1<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V3.1//EN"[]> 2 3<book id="V4LGuide"> 4 <bookinfo> 5 <title>Video4Linux Programming</title> 6 7 <authorgroup> 8 <author> 9 <firstname>Alan</firstname> 10 <surname>Cox</surname> 11 <affiliation> 12 <address> 13 <email>alan@redhat.com</email> 14 </address> 15 </affiliation> 16 </author> 17 </authorgroup> 18 19 <copyright> 20 <year>2000</year> 21 <holder>Alan Cox</holder> 22 </copyright> 23 24 <legalnotice> 25 <para> 26 This documentation is free software; you can redistribute 27 it and/or modify it under the terms of the GNU General Public 28 License as published by the Free Software Foundation; either 29 version 2 of the License, or (at your option) any later 30 version. 31 </para> 32 33 <para> 34 This program is distributed in the hope that it will be 35 useful, but WITHOUT ANY WARRANTY; without even the implied 36 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 37 See the GNU General Public License for more details. 38 </para> 39 40 <para> 41 You should have received a copy of the GNU General Public 42 License along with this program; if not, write to the Free 43 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, 44 MA 02111-1307 USA 45 </para> 46 47 <para> 48 For more details see the file COPYING in the source 49 distribution of Linux. 50 </para> 51 </legalnotice> 52 </bookinfo> 53 54<toc></toc> 55 56 <chapter id="intro"> 57 <title>Introduction</title> 58 <para> 59 Parts of this document first appeared in Linux Magazine under a 60 ninety day exclusivity. 61 </para> 62 <para> 63 Video4Linux is intended to provide a common programming interface 64 for the many TV and capture cards now on the market, as well as 65 parallel port and USB video cameras. Radio, teletext decoders and 66 vertical blanking data interfaces are also provided. 67 </para> 68 </chapter> 69 <chapter id="radio"> 70 <title>Radio Devices</title> 71 <para> 72 There are a wide variety of radio interfaces available for PC's, and these 73 are generally very simple to program. The biggest problem with supporting 74 such devices is normally extracting documentation from the vendor. 75 </para> 76 <para> 77 The radio interface supports a simple set of control ioctls standardised 78 across all radio and tv interfaces. It does not support read or write, which 79 are used for video streams. The reason radio cards do not allow you to read 80 the audio stream into an application is that without exception they provide 81 a connection on to a soundcard. Soundcards can be used to read the radio 82 data just fine. 83 </para> 84 <sect1 id="registerradio"> 85 <title>Registering Radio Devices</title> 86 <para> 87 The Video4linux core provides an interface for registering devices. The 88 first step in writing our radio card driver is to register it. 89 </para> 90 <programlisting> 91 92 93static struct video_device my_radio 94{ 95 "My radio", 96 VID_TYPE_TUNER, 97 VID_HARDWARE_MYRADIO, 98 radio_open. 99 radio_close, 100 NULL, /* no read */ 101 NULL, /* no write */ 102 NULL, /* no poll */ 103 radio_ioctl, 104 NULL, /* no special init function */ 105 NULL /* no private data */ 106}; 107 108 109 </programlisting> 110 <para> 111 This declares our video4linux device driver interface. The VID_TYPE_ value 112 defines what kind of an interface we are, and defines basic capabilities. 113 </para> 114 <para> 115 The only defined value relevant for a radio card is VID_TYPE_TUNER which 116 indicates that the device can be tuned. Clearly our radio is going to have some 117 way to change channel so it is tuneable. 118 </para> 119 <para> 120 The VID_HARDWARE_ types are unique to each device. Numbers are assigned by 121 <email>alan@redhat.com</email> when device drivers are going to be released. Until then you 122 can pull a suitably large number out of your hat and use it. 10000 should be 123 safe for a very long time even allowing for the huge number of vendors 124 making new and different radio cards at the moment. 125 </para> 126 <para> 127 We declare an open and close routine, but we do not need read or write, 128 which are used to read and write video data to or from the card itself. As 129 we have no read or write there is no poll function. 130 </para> 131 <para> 132 The private initialise function is run when the device is registered. In 133 this driver we've already done all the work needed. The final pointer is a 134 private data pointer that can be used by the device driver to attach and 135 retrieve private data structures. We set this field "priv" to NULL for 136 the moment. 137 </para> 138 <para> 139 Having the structure defined is all very well but we now need to register it 140 with the kernel. 141 </para> 142 <programlisting> 143 144 145static int io = 0x320; 146 147int __init myradio_init(struct video_init *v) 148{ 149 if(!request_region(io, MY_IO_SIZE, "myradio")) 150 { 151 printk(KERN_ERR 152 "myradio: port 0x%03X is in use.\n", io); 153 return -EBUSY; 154 } 155 156 if(video_device_register(&my_radio, VFL_TYPE_RADIO)==-1) { 157 release_region(io, MY_IO_SIZE); 158 return -EINVAL; 159 } 160 return 0; 161} 162 163 </programlisting> 164 <para> 165 The first stage of the initialisation, as is normally the case, is to check 166 that the I/O space we are about to fiddle with doesn't belong to some other 167 driver. If it is we leave well alone. If the user gives the address of the 168 wrong device then we will spot this. These policies will generally avoid 169 crashing the machine. 170 </para> 171 <para> 172 Now we ask the Video4Linux layer to register the device for us. We hand it 173 our carefully designed video_device structure and also tell it which group 174 of devices we want it registered with. In this case VFL_TYPE_RADIO. 175 </para> 176 <para> 177 The types available are 178 </para> 179 <table frame=all><title>Device Types</title> 180 <tgroup cols=3 align=left> 181 <tbody> 182 <row> 183 <entry>VFL_TYPE_RADIO</><entry>/dev/radio{n}</><entry> 184 185 Radio devices are assigned in this block. As with all of these 186 selections the actual number assignment is done by the video layer 187 accordijng to what is free.</entry> 188 </row><row> 189 <entry>VFL_TYPE_GRABBER</><entry>/dev/video{n}</><entry> 190 Video capture devices and also -- counter-intuitively for the name -- 191 hardware video playback devices such as MPEG2 cards.</entry> 192 </row><row> 193 <entry>VFL_TYPE_VBI</><entry>/dev/vbi{n}</><entry> 194 The VBI devices capture the hidden lines on a television picture 195 that carry further information like closed caption data, teletext 196 (primarily in Europe) and now Intercast and the ATVEC internet 197 television encodings.</entry> 198 </row><row> 199 <entry>VFL_TYPE_VTX</><entry>/dev/vtx[n}</><entry> 200 VTX is 'Videotext' also known as 'Teletext'. This is a system for 201 sending numbered, 40x25, mostly textual page images over the hidden 202 lines. Unlike the /dev/vbi interfaces, this is for 'smart' decoder 203 chips. (The use of the word smart here has to be taken in context, 204 the smartest teletext chips are fairly dumb pieces of technology). 