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="V4LGuide"> 6 <bookinfo> 7 <title>Video4Linux Programming</title> 8 9 <authorgroup> 10 <author> 11 <firstname>Alan</firstname> 12 <surname>Cox</surname> 13 <affiliation> 14 <address> 15 <email>alan@redhat.com</email> 16 </address> 17 </affiliation> 18 </author> 19 </authorgroup> 20 21 <copyright> 22 <year>2000</year> 23 <holder>Alan Cox</holder> 24 </copyright> 25 26 <legalnotice> 27 <para> 28 This documentation is free software; you can redistribute 29 it and/or modify it under the terms of the GNU General Public 30 License as published by the Free Software Foundation; either 31 version 2 of the License, or (at your option) any later 32 version. 33 </para> 34 35 <para> 36 This program is distributed in the hope that it will be 37 useful, but WITHOUT ANY WARRANTY; without even the implied 38 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 39 See the GNU General Public License for more details. 40 </para> 41 42 <para> 43 You should have received a copy of the GNU General Public 44 License along with this program; if not, write to the Free 45 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, 46 MA 02111-1307 USA 47 </para> 48 49 <para> 50 For more details see the file COPYING in the source 51 distribution of Linux. 52 </para> 53 </legalnotice> 54 </bookinfo> 55 56<toc></toc> 57 58 <chapter id="intro"> 59 <title>Introduction</title> 60 <para> 61 Parts of this document first appeared in Linux Magazine under a 62 ninety day exclusivity. 63 </para> 64 <para> 65 Video4Linux is intended to provide a common programming interface 66 for the many TV and capture cards now on the market, as well as 67 parallel port and USB video cameras. Radio, teletext decoders and 68 vertical blanking data interfaces are also provided. 69 </para> 70 </chapter> 71 <chapter id="radio"> 72 <title>Radio Devices</title> 73 <para> 74 There are a wide variety of radio interfaces available for PC's, and these 75 are generally very simple to program. The biggest problem with supporting 76 such devices is normally extracting documentation from the vendor. 77 </para> 78 <para> 79 The radio interface supports a simple set of control ioctls standardised 80 across all radio and tv interfaces. It does not support read or write, which 81 are used for video streams. The reason radio cards do not allow you to read 82 the audio stream into an application is that without exception they provide 83 a connection on to a soundcard. Soundcards can be used to read the radio 84 data just fine. 85 </para> 86 <sect1 id="registerradio"> 87 <title>Registering Radio Devices</title> 88 <para> 89 The Video4linux core provides an interface for registering devices. The 90 first step in writing our radio card driver is to register it. 91 </para> 92 <programlisting> 93 94 95static struct video_device my_radio 96{ 97 "My radio", 98 VID_TYPE_TUNER, 99 VID_HARDWARE_MYRADIO, 100 radio_open. 101 radio_close, 102 NULL, /* no read */ 103 NULL, /* no write */ 104 NULL, /* no poll */ 105 radio_ioctl, 106 NULL, /* no special init function */ 107 NULL /* no private data */ 108}; 109 110 111 </programlisting> 112 <para> 113 This declares our video4linux device driver interface. The VID_TYPE_ value 114 defines what kind of an interface we are, and defines basic capabilities. 115 </para> 116 <para> 117 The only defined value relevant for a radio card is VID_TYPE_TUNER which 118 indicates that the device can be tuned. Clearly our