemon, enable the gadget
 174 On the host: use the USB function provided by the device
 175
 176 6. HID function
 177 ===============
 178
 179 The function is provided by usb_f_hid.ko module.
 180
 181 Function-specific configfs interface
 182 ------------------------------------
 183
 184 The function name to use when creating the function directory is "hid".
 185 The HID function provides these attributes in its function directory:
 186
 187         protocol        - HID protocol to use
 188         report_desc     - data to be used in HID reports, except data
 189                         passed with /dev/hidg<X>
 190         report_length   - HID report length
 191         subclass        - HID subclass to use
 192
 193 For a keyboard the protocol and the subclass are 1, the report_length is 8,
 194 while the report_desc is:
 195
 196 $ hd my_report_desc
 197 00000000  05 01 09 06 a1 01 05 07  19 e0 29 e7 15 00 25 01  |..........)...%.|
 198 00000010  75 01 95 08 81 02 95 01  75 08 81 03 95 05 75 01  |u.......u.....u.|
 199 00000020  05 08 19 01 29 05 91 02  95 01 75 03 91 03 95 06  |....).....u.....|
 200 00000030  75 08 15 00 25 65 05 07  19 00 29 65 81 00 c0     |u...%e....)e...|
 201 0000003f
 202
 203 Such a sequence of bytes can be stored to the attribute with echo:
 204
 205 $ echo -ne \\x05\\x01\\x09\\x06\\xa1.....
 206
 207 Testing the HID function
 208 ------------------------
 209
 210 Device:
 211 - create the gadget
 212 - connect the gadget to a host, preferably not the one used
 213 to control the gadget
 214 - run a program which writes to /dev/hidg<N>, e.g.
 215 a userspace program found in Documentation/usb/gadget_hid.txt:
 216
 217 $ ./hid_gadget_test /dev/hidg0 keyboard
 218
 219 Host:
 220 - observe the keystrokes from the gadget
 221
 222 7. LOOPBACK function
 223 ====================
 224
 225 The function is provided by usb_f_ss_lb.ko module.
 226
 227 Function-specific configfs interface
 228 ------------------------------------
 229
 230 The function name to use when creating the function directory is "Loopback".
 231 The LOOPBACK function provides these attributes in its function directory:
 232
 233         qlen            - depth of loopback queue
 234         bulk_buflen     - buffer length
 235
 236 Testing the LOOPBACK function
 237 -----------------------------
 238
 239 device: run the gadget
 240 host: test-usb (tools/usb/testusb.c)
 241
 242 8. MASS STORAGE function
 243 ========================
 244
 245 The function is provided by usb_f_mass_storage.ko module.
 246
 247 Function-specific configfs interface
 248 ------------------------------------
 249
 250 The function name to use when creating the function directory is "mass_storage".
 251 The MASS STORAGE function provides these attributes in its directory:
 252 files:
 253
 254         stall           - Set to permit function to halt bulk endpoints.
 255                         Disabled on some USB devices known not to work
 256                         correctly. You should set it to true.
 257         num_buffers     - Number of pipeline buffers. Valid numbers
 258                         are 2..4. Available only if
 259                         CONFIG_USB_GADGET_DEBUG_FILES is set.
 260
 261 and a default lun.0 directory corresponding to SCSI LUN #0.
 262
 263 A new lun can be added with mkdir:
 264
 265 $ mkdir functions/mass_storage.0/partition.5
 266
 267 Lun numbering does not have to be continuous, except for lun #0 which is
 268 created by default. A maximum of 8 luns can be specified and they all must be
 269 named following the <name>.<number> scheme. The numbers can be 0..8.
 270 Probably a good convention is to name the luns "lun.<number>",
 271 although it is not mandatory.
 272
 273 In each lun directory there are the following attribute files:
 274
 275         file            - The path to the backing file for the LUN.
 276                         Required if LUN is not marked as removable.
 277         ro              - Flag specifying access to the LUN shall be
 278                         read-only. This is implied if CD-ROM emulation
 279                         is enabled as well as when it was impossible
 280                         to open "filename" in R/W mode.
 281         removable       - Flag specifying that LUN shall be indicated as
 282                         being removable.