205 </entry> 206 </row> 207 </tbody> 208 </tgroup> 209 </table> 210 <para> 211 We are most definitely a radio. 212 </para> 213 <para> 214 Finally we allocate our I/O space so that nobody treads on us and return 0 215 to signify general happiness with the state of the universe. 216 </para> 217 </sect1> 218 <sect1 id="openradio"> 219 <title>Opening And Closing The Radio</title> 220 221 <para> 222 The functions we declared in our video_device are mostly very simple. 223 Firstly we can drop in what is basically standard code for open and close. 224 </para> 225 <programlisting> 226 227 228static int users = 0; 229 230static int radio_open(stuct video_device *dev, int flags) 231{ 232 if(users) 233 return -EBUSY; 234 users++; 235 MOD_INC_USE_COUNT; 236 return 0; 237} 238 239 </programlisting> 240 <para> 241 At open time we need to do nothing but check if someone else is also using 242 the radio card. If nobody is using it we make a note that we are using it, 243 then we ensure that nobody unloads our driver on us. 244 </para> 245 <programlisting> 246 247 248static int radio_close(struct video_device *dev) 249{ 250 users--; 251 MOD_DEC_USE_COUNT; 252} 253 254 </programlisting> 255 <para> 256 At close time we simply need to reduce the user count and allow the module 257 to become unloadable. 258 </para> 259 <para> 260 If you are sharp you will have noticed neither the open nor the close 261 routines attempt to reset or change the radio settings. This is intentional. 262 It allows an application to set up the radio and exit. It avoids a user 263 having to leave an application running all the time just to listen to the 264 radio. 265 </para> 266 </sect1> 267 <sect1 id="ioctlradio"> 268 <title>The Ioctl Interface</title> 269 <para> 270 This leaves the ioctl routine, without which the driver will not be 271 terribly useful to anyone. 272 </para> 273 <programlisting> 274 275 276static int radio_ioctl(struct video_device *dev, unsigned int cmd, void *arg) 277{ 278 switch(cmd) 279 { 280 case VIDIOCGCAP: 281 { 282 struct video_capability v; 283 v.type = VID_TYPE_TUNER; 284 v.channels = 1; 285 v.audios = 1; 286 v.maxwidth = 0; 287 v.minwidth = 0; 288 v.maxheight = 0; 289 v.minheight = 0; 290 strcpy(v.name, "My Radio"); 291 if(copy_to_user(arg, &v, sizeof(v))) 292 return -EFAULT; 293 return 0; 294 } 295 296 </programlisting> 297 <para> 298 VIDIOCGCAP is the first ioctl all video4linux devices must support. It 299 allows the applications to find out what sort of a card they have found and 300 to figure out what they want to do about it. The fields in the structure are 301 </para> 302 <table frame=all><title>struct video_capability fields</title> 303 <tgroup cols=2 align=left> 304 <tbody> 305 <row> 306 <entry>name</><entry>The device text name. This is intended for the user.</> 307 </row><row> 308 <entry>channels</><entry>The number of different channels you can tune on 309 this card. It could even by zero for a card that has 310 no tuning capability. For our simple FM radio it is 1. 311 An AM/FM radio would report 2.</entry> 312 </row><row> 313 <entry>audios</><entry>The number of audio inputs on this device. For our 314 radio there is only one audio input.</entry> 315 </row><row> 316 <entry>minwidth,minheight</><entry>The smallest size the card is capable of capturing 317 images in. We set these to zero. Radios do not 318 capture pictures</entry> 319 </row><row> 320 <entry>maxwidth,maxheight</><entry>The largest image size the card is capable of 321 capturing. For our radio we report 0. 322 </entry> 323 </row><row> 324 <entry>type</><entry>This reports the capabilities of the device, and 325 matches the field we filled in in the struct 326 video_device when registering.</entry> 327 </row> 328 </tbody> 329 </tgroup> 330 </table> 331 <para> 332 Having filled in the fields, we use copy_to_user to copy the structure into 333 the users buffer. If the copy fails we return an EFAULT to the application 334 so that it knows it tried to feed us garbage. 335 </para> 336 <para> 337 The next pair of ioctl operations select which tuner is to be used and let 338 the application find the tuner properties. We have only a single FM band 339 tuner in our example device. 340 </para> 341 <programlisting> 342 343 344 case VIDIOCGTUNER: 345 { 346 struct video_tuner v; 347 if(copy_from_user(&v, arg, sizeof(v))!=0) 348 return -EFAULT; 349 if(v.tuner) 350 return -EINVAL; 351 v.rangelow=(87*16000); 352 v.rangehigh=(108*16000); 353 v.flags = VIDEO_TUNER_LOW; 354 v.mode = VIDEO_MODE_AUTO; 355 v.signal = 0xFFFF; 356 strcpy(v.name, "FM"); 357 if(copy_to_user(&v, arg, sizeof(v))!=0) 358 return -EFAULT; 359 return 0; 360 } 361 362 </programlisting> 363 <para> 364 The VIDIOCGTUNER ioctl allows applications to query a tuner. The application 365 sets the tuner field to the tuner number it wishes to query. The query does 366 not change the tuner that is being used, it merely enquires about the tuner 367 in question. 368 </para> 369 <para> 370 We have exactly one tuner so after copying the user buffer to our temporary 371 structure we complain if they asked for a tuner other than tuner 0. 372 </para> 373 <para> 374 The video_tuner structure has the following fields 375 </para> 376 <table frame=all><title>struct video_tuner fields</title> 377 <tgroup cols=2 align=left> 378 <tbody> 379 <row> 380 <entry>int tuner</><entry>The number of the tuner in question</entry> 381 </row><row> 382 <entry>char name[32]</><entry>A text description of this tuner. "FM" will do fine. 383 This is intended for the application.</entry> 384 </row><row> 385 <entry>u32 flags</> 386 <entry>Tuner capability flags</entry> 387 </row> 388 <row> 389 <entry>u16 mode</><entry>The current reception mode</entry> 390 391 </row><row> 392 <entry>u16 signal</><entry>The signal strength scaled between 0 and 65535. If 393 a device cannot tell the signal strength it should 394 report 65535. Many simple cards contain only a 395 signal/no signal bit. Such cards will report either 396 0 or 65535.</entry> 397 398 </row><row> 399 <entry>u32 rangelow, rangehigh</><entry> 400 The range of frequencies supported by the radio 401 or TV. It is scaled according to the VIDEO_TUNER_LOW 402 flag.</entry> 403 404 </row> 405 </tbody> 406 </tgroup> 407 </table> 408 409 <table frame=all><title>struct video_tuner flags</title> 410 <tgroup cols=2 align=left> 411 <tbody> 412 <row> 413 <entry>VIDEO_TUNER_PAL</><entry>A PAL TV tuner</entry> 414 </row><row> 415 <entry>VIDEO_TUNER_NTSC</><entry>An NTSC (US) TV tuner</entry> 416 </row><row> 417 <entry>VIDEO_TUNER_SECAM</><entry>A SECAM (French) TV tuner</entry> 418 </row><row> 419 <entry>VIDEO_TUNER_LOW</><entry> 420 The tuner frequency is scaled in 1/16th of a KHz 421 steps. If not it is in 1/16th of a MHz steps 422 </entry> 423 </row><row> 424 <entry>VIDEO_TUNER_NORM</><entry>The tuner can set its format</entry> 425 </row><row> 426 <entry>VIDEO_TUNER_STEREO_ON</><entry>The tuner is currently receiving a stereo signal</entry> 427 </row> 428 </tbody> 429 </tgroup> 430 </table> 431 432 <table frame=all><title>struct video_tuner modes</title> 433 <tgroup cols=2 align=left> 434 <tbody> 435 <row> 436 <entry>VIDEO_MODE_PAL</><entry>PAL Format</entry> 437 </row><row> 438 <entry>VIDEO_MODE_NTSC</><entry>NTSC Format (USA)</entry> 439 </row><row> 440 <entry>VIDEO_MODE_SECAM</><entry>French Format</entry> 441 </row><row> 442 <entry>VIDEO_MODE_AUTO</><entry>A device that does not need to do 443 TV format switching</entry> 444 </row> 445 </tbody> 446 </tgroup> 447 </table> 448 <para> 449 The settings for the radio card are thus fairly simple. We report that we 450 are a tuner called "FM" for FM radio. In order to get the best tuning 451 resolution we report VIDEO_TUNER_LOW and select tuning to 1/16th of KHz. Its 452 unlikely our card can do that resolution but it is a fair bet the card can 453 do better than 1/16th of a MHz. VIDEO_TUNER_LOW is appropriate to almost all 454 radio usage. 455 </para> 456 <para> 457 We report that the tuner automatically handles deciding what format it is 458 receiving - true enough as it only handles FM radio. Our example card is 459 also incapable of detecting stereo or signal strengths so it reports a 460 strength of 0xFFFF (maximum) and no stereo detected. 461 </para> 462 <para> 463 To finish off we set the range that can be tuned to be 87-108Mhz, the normal 464 FM broadcast radio range. It is important to find out what the card is 465 actually capable of tuning. It is easy enough to simply use the FM broadcast 466 range. Unfortunately if you do this you will discover the FM broadcast 467 ranges in the USA, Europe and Japan are all subtly different and some users 468 cannot receive all the stations they wish. 469 </para> 470 <para> 471 The application also needs to be able to set the tuner it wishes to use. In 472 our case, with a single tuner this is rather simple to arrange. 473 </para> 474 <programlisting> 475 476 case VIDIOCSTUNER: 477 { 478 struct video_tuner v; 479 if(copy_from_user(&v, arg, sizeof(v))) 480 return -EFAULT; 481 if(v.tuner != 0) 482 return -EINVAL; 483 return 0; 484 } 485 486 </programlisting> 487 <para> 488 We copy the user supplied structure into kernel memory so we can examine it. 489 If the user has selected a tuner other than zero we reject the request. If 490 they wanted tuner 0 then, surprisingly enough, that is the current tuner already. 491 </para> 492 <para> 493 The next two ioctls we need to provide are to get and set the frequency of 494 the radio. These both use an unsigned long argument which is the frequency. 495 The scale of the frequency depends on the VIDEO_TUNER_LOW flag as I 496 mentioned earlier on. Since we have VIDEO_TUNER_LOW set this will be in 497 1/16ths of a KHz. 498 </para> 499 <programlisting> 500 501static unsigned long current_freq; 502 503 504 505 case VIDIOCGFREQ: 506 if(copy_to_user(arg, &current_freq, 507 sizeof(unsigned long)) 508 return -EFAULT; 509 return 0; 510 511 </programlisting> 512 <para> 513 Querying the frequency in our case is relatively simple. Our radio card is 514 too dumb to let us query the signal strength so we remember our setting if 515 we know it. All we have to do is copy it to the user. 516 </para> 517 <programlisting> 518 519 520 case VIDIOCSFREQ: 521 { 522 u32 freq; 523 if(copy_from_user(arg, &freq, 524 sizeof(unsigned long))!=0) 525 return -EFAULT; 526 if(hardware_set_freq(freq)<0) 527 return -EINVAL; 528 current_freq = freq; 529 return 0; 530 } 531 532 </programlisting> 533 <para> 534 Setting the frequency is a little more complex. We begin by copying the 535 desired frequency into kernel space. Next we call a hardware specific routine 536 to set the radio up. This might be as simple as some scaling and a few 537 writes to an I/O port. For most radio cards it turns out a good deal more 538 complicated and may involve programming things like a phase locked loop on 539 the card. This is what documentation is for. 540 </para> 541 <para> 542 The final set of operations we need to provide for our radio are the 543 volume controls. Not all radio cards can even do volume control. After all 544 there is a perfectly good volume control on the sound card. We will assume 545 our radio card has a simple 4 step volume control. 546 </para> 547 <para> 548 There are two ioctls with audio we need to support 549 </para> 550 <programlisting> 551 552static int current_volume=0; 553 554 case VIDIOCGAUDIO: 555 { 556 struct video_audio v; 557 if(copy_from_user(&v, arg, sizeof(v))) 558 return -EFAULT; 559 if(v.audio != 0) 560 return -EINVAL; 561 v.volume = 16384*current_volume; 562 v.step = 16384; 563 strcpy(v.name, "Radio"); 564 v.mode = VIDEO_SOUND_MONO; 565 v.balance = 0; 566 v.base = 0; 567 v.treble = 0; 568 569 if(copy_to_user(arg. &v, sizeof(v))) 570 return -EFAULT; 571 return 0; 572 } 573 574 </programlisting> 575 <para> 576 Much like the tuner we start by copying the user structure into kernel 577 space. Again we check if the user has asked for a valid audio input. We have 578 only input 0 and we punt if they ask for another input. 579 </para> 580 <para> 581 Then we fill in the video_audio structure. This has the following format 582 </para> 583 <table frame=all><title>struct video_audio fields</title> 584 <tgroup cols=2 align=left> 585 <tbody> 586 <row> 587 <entry>audio</><entry>The input the user wishes to query</> 588 </row><row> 589 <entry>volume</><entry>The volume setting on a scale of 0-65535</> 590 </row><row> 591 <entry>base</><entry>The base level on a scale of 0-65535</> 592 </row><row> 593 <entry>treble</><entry>The treble level on a scale of 0-65535</> 594 </row><row> 595 <entry>flags</><entry>The features this audio device supports 596 </entry> 597 </row><row> 598 <entry>name</><entry>A text name to display to the user. We picked 599 "Radio" as it explains things quite nicely.</> 600 </row><row> 601 <entry>mode</><entry>The current reception mode for the audio 602 603 We report MONO because our card is too stupid to know if it is in 604 mono or stereo. 605 </entry> 606 </row><row> 607 <entry>balance</><entry>The stereo balance on a scale of 0-65535, 32768 is 608 middle.</> 609 </row><row> 610 <entry>step</><entry>The step by which the volume control jumps. This is 611 used to help make it easy for applications to set 612 slider behaviour.</> 613 </row> 614 </tbody> 615 </tgroup> 616 </table> 617 618 <table frame=all><title>struct video_audio flags</title> 619 <tgroup cols=2 align=left> 620 <tbody> 621 <row> 622 <entry>VIDEO_AUDIO_MUTE</><entry>The audio is currently muted. We 623 could fake this in our driver but we 624 choose not to bother.