 283         cdrom           - Flag specifying that LUN shall be reported as
 284                         being a CD-ROM.
 285         nofua           - Flag specifying that FUA flag
 286                         in SCSI WRITE(10,12)
 287
 288 Testing the MASS STORAGE function
 289 ---------------------------------
 290
 291 device: connect the gadget, enable it
 292 host: dmesg, see the USB drives appear (if system configured to automatically
 293 mount)
 294
 295 9. MIDI function
 296 ================
 297
 298 The function is provided by usb_f_midi.ko module.
 299
 300 Function-specific configfs interface
 301 ------------------------------------
 302
 303 The function name to use when creating the function directory is "midi".
 304 The MIDI function provides these attributes in its function directory:
 305
 306         buflen          - MIDI buffer length
 307         id              - ID string for the USB MIDI adapter
 308         in_ports        - number of MIDI input ports
 309         index           - index value for the USB MIDI adapter
 310         out_ports       - number of MIDI output ports
 311         qlen            - USB read request queue length
 312
 313 Testing the MIDI function
 314 -------------------------
 315
 316 There are two cases: playing a mid from the gadget to
 317 the host and playing a mid from the host to the gadget.
 318
 319 1) Playing a mid from the gadget to the host
 320 host)
 321
 322 $ arecordmidi -l
 323  Port    Client name                      Port name
 324  14:0    Midi Through                     Midi Through Port-0
 325  24:0    MIDI Gadget                      MIDI Gadget MIDI 1
 326 $ arecordmidi -p 24:0 from_gadget.mid
 327
 328 gadget)
 329
 330 $ aplaymidi -l
 331  Port    Client name                      Port name
 332  20:0    f_midi                           f_midi
 333
 334 $ aplaymidi -p 20:0 to_host.mid
 335
 336 2) Playing a mid from the host to the gadget
 337 gadget)
 338
 339 $ arecordmidi -l
 340  Port    Client name                      Port name
 341  20:0    f_midi                           f_midi
 342
 343 $ arecordmidi -p 20:0 from_host.mid
 344
 345 host)
 346
 347 $ aplaymidi -l
 348  Port    Client name                      Port name
 349  14:0    Midi Through                     Midi Through Port-0
 350  24:0    MIDI Gadget                      MIDI Gadget MIDI 1
 351
 352 $ aplaymidi -p24:0 to_gadget.mid
 353
 354 The from_gadget.mid should sound identical to the to_host.mid.
 355 The from_host.id should sound identical to the to_gadget.mid.
 356
 357 MIDI files can be played to speakers/headphones with e.g. timidity installed
 358
 359 $ aplaymidi -l
 360  Port    Client name                      Port name
 361  14:0    Midi Through                     Midi Through Port-0
 362  24:0    MIDI Gadget                      MIDI Gadget MIDI 1
 363 128:0    TiMidity                         TiMidity port 0
 364 128:1    TiMidity                         TiMidity port 1
 365 128:2    TiMidity                         TiMidity port 2
 366 128:3    TiMidity                         TiMidity port 3
 367
 368 $ aplaymidi -p 128:0 file.mid
 369
 370 MIDI ports can be logically connected using the aconnect utility, e.g.:
 371
 372 $ aconnect 24:0 128:0 # try it on the host
 373
 374 After the gadget's MIDI port is connected to timidity's MIDI port,
 375 whatever is played at the gadget side with aplaymidi -l is audible
 376 in host's speakers/headphones.
 377
 378 10. NCM function
 379 ================
 380
 381 The function is provided by usb_f_ncm.ko module.
 382
 383 Function-specific configfs interface
 384 ------------------------------------
 385
 386 The function name to use when creating the function directory is "ncm".
 387 The NCM function provides these attributes in its function directory:
 388
 389         ifname          - network device interface name associated with this
 390                         function instance
 391         qmult           - queue length multiplier for high and super speed
 392         host_addr       - MAC address of host's end of this
 393                         Ethernet over USB link
 394         dev_addr        - MAC address of device's end of this
 395                         Ethernet over USB link
 396
 397 and after creating the functions/ncm.<instance name> they contain default
 398 values: qmult is 5, dev_addr and host_addr are randomly selected.