</entry> 625 </row><row> 626 <entry>VIDEO_AUDIO_MUTABLE</><entry>The input has a mute option</entry> 627 </row><row> 628 <entry>VIDEO_AUDIO_TREBLE</><entry>The input has a treble control</entry> 629 </row><row> 630 <entry>VIDEO_AUDIO_BASS</><entry>The input has a base control</entry> 631 </row> 632 </tbody> 633 </tgroup> 634 </table> 635 636 <table frame=all><title>struct video_audio modes</title> 637 <tgroup cols=2 align=left> 638 <tbody> 639 <row> 640 <entry>VIDEO_SOUND_MONO</><entry>Mono sound</entry> 641 </row><row> 642 <entry>VIDEO_SOUND_STEREO</><entry>Stereo sound</entry> 643 </row><row> 644 <entry>VIDEO_SOUND_LANG1</><entry>Alternative language 1 (TV specific)</entry> 645 </row><row> 646 <entry>VIDEO_SOUND_LANG2</><entry>Alternative language 2 (TV specific)</entry> 647 </row> 648 </tbody> 649 </tgroup> 650 </table> 651 <para> 652 Having filled in the structure we copy it back to user space. 653 </para> 654 <para> 655 The VIDIOCSAUDIO ioctl allows the user to set the audio parameters in the 656 video_audio structure. The driver does its best to honour the request. 657 </para> 658 <programlisting> 659 660 case VIDIOCSAUDIO: 661 { 662 struct video_audio v; 663 if(copy_from_user(&v, arg, sizeof(v))) 664 return -EFAULT; 665 if(v.audio) 666 return -EINVAL; 667 current_volume = v/16384; 668 hardware_set_volume(current_volume); 669 return 0; 670 } 671 672 </programlisting> 673 <para> 674 In our case there is very little that the user can set. The volume is 675 basically the limit. Note that we could pretend to have a mute feature 676 by rewriting this to 677 </para> 678 <programlisting> 679 680 case VIDIOCSAUDIO: 681 { 682 struct video_audio v; 683 if(copy_from_user(&v, arg, sizeof(v))) 684 return -EFAULT; 685 if(v.audio) 686 return -EINVAL; 687 current_volume = v/16384; 688 if(v.flags&VIDEO_AUDIO_MUTE) 689 hardware_set_volume(0); 690 else 691 hardware_set_volume(current_volume); 692 current_muted = v.flags & 693 VIDEO_AUDIO_MUTE; 694 return 0; 695 } 696 697 </programlisting> 698 <para> 699 This with the corresponding changes to the VIDIOCGAUDIO code to report the 700 state of the mute flag we save and to report the card has a mute function, 701 will allow applications to use a mute facility with this card. It is 702 questionable whether this is a good idea however. User applications can already 703 fake this themselves and kernel space is precious. 704 </para> 705 <para> 706 We now have a working radio ioctl handler. So we just wrap up the function 707 </para> 708 <programlisting> 709 710 711 } 712 return -ENOIOCTLCMD; 713} 714 715 </programlisting> 716 <para> 717 and pass the Video4Linux layer back an error so that it knows we did not 718 understand the request we got passed. 719 </para> 720 </sect1> 721 <sect1 id="modradio"> 722 <title>Module Wrapper</title> 723 <para> 724 Finally we add in the usual module wrapping and the driver is done. 725 </para> 726 <programlisting> 727 728#ifndef MODULE 729 730static int io = 0x300; 731 732#else 733 734static int io = -1; 735 736 737MODULE_AUTHOR("Alan Cox"); 738MODULE_DESCRIPTION("A driver for an imaginary radio card."); 739MODULE_PARM(io, "i"); 740MODULE_PARM_DESC(io, "I/O address of the card."); 741 742EXPORT_NO_SYMBOLS; 743 744int init_module(void) 745{ 746 if(io==-1) 747 { 748 printk(KERN_ERR 749 "You must set an I/O address with io=0x???\n"); 750 return -EINVAL; 751 } 752 return myradio_init(NULL); 753} 754 755void cleanup_module(void) 756{ 757 video_unregister_device(&my_radio); 758 release_region(io, MY_IO_SIZE); 759} 760 761#endif 762 763 </programlisting> 764 <para> 765 In this example we set the IO base by default if the driver is compiled into 766 the kernel where you cannot pass a parameter. For the module we require the 767 user sets the parameter. We set io to a nonsense port (-1) so that we can 768 tell if the user supplied an io parameter or not. 769 </para> 770 <para> 771 We use MODULE_ defines to give an author for the card driver and a 772 description. We also use them to declare that io is an integer and it is the 773 address of the card. 774 </para> 775 <para> 776 The clean-up routine unregisters the video_device we registered, and frees 777 up the I/O space. Note that the unregister takes the actual video_device 778 structure as its argument. Unlike the file operations structure which can be 779 shared by all instances of a device a video_device structure as an actual 780 instance of the device. If you are registering multiple radio devices you 781 need to fill in one structure per device (most likely by setting up a 782 template and copying it to each of the actual device structures). 783 </para> 784 </sect1> 785 </chapter> 786 <chapter> 787 <title>Video Capture Devices</title> 788 <sect1 id="introvid"> 789 <title>Video Capture Device Types</title> 790 <para> 791 The video capture devices share the same interfaces as radio devices. In 792 order to explain the video capture interface I will use the example of a 793 camera that has no tuners or audio input. This keeps the example relatively 794 clean. To get both combine the two driver examples. 795 </para> 796 <para> 797 Video capture devices divide into four categories. A little technology 798 backgrounder. Full motion video even at television resolution (which is 799 actually fairly low) is pretty resource-intensive. You are continually 800 passing megabytes of data every second from the capture card to the display. 801 several alternative approaches have emerged because copying this through the 802 processor and the user program is a particularly bad idea . 803 </para> 804 <para> 805 The first is to add the television image onto the video output directly. 806 This is also how some 3D cards work. These basic cards can generally drop the 807 video into any chosen rectangle of the display. Cards like this, which 808 include most mpeg1 cards that used the feature connector, aren't very 809 friendly in a windowing environment. They don't understand windows or 810 clipping. The video window is always on the top of the display. 811 </para> 812 <para> 813 Chroma keying is a technique used by cards to get around this. It is an old 814 television mixing trick where you mark all the areas you wish to replace 815 with a single clear colour that isn't used in the image - TV people use an 816 incredibly bright blue while computing people often use a particularly 817 virulent purple. Bright blue occurs on the desktop. Anyone with virulent 818 purple windows has another problem besides their TV overlay. 819 </para> 820 <para> 821 The third approach is to copy the data from the capture card to the video 822 card, but to do it directly across the PCI bus. This relieves the processor 823 from doing the work but does require some smartness on the part of the video 824 capture chip, as well as a suitable video card. Programming this kind of 825 card and more so debugging it can be extremely tricky. There are some quite 826 complicated interactions with the display and you may also have to cope with 827 various chipset bugs that show up when PCI cards start talking to each 828 other. 829 </para> 830 <para> 831 To keep our example fairly simple we will assume a card that supports 832 overlaying a flat rectangular image onto the frame buffer output, and which 833 can also capture stuff into processor memory. 834 </para> 835 </sect1> 836 <sect1 id="regvid"> 837 <title>Registering Video Capture Devices</title> 838 <para> 839 This time we need to add more functions for our camera device. 840 </para> 841 <programlisting> 842static struct video_device my_camera 843{ 844 "My Camera", 845 VID_TYPE_OVERLAY|VID_TYPE_SCALES|\ 846 VID_TYPE_CAPTURE|VID_TYPE_CHROMAKEY, 847 VID_HARDWARE_MYCAMERA, 848 camera_open. 849 camera_close, 850 camera_read, /* no read */ 851 NULL, /* no write */ 852 camera_poll, /* no poll */ 853 camera_ioctl, 854 NULL, /* no special init function */ 855 NULL /* no private data */ 856}; 857 </programlisting> 858 <para> 859 We need a read() function which is used for capturing data from 860 the card, and we need a poll function so that a driver can wait for the next 861 frame to be captured. 862 </para> 863 <para> 864 We use the extra video capability flags that did not apply to the 865 radio interface. The video related flags are 866 </para> 867 <table frame=all><title>Capture Capabilities</title> 868 <tgroup cols=2 align=left> 869 <tbody> 870 <row> 871<entry>VID_TYPE_CAPTURE</><entry>We support image capture</> 872</row><row> 873<entry>VID_TYPE_TELETEXT</><entry>A teletext capture device (vbi{n])</> 874</row><row> 875<entry>VID_TYPE_OVERLAY</><entry>The image can be directly overlaid onto the 876 frame buffer</> 877</row><row> 878<entry>VID_TYPE_CHROMAKEY</><entry>Chromakey can be used to select which parts 879 of the image to display</> 880</row><row> 881<entry>VID_TYPE_CLIPPING</><entry>It is possible to give the board a list of 882 rectangles to draw around. </> 883</row><row> 884<entry>VID_TYPE_FRAMERAM</><entry>The video capture goes into the video memory 885 and actually changes it. Applications need 886 to know this so they can clean up after the 887 card</> 888</row><row> 889<entry>VID_TYPE_SCALES</><entry>The image can be scaled to various sizes, 890 rather than being a single fixed size.</> 891</row><row> 892<entry>VID_TYPE_MONOCHROME</><entry>The capture will be monochrome. This isn't a 893 complete answer to the question since a mono 894 camera on a colour capture card will still 895 produce mono output.</> 896</row><row> 897<entry>VID_TYPE_SUBCAPTURE</><entry>The card allows only part of its field of 898 view to be captured. This enables 899 applications to avoid copying all of a large 900 image into memory when only some section is 901 relevant.</> 902 </row> 903 </tbody> 904 </tgroup> 905 </table> 906 <para> 907 We set VID_TYPE_CAPTURE so that we are seen as a capture card, 908 VID_TYPE_CHROMAKEY so the application knows it is time to draw in virulent 909 purple, and VID_TYPE_SCALES because we can be resized. 910 </para> 911 <para> 912 Our setup is fairly similar. This time we also want an interrupt line 913 for the 'frame captured' signal. Not all cards have this so some of them 914 cannot handle poll(). 915 </para> 916 <programlisting> 917 918 919static int io = 0x320; 920static int irq = 11; 921 922int __init mycamera_init(struct video_init *v) 923{ 924 if(!request_region(io, MY_IO_SIZE, "mycamera")) 925 { 926 printk(KERN_ERR 927 "mycamera: port 0x%03X is in use.\n", io); 928 return -EBUSY; 929 } 930 931 if(video_device_register(&my_camera, 932 VFL_TYPE_GRABBER)==-1) { 933 release_region(io, MY_IO_SIZE); 934 return -EINVAL; 935 } 936 return 0; 937} 938 939 </programlisting> 940 <para> 941 This is little changed from the needs of the radio card. We specify 942 VFL_TYPE_GRABBER this time as we want to be allocated a /dev/video name. 943 </para> 944 </sect1> 945 <sect1 id="opvid"> 946 <title>Opening And Closing The Capture Device</title> 947 <programlisting> 948 949 950static int users = 0; 951 952static int camera_open(stuct video_device *dev, int flags) 953{ 954 if(users) 955 return -EBUSY; 956 if(request_irq(irq, camera_irq, 0, "camera", dev)<0) 957 return -EBUSY; 958 users++; 959 MOD_INC_USE_COUNT; 960 return 0; 961} 962 963 964static int camera_close(struct video_device *dev) 965{ 966 users--; 967 free_irq(irq, dev); 968 MOD_DEC_USE_COUNT; 969} 970 </programlisting> 971 <para> 972 The open and close routines are also quite similar. The only real change is 973 that we now request an interrupt for the camera device interrupt line. If we 974 cannot get the interrupt we report EBUSY to the application and give up. 975 </para> 976 </sect1> 977 <sect1 id="irqvid"> 978 <title>Interrupt Handling</title> 979 <para> 980 Our example handler is for an ISA bus device. If it was PCI you would be 981 able to share the interrupt and would have set SA_SHIRQ to indicate a 982 shared IRQ. We pass the device pointer as the interrupt routine argument. We 983 don't need to since we only support one card but doing this will make it 984 easier to upgrade the driver for multiple devices in the future. 985 </para> 986 <para> 987 Our interrupt routine needs to do little if we assume the card can simply 988 queue one frame to be read after it captures it. 989 </para> 990 <programlisting> 991 992 993static struct wait_queue *capture_wait; 994static int capture_ready = 0; 995 996static void camera_irq(int irq, void *dev_id, 997 struct pt_regs *regs) 998{ 999 capture_ready=1; 1000 wake_up_interruptible(&capture_wait); 1001} 1002 </programlisting> 1003 <para> 1004 The interrupt handler is nice and simple for this card as we are assuming 1005 the card is buffering the frame for us. This means we have little to do but 1006 wake up anybody interested. We also set a capture_ready flag, as we may 1007 capture a frame before an application needs it. In this case we need to know 1008 that a frame is ready. If we had to collect the frame on the interrupt life 1009 would be more complex. 1010 </para> 1011 <para> 1012 The two new routines we need to supply are camera_read which returns a 1013 frame, and camera_poll which waits for a frame to become ready. 1014 </para> 1015 <programlisting> 1016 1017 1018static int camera_poll(struct video_device *dev, 1019 struct file *file, struct poll_table *wait) 1020{ 1021 poll_wait(file, &capture_wait, wait); 1022 if(capture_read) 1023 return POLLIN|POLLRDNORM; 1024 return 0; 1025} 1026 1027 </programlisting> 1028 <para> 1029 Our wait queue for polling is the capture_wait queue. This will cause the 1030 task to be woken up by our camera_irq routine. We check capture_read to see 1031 if there is an image present and if so report that it is readable. 