 399 Except for ifname they can be written to until the function is linked to a
 400 configuration. The ifname is read-only and contains the name of the interface
 401 which was assigned by the net core, e. g. usb0.
 402
 403 Testing the NCM function
 404 ------------------------
 405
 406 Configure IP addresses of the device and the host. Then:
 407
 408 On the device: ping <host's IP>
 409 On the host: ping <device's IP>
 410
 411 11. OBEX function
 412 =================
 413
 414 The function is provided by usb_f_obex.ko module.
 415
 416 Function-specific configfs interface
 417 ------------------------------------
 418
 419 The function name to use when creating the function directory is "obex".
 420 The OBEX function provides just one attribute in its function directory:
 421
 422         port_num
 423
 424 The attribute is read-only.
 425
 426 There can be at most 4 ACM/generic serial/OBEX ports in the system.
 427
 428 Testing the OBEX function
 429 -------------------------
 430
 431 On device: seriald -f /dev/ttyGS<Y> -s 1024
 432 On host: serialc -v <vendorID> -p <productID> -i<interface#> -a1 -s1024 \
 433              -t<out endpoint addr> -r<in endpoint addr>
 434
 435 where seriald and serialc are Felipe's utilities found here:
 436
 437 https://github.com/felipebalbi/usb-tools.git master
 438
 439 12. PHONET function
 440 ===================
 441
 442 The function is provided by usb_f_phonet.ko module.
 443
 444 Function-specific configfs interface
 445 ------------------------------------
 446
 447 The function name to use when creating the function directory is "phonet".
 448 The PHONET function provides just one attribute in its function directory:
 449
 450         ifname          - network device interface name associated with this
 451                         function instance
 452
 453 Testing the PHONET function
 454 ---------------------------
 455
 456 It is not possible to test the SOCK_STREAM protocol without a specific piece
 457 of hardware, so only SOCK_DGRAM has been tested. For the latter to work,
 458 in the past I had to apply the patch mentioned here:
 459
 460 http://www.spinics.net/lists/linux-usb/msg85689.html
 461
 462 These tools are required:
 463
 464 git://git.gitorious.org/meego-cellular/phonet-utils.git
 465
 466 On the host:
 467
 468 $ ./phonet -a 0x10 -i usbpn0
 469 $ ./pnroute add 0x6c usbpn0
 470 $./pnroute add 0x10 usbpn0
 471 $ ifconfig usbpn0 up
 472
 473 On the device:
 474
 475 $ ./phonet -a 0x6c -i upnlink0
 476 $ ./pnroute add 0x10 upnlink0
 477 $ ifconfig upnlink0 up
 478
 479 Then a test program can be used:
 480
 481 http://www.spinics.net/lists/linux-usb/msg85690.html
 482
 483 On the device:
 484
 485 $ ./pnxmit -a 0x6c -r
 486
 487 On the host:
 488
 489 $ ./pnxmit -a 0x10 -s 0x6c
 490
 491 As a result some data should be sent from host to device.
 492 Then the other way round:
 493
 494 On the host:
 495
 496 $ ./pnxmit -a 0x10 -r
 497
 498 On the device:
 499
 500 $ ./pnxmit -a 0x6c -s 0x10
 501
 502 13. RNDIS function
 503 ==================
 504
 505 The function is provided by usb_f_rndis.ko module.
 506
 507 Function-specific configfs interface
 508 ------------------------------------
 509
 510 The function name to use when creating the function directory is "rndis".
 511 The RNDIS function provides these attributes in its function directory:
 512
 513         ifname          - network device interface name associated with this
 514                         function instance
 515         qmult           - queue length multiplier for high and super speed
 516         host_addr       - MAC address of host's end of this
 517                         Ethernet over USB link
 518         dev_addr        - MAC address of device's end of this
 519                         Ethernet over USB link
 520
 521 and after creating the functions/rndis.<instance name> they contain default
 522 values: qmult is 5, dev_addr and host_addr are randomly selected.
 523 Except for ifname they can be written to until the function is linked to a
 524 configuration. The ifname is read-only and contains the name of the interface
 525 which was assigned by the net core, e. g. usb0.