1032 </para> 1033 </sect1> 1034 <sect1 id="rdvid"> 1035 <title>Reading The Video Image</title> 1036 <programlisting> 1037 1038 1039static long camera_read(struct video_device *dev, char *buf, 1040 unsigned long count) 1041{ 1042 struct wait_queue wait = { current, NULL }; 1043 u8 *ptr; 1044 int len; 1045 int i; 1046 1047 add_wait_queue(&capture_wait, &wait); 1048 1049 while(!capture_ready) 1050 { 1051 if(file->flags&O_NDELAY) 1052 { 1053 remove_wait_queue(&capture_wait, &wait); 1054 current->state = TASK_RUNNING; 1055 return -EWOULDBLOCK; 1056 } 1057 if(signal_pending(current)) 1058 { 1059 remove_wait_queue(&capture_wait, &wait); 1060 current->state = TASK_RUNNING; 1061 return -ERESTARTSYS; 1062 } 1063 schedule(); 1064 current->state = TASK_INTERRUPTIBLE; 1065 } 1066 remove_wait_queue(&capture_wait, &wait); 1067 current->state = TASK_RUNNING; 1068 1069 </programlisting> 1070 <para> 1071 The first thing we have to do is to ensure that the application waits until 1072 the next frame is ready. The code here is almost identical to the mouse code 1073 we used earlier in this chapter. It is one of the common building blocks of 1074 Linux device driver code and probably one which you will find occurs in any 1075 drivers you write. 1076 </para> 1077 <para> 1078 We wait for a frame to be ready, or for a signal to interrupt our waiting. If a 1079 signal occurs we need to return from the system call so that the signal can 1080 be sent to the application itself. We also check to see if the user actually 1081 wanted to avoid waiting - ie if they are using non-blocking I/O and have other things 1082 to get on with. 1083 </para> 1084 <para> 1085 Next we copy the data from the card to the user application. This is rarely 1086 as easy as our example makes out. We will add capture_w, and capture_h here 1087 to hold the width and height of the captured image. We assume the card only 1088 supports 24bit RGB for now. 1089 </para> 1090 <programlisting> 1091 1092 1093 1094 capture_ready = 0; 1095 1096 ptr=(u8 *)buf; 1097 len = capture_w * 3 * capture_h; /* 24bit RGB */ 1098 1099 if(len>count) 1100 len=count; /* Doesn't all fit */ 1101 1102 for(i=0; i<len; i++) 1103 { 1104 put_user(inb(io+IMAGE_DATA), ptr); 1105 ptr++; 1106 } 1107 1108 hardware_restart_capture(); 1109 1110 return i; 1111} 1112 1113 </programlisting> 1114 <para> 1115 For a real hardware device you would try to avoid the loop with put_user(). 1116 Each call to put_user() has a time overhead checking whether the accesses to user 1117 space are allowed. It would be better to read a line into a temporary buffer 1118 then copy this to user space in one go. 1119 </para> 1120 <para> 1121 Having captured the image and put it into user space we can kick the card to 1122 get the next frame acquired. 1123 </para> 1124 </sect1> 1125 <sect1 id="iocvid"> 1126 <title>Video Ioctl Handling</title> 1127 <para> 1128 As with the radio driver the major control interface is via the ioctl() 1129 function. Video capture devices support the same tuner calls as a radio 1130 device and also support additional calls to control how the video functions 1131 are handled. In this simple example the card has no tuners to avoid making 1132 the code complex. 1133 </para> 1134 <programlisting> 1135 1136 1137 1138static int camera_ioctl(struct video_device *dev, unsigned int cmd, void *arg) 1139{ 1140 switch(cmd) 1141 { 1142 case VIDIOCGCAP: 1143 { 1144 struct video_capability v; 1145 v.type = VID_TYPE_CAPTURE|\ 1146 VID_TYPE_CHROMAKEY|\ 1147 VID_TYPE_SCALES|\ 1148 VID_TYPE_OVERLAY; 1149 v.channels = 1; 1150 v.audios = 0; 1151 v.maxwidth = 640; 1152 v.minwidth = 16; 1153 v.maxheight = 480; 1154 v.minheight = 16; 1155 strcpy(v.name, "My Camera"); 1156 if(copy_to_user(arg, &v, sizeof(v))) 1157 return -EFAULT; 1158 return 0; 1159 } 1160 1161 1162 </programlisting> 1163 <para> 1164 The first ioctl we must support and which all video capture and radio 1165 devices are required to support is VIDIOCGCAP. This behaves exactly the same 1166 as with a radio device. This time, however, we report the extra capabilities 1167 we outlined earlier on when defining our video_dev structure. 1168 </para> 1169 <para> 1170 We now set the video flags saying that we support overlay, capture, 1171 scaling and chromakey. We also report size limits - our smallest image is 1172 16x16 pixels, our largest is 640x480. 1173 </para> 1174 <para> 1175 To keep things simple we report no audio and no tuning capabilities at all. 1176 </para> 1177 <programlisting> 1178 1179 case VIDIOCGCHAN: 1180 { 1181 struct video_channel v; 1182 if(copy_from_user(&v, arg, sizeof(v))) 1183 return -EFAULT; 1184 if(v.channel != 0) 1185 return -EINVAL; 1186 v.flags = 0; 1187 v.tuners = 0; 1188 v.type = VIDEO_TYPE_CAMERA; 1189 v.norm = VIDEO_MODE_AUTO; 1190 strcpy(v.name, "Camera Input");break; 1191 if(copy_to_user(&v, arg, sizeof(v))) 1192 return -EFAULT; 1193 return 0; 1194 } 1195 1196 1197 </programlisting> 1198 <para> 1199 This follows what is very much the standard way an ioctl handler looks 1200 in Linux. We copy the data into a kernel space variable and we check that the 1201 request is valid (in this case that the input is 0). Finally we copy the 1202 camera info back to the user. 1203 </para> 1204 <para> 1205 The VIDIOCGCHAN ioctl allows a user to ask about video channels (that is 1206 inputs to the video card). Our example card has a single camera input. The 1207 fields in the structure are 1208 </para> 1209 <table frame=all><title>struct video_channel fields</title> 1210 <tgroup cols=2 align=left> 1211 <tbody> 1212 <row> 1213 1214 <entry>channel</><entry>The channel number we are selecting</entry> 1215 </row><row> 1216 <entry>name</><entry>The name for this channel. This is intended 1217 to describe the port to the user. 1218 Appropriate names are therefore things like 1219 "Camera" "SCART input"</entry> 1220 </row><row> 1221 <entry>flags</><entry>Channel properties</entry> 1222 </row><row> 1223 <entry>type</><entry>Input type</entry> 1224 </row><row> 1225 <entry>norm</><entry>The current television encoding being used 1226 if relevant for this channel. 1227 </entry> 1228 </row> 1229 </tbody> 1230 </tgroup> 1231 </table> 1232 <table frame=all><title>struct video_channel flags</title> 1233 <tgroup cols=2 align=left> 1234 <tbody> 1235 <row> 1236 <entry>VIDEO_VC_TUNER</><entry>Channel has a tuner.</entry> 1237 </row><row> 1238 <entry>VIDEO_VC_AUDIO</><entry>Channel has audio.</entry> 1239 </row> 1240 </tbody> 1241 </tgroup> 1242 </table> 1243 <table frame=all><title>struct video_channel types</title> 1244 <tgroup cols=2 align=left> 1245 <tbody> 1246 <row> 1247 <entry>VIDEO_TYPE_TV</><entry>Television input.</entry> 1248 </row><row> 1249 <entry>VIDEO_TYPE_CAMERA</><entry>Fixed camera input.</entry> 1250 </row><row> 1251 <entry>0</><entry>Type is unknown.</entry> 1252 </row> 1253 </tbody> 1254 </tgroup> 1255 </table> 1256 <table frame=all><title>struct video_channel norms</title> 1257 <tgroup cols=2 align=left> 1258 <tbody> 1259 <row> 1260 <entry>VIDEO_MODE_PAL</><entry>PAL encoded Television</entry> 1261 </row><row> 1262 <entry>VIDEO_MODE_NTSC</><entry>NTSC (US) encoded Television</entry> 1263 </row><row> 1264 <entry>VIDEO_MODE_SECAM</><entry>SECAM (French) Television </entry> 1265 </row><row> 1266 <entry>VIDEO_MODE_AUTO</><entry>Automatic switching, or format does not 1267 matter</entry> 1268 </row> 1269 </tbody> 1270 </tgroup> 1271 </table> 1272 <para> 1273 The corresponding VIDIOCSCHAN ioctl allows a user to change channel and to 1274 request the norm is changed - for example to switch between a PAL or an NTSC 1275 format camera. 1276 </para> 1277 <programlisting> 1278 1279 1280 case VIDIOCSCHAN: 1281 { 1282 struct video_channel v; 1283 if(copy_from_user(&v, arg, sizeof(v))) 1284 return -EFAULT; 1285 if(v.channel != 0) 1286 return -EINVAL; 1287 if(v.norm != VIDEO_MODE_AUTO) 1288 return -EINVAL; 1289 return 0; 1290 } 1291 1292 1293 </programlisting> 1294 <para> 1295 The implementation of this call in our driver is remarkably easy. Because we 1296 are assuming fixed format hardware we need only check that the user has not 1297 tried to change anything. 1298 </para> 1299 <para> 1300 The user also needs to be able to configure and adjust the picture they are 1301 seeing. This is much like adjusting a television set. A user application 1302 also needs to know the palette being used so that it knows how to display 1303 the image that has been captured. The VIDIOCGPICT and VIDIOCSPICT ioctl 1304 calls provide this information. 1305 </para> 1306 <programlisting> 1307 1308 1309 case VIDIOCGPICT 1310 { 1311 struct video_picture v; 1312 v.brightness = hardware_brightness(); 1313 v.hue = hardware_hue(); 1314 v.colour = hardware_saturation(); 1315 v.contrast = hardware_brightness(); 1316 /* Not settable */ 1317 v.whiteness = 32768; 1318 v.depth = 24; /* 24bit */ 1319 v.palette = VIDEO_PALETTE_RGB24; 1320 if(copy_to_user(&v, arg, 1321 sizeof(v))) 1322 return -EFAULT; 1323 return 0; 1324 } 1325 1326 1327 </programlisting> 1328 <para> 1329 The brightness, hue, color, and contrast provide the picture controls that 1330 are akin to a conventional television. Whiteness provides additional 1331 control for greyscale images. All of these values are scaled between 0-65535 1332 and have 32768 as the mid point setting. The scaling means that applications 1333 do not have to worry about the capability range of the hardware but can let 1334 it make a best effort attempt. 1335 </para> 1336 <para> 1337 Our depth is 24, as this is in bits. We will be returning RGB24 format. This 1338 has one byte of red, then one of green, then one of blue. This then repeats 1339 for every other pixel in the image. The other common formats the interface 1340 defines are 1341 </para> 1342 <table frame=all><title>Framebuffer Encodings</title> 1343 <tgroup cols=2 align=left> 1344 <tbody> 1345 <row> 1346 <entry>GREY</><entry>Linear greyscale. This is for simple cameras and the 1347 like</> 1348 </row><row> 1349 <entry>RGB565</><entry>The top 5 bits hold 32 red levels, the next six bits 1350 hold green and the low 5 bits hold blue. </> 1351 </row><row> 1352 <entry>RGB555</><entry>The top bit is clear. The red green and blue levels 1353 each occupy five bits.</> 1354 </row> 1355 </tbody> 1356 </tgroup> 1357 </table> 1358 <para> 1359 Additional modes are support for YUV capture formats. These are common for 1360 TV and video conferencing applications. 1361 </para> 1362 <para> 1363 The VIDIOCSPICT ioctl allows a user to set some of the picture parameters. 1364 Exactly which ones are supported depends heavily on the card itself. It is 1365 possible to support many modes and effects in software. In general doing 1366 this in the kernel is a bad idea. Video capture is a performance-sensitive 1367 application and the programs can often do better if they aren't being 1368 'helped' by an overkeen driver writer. Thus for our device we will report 1369 RGB24 only and refuse to allow a change. 1370 </para> 1371 <programlisting> 1372 1373 1374 case VIDIOCSPICT: 1375 { 1376 struct video_picture v; 1377 if(copy_from_user(&v, arg, sizeof(v))) 1378 return -EFAULT; 1379 if(v.depth!=24 || 1380 v.palette != VIDEO_PALETTE_RGB24) 1381 return -EINVAL; 1382 set_hardware_brightness(v.brightness); 1383 set_hardware_hue(v.hue); 1384 set_hardware_saturation(v.colour); 1385 set_hardware_brightness(v.contrast); 1386 return 0; 1387 } 1388 1389 1390 </programlisting> 1391 <para> 1392 We check the user has not tried to change the palette or the depth. We do 1393 not want to carry out some of the changes and then return an error. This may 1394 confuse the application which will be assuming no change occurred. 1395 </para> 1396 <para> 1397 In much the same way as you need to be able to set the picture controls to 1398 get the right capture images, many cards need to know what they are 1399 displaying onto when generating overlay output. In some cases getting this 1400 wrong even makes a nasty mess or may crash the computer. For that reason 1401 the VIDIOCSBUF ioctl used to set up the frame buffer information may well 1402 only be usable by root. 1403 </para> 1404 <para> 1405 We will assume our card is one of the old ISA devices with feature connector 1406 and only supports a couple of standard video modes. Very common for older 1407 cards although the PCI devices are way smarter than this. 1408 </para> 1409 <programlisting> 1410 1411 1412static struct video_buffer capture_fb; 1413 1414 case VIDIOCGFBUF: 1415 { 1416 if(copy_to_user(arg, &capture_fb, 1417 sizeof(capture_fb))) 1418 return -EFAULT; 1419 return 0; 1420 1421 } 1422 1423 1424 </programlisting> 1425 <para> 1426 We keep the frame buffer information in the format the ioctl uses. This 1427 makes it nice and easy to work with in the ioctl calls. 1428 </para> 1429 <programlisting> 1430 1431 case VIDIOCSFBUF: 1432 { 1433 struct video_buffer v; 1434 1435 if(!capable(CAP_SYS_ADMIN)) 1436 return -EPERM; 1437 1438 if(copy_from_user(&v, arg, sizeof(v))) 1439 return -EFAULT; 1440 if(v.width!=320 && v.width!=640) 1441 return -EINVAL; 1442 if(v.height!=200 && v.height!=240 1443 && v.height!=400 1444 && v.height !=480) 1445 return -EINVAL; 1446 memcpy(&capture_fb, &v, sizeof(v)); 1447 hardware_set_fb(&v); 1448 return 0; 1449 } 1450 1451 1452 1453 </programlisting> 1454 <para> 1455 The capable() function checks a user has the required capability. The Linux 1456 operating system has a set of about 30 capabilities indicating privileged 1457 access to services. The default set up gives the superuser (uid 0) all of 1458 them and nobody else has any. 1459 </para> 1460 <para> 1461 We check that the user has the SYS_ADMIN capability, that is they are 1462 allowed to operate as the machine administrator. We don't want anyone but 1463 the administrator making a mess of the display. 