 526
 527 Testing the RNDIS function
 528 --------------------------
 529
 530 Configure IP addresses of the device and the host. Then:
 531
 532 On the device: ping <host's IP>
 533 On the host: ping <device's IP>
 534
 535 14. SERIAL function
 536 ===================
 537
 538 The function is provided by usb_f_gser.ko module.
 539
 540 Function-specific configfs interface
 541 ------------------------------------
 542
 543 The function name to use when creating the function directory is "gser".
 544 The SERIAL function provides just one attribute in its function directory:
 545
 546         port_num
 547
 548 The attribute is read-only.
 549
 550 There can be at most 4 ACM/generic serial/OBEX ports in the system.
 551
 552 Testing the SERIAL function
 553 ---------------------------
 554
 555 On host: insmod usbserial
 556          echo VID PID >/sys/bus/usb-serial/drivers/generic/new_id
 557 On host: cat > /dev/ttyUSB<X>
 558 On target: cat /dev/ttyGS<Y>
 559
 560 then the other way round
 561
 562 On target: cat > /dev/ttyGS<Y>
 563 On host: cat /dev/ttyUSB<X>
 564
 565 15. SOURCESINK function
 566 =======================
 567
 568 The function is provided by usb_f_ss_lb.ko module.
 569
 570 Function-specific configfs interface
 571 ------------------------------------
 572
 573 The function name to use when creating the function directory is "SourceSink".
 574 The SOURCESINK function provides these attributes in its function directory:
 575
 576         pattern         - 0 (all zeros), 1 (mod63), 2 (none)
 577         isoc_interval   - 1..16
 578         isoc_maxpacket  - 0 - 1023 (fs), 0 - 1024 (hs/ss)
 579         isoc_mult       - 0..2 (hs/ss only)
 580         isoc_maxburst   - 0..15 (ss only)
 581         bulk_buflen     - buffer length
 582         bulk_qlen       - depth of queue for bulk
 583         iso_qlen        - depth of queue for iso
 584
 585 Testing the SOURCESINK function
 586 -------------------------------
 587
 588 device: run the gadget
 589 host: test-usb (tools/usb/testusb.c)
 590
 591
 592 16. UAC1 function
 593 =================
 594
 595 The function is provided by usb_f_uac1.ko module.
 596
 597 Function-specific configfs interface
 598 ------------------------------------
 599
 600 The function name to use when creating the function directory is "uac1".
 601 The uac1 function provides these attributes in its function directory:
 602
 603         audio_buf_size - audio buffer size
 604         fn_cap - capture pcm device file name
 605         fn_cntl - control device file name
 606         fn_play - playback pcm device file name
 607         req_buf_size - ISO OUT endpoint request buffer size
 608         req_count - ISO OUT endpoint request count
 609
 610 The attributes have sane default values.
 611
 612 Testing the UAC1 function
 613 -------------------------
 614
 615 device: run the gadget
 616 host: aplay -l # should list our USB Audio Gadget
 617
 618 17. UAC2 function
 619 =================
 620
 621 The function is provided by usb_f_uac2.ko module.
 622
 623 Function-specific configfs interface
 624 ------------------------------------
 625
 626 The function name to use when creating the function directory is "uac2".
 627 The uac2 function provides these attributes in its function directory:
 628
 629         c_chmask - capture channel mask
 630         c_srate - capture sampling rate
 631         c_ssize - capture sample size (bytes)
 632         p_chmask - playback channel mask
 633         p_srate - playback sampling rate
 634         p_ssize - playback sample size (bytes)
 635
 636 The attributes have sane default values.
 637
 638 Testing the UAC2 function
 639 -------------------------
 640
 641 device: run the gadget
 642 host: aplay -l # should list our USB Audio Gadget
 643
 644 This function does not require real hardware support, it just
 645 sends a stream of audio data to/from the host. In order to
 646 actually hear something at the device side, a command similar
 647 to this must be used at the device side:
 648
 649 $ arecord -f dat -t wav -D hw:2,0 | aplay -D hw:0,0 &
 650
 651 e.g.:
 652
 653 $ arecord -f dat -t wav -D hw:CARD=UAC2Gadget,DEV=0 | \
 654 aplay -D default:CARD=OdroidU3
 655
 656 18. UVC function
 657 ================
 658
 659 The function is provided by usb_f_uvc.ko module.
 660
 661 Function-specific configfs interface
 662 ------------------------------------
 663
 664 The function name to use when creating the function directory is "uvc".
 665 The uvc function provides these attributes in its function directory:
 666
 667         streaming_interval - interval for polling endpoint for data transfers
 668         streaming_maxburst - bMaxBurst for super speed companion descriptor
 669         streaming_maxpacket - maximum packet size this endpoint is capable of
 670                               sending or receiving when this configuration is
 671                               selected
 672
 673 There are also "control" and "streaming" subdirectories, each of which contain
 674 a number of their subdirectories. There are some sane defaults provided, but
 675 the user must provide the following:
 676
 677         control header - create in control/header, link from control/class/fs
 678                         and/or control/class/ss
 679         streaming header - create in streaming/header, link from
 680                         streaming/class/fs and/or streaming/class/hs and/or
 681                         streaming/class/ss
 682         format description - create in streaming/mjpeg and/or
 683                         streaming/uncompressed
 684         frame description - create in streaming/mjpeg/<format> and/or in
 685                         streaming/uncompressed/<format>
 686
 687 Each frame description contains frame interval specification, and each
 688 such specification consists of a number of lines with an inverval value
 689 in each line. The rules stated above are best illustrated with an example:
 690
 691 # mkdir functions/uvc.usb0/control/header/h
 692 # cd functions/uvc.usb0/control/header/h
 693 # ln -s header/h class/fs
 694 # ln -s header/h class/ss
 695 # mkdir -p functions/uvc.usb0/streaming/uncompressed/u/360p
 696 # cat <<EOF > functions/uvc.usb0/streaming/uncompressed/u/360p/dwFrameInterval
 697 666666
 698 1000000
 699 5000000
 700 EOF
 701 # cd $GADGET_CONFIGFS_ROOT
 702 # mkdir functions/uvc.usb0/streaming/header/h
 703 # cd functions/uvc.usb0/streaming/header/h
 704 # ln -s ../../uncompressed/u
 705 # cd ../../class/fs
 706 # ln -s ../../header/h
 707 # cd ../../class/hs
 708 # ln -s ../../header/h
 709 # cd ../../class/ss
 710 # ln -s ../../header/h
 711
 712
 713 Testing the UVC function
 714 ------------------------
 715
 716 device: run the gadget, modprobe vivid
 717
 718 # uvc-gadget -u /dev/video<uvc video node #> -v /dev/video<vivid video node #>
 719
 720 where uvc-gadget is this program:
 721 http://git.ideasonboard.org/uvc-gadget.git
 722
 723 with these patches:
 724 http://www.spinics.net/lists/linux-usb/msg99220.html
 725
 726 host: luvcview -f yuv
 727
 728 19. PRINTER function
 729 ====================
 730
 731 The function is provided by usb_f_printer.ko module.
 732
 733 Function-specific configfs interface
 734 ------------------------------------
 735
 736 The function name to use when creating the function directory is "printer".
 737 The printer function provides these attributes in its function directory:
 738
 739         pnp_string      - Data to be passed to the host in pnp string
 740         q_len           - Number of requests per endpoint
 741
 742 Testing the PRINTER function
 743 ----------------------------
 744
 745 The most basic testing:
 746
 747 device: run the gadget
 748 # ls -l /devices/virtual/usb_printer_gadget/
 749
 750 should show g_printer<number>.
 751
 752 If udev is active, then /dev/g_printer<number> should appear automatically.
 753
 754 host:
 755
 756 If udev is active, then e.g. /dev/usb/lp0 should appear.
 757
 758 host->device transmission:
 759
 760 device:
 761 # cat /dev/g_printer<number>
 762 host:
 763 # cat > /dev/usb/lp0
 764
 765 device->host transmission:
 766
 767 # cat > /dev/g_printer<number>
 768 host:
 769 # cat /dev/usb/lp0
 770
 771 More advanced testing can be done with the prn_example
 772 described in Documentation/usb/gadget-printer.txt.