1464 </para> 1465 <para> 1466 Next we check for standard PC video modes (320 or 640 wide with either 1467 EGA or VGA depths). If the mode is not a standard video mode we reject it as 1468 not supported by our card. If the mode is acceptable we save it so that 1469 VIDIOCFBUF will give the right answer next time it is called. The 1470 hardware_set_fb() function is some undescribed card specific function to 1471 program the card for the desired mode. 1472 </para> 1473 <para> 1474 Before the driver can display an overlay window it needs to know where the 1475 window should be placed, and also how large it should be. If the card 1476 supports clipping it needs to know which rectangles to omit from the 1477 display. The video_window structure is used to describe the way the image 1478 should be displayed. 1479 </para> 1480 <table frame=all><title>struct video_window fields</title> 1481 <tgroup cols=2 align=left> 1482 <tbody> 1483 <row> 1484 <entry>width</><entry>The width in pixels of the desired image. The card 1485 may use a smaller size if this size is not available</> 1486 </row><row> 1487 <entry>height</><entry>The height of the image. The card may use a smaller 1488 size if this size is not available.</> 1489 </row><row> 1490 <entry>x</><entry> The X position of the top left of the window. This 1491 is in pixels relative to the left hand edge of the 1492 picture. Not all cards can display images aligned on 1493 any pixel boundary. If the position is unsuitable 1494 the card adjusts the image right and reduces the 1495 width.</> 1496 </row><row> 1497 <entry>y</><entry> The Y position of the top left of the window. This 1498 is counted in pixels relative to the top edge of the 1499 picture. As with the width if the card cannot 1500 display starting on this line it will adjust the 1501 values.</> 1502 </row><row> 1503 <entry>chromakey</><entry>The colour (expressed in RGB32 format) for the 1504 chromakey colour if chroma keying is being used. </> 1505 </row><row> 1506 <entry>clips</><entry>An array of rectangles that must not be drawn 1507 over.</> 1508 </row><row> 1509 <entry>clipcount</><entry>The number of clips in this array.</> 1510 </row> 1511 </tbody> 1512 </tgroup> 1513 </table> 1514 <para> 1515 Each clip is a struct video_clip which has the following fields 1516 </para> 1517 <table frame=all><title>video_clip fields</title> 1518 <tgroup cols=2 align=left> 1519 <tbody> 1520 <row> 1521 <entry>x, y</><entry>Co-ordinates relative to the display</> 1522 </row><row> 1523 <entry>width, height</><entry>Width and height in pixels</> 1524 </row><row> 1525 <entry>next</><entry>A spare field for the application to use</> 1526 </row> 1527 </tbody> 1528 </tgroup> 1529 </table> 1530 <para> 1531 The driver is required to ensure it always draws in the area requested or a smaller area, and that it never draws in any of the areas that are clipped. 1532 This may well mean it has to leave alone. small areas the application wished to be 1533 drawn. 1534 </para> 1535 <para> 1536 Our example card uses chromakey so does not have to address most of the 1537 clipping. We will add a video_window structure to our global variables to 1538 remember our parameters, as we did with the frame buffer. 1539 </para> 1540 <programlisting> 1541 1542 1543 case VIDIOCGWIN: 1544 { 1545 if(copy_to_user(arg, &capture_win, 1546 sizeof(capture_win))) 1547 return -EFAULT; 1548 return 0; 1549 } 1550 1551 1552 case VIDIOCSWIN: 1553 { 1554 struct video_window v; 1555 if(copy_from_user(&v, arg, sizeof(v))) 1556 return -EFAULT; 1557 if(v.width > 640 || v.height > 480) 1558 return -EINVAL; 1559 if(v.width < 16 || v.height < 16) 1560 return -EINVAL; 1561 hardware_set_key(v.chromakey); 1562 hardware_set_window(v); 1563 memcpy(&capture_win, &v, sizeof(v)); 1564 capture_w = v.width; 1565 capture_h = v.height; 1566 return 0; 1567 } 1568 1569 1570 </programlisting> 1571 <para> 1572 Because we are using Chromakey our setup is fairly simple. Mostly we have to 1573 check the values are sane and load them into the capture card. 1574 </para> 1575 <para> 1576 With all the setup done we can now turn on the actual capture/overlay. This 1577 is done with the VIDIOCCAPTURE ioctl. This takes a single integer argument 1578 where 0 is on and 1 is off. 1579 </para> 1580 <programlisting> 1581 1582 1583 case VIDIOCCAPTURE: 1584 { 1585 int v; 1586 if(get_user(v, (int *)arg)) 1587 return -EFAULT; 1588 if(v==0) 1589 hardware_capture_off(); 1590 else 1591 { 1592 if(capture_fb.width == 0 1593 || capture_w == 0) 1594 return -EINVAL; 1595 hardware_capture_on(); 1596 } 1597 return 0; 1598 } 1599 1600 1601 </programlisting> 1602 <para> 1603 We grab the flag from user space and either enable or disable according to 1604 its value. There is one small corner case we have to consider here. Suppose 1605 that the capture was requested before the video window or the frame buffer 1606 had been set up. In those cases there will be unconfigured fields in our 1607 card data, as well as unconfigured hardware settings. We check for this case and 1608 return an error if the frame buffer or the capture window width is zero. 1609 </para> 1610 <programlisting> 1611 1612 1613 default: 1614 return -ENOIOCTLCMD; 1615 } 1616} 1617 </programlisting> 1618 <para> 1619 1620 We don't need to support any other ioctls, so if we get this far, it is time 1621 to tell the video layer that we don't now what the user is talking about. 1622 </para> 1623 </sect1> 1624 <sect1 id="endvid"> 1625 <title>Other Functionality</title> 1626 <para> 1627 The Video4Linux layer supports additional features, including a high 1628 performance mmap() based capture mode and capturing part of the image. 1629 These features are out of the scope of the book. You should however have enough 1630 example code to implement most simple video4linux devices for radio and TV 1631 cards. 1632 </para> 1633 </sect1> 1634 </chapter> 1635 <chapter id="bugs"> 1636 <title>Known Bugs And Assumptions</title> 1637 <para> 1638 <variablelist> 1639 <varlistentry><term>Multiple Opens</term> 1640 <listitem> 1641 <para> 1642 The driver assumes multiple opens should not be allowed. A driver 1643 can work around this but not cleanly. 1644 </para> 1645 </listitem></varlistentry> 1646 1647 <varlistentry><term>API Deficiencies</term> 1648 <listitem> 1649 <para> 1650 The existing API poorly reflects compression capable devices. There 1651 are plans afoot to merge V4L, V4L2 and some other ideas into a 1652 better interface. 1653 </para> 1654 </listitem></varlistentry> 1655 </variablelist> 1656 1657 </para> 1658 </chapter> 1659 1660 <chapter id="pubfunctions"> 1661 <title>Public Functions Provided</title> 1662!Edrivers/media/video/videodev.c 1663 </chapter> 1664 1665</book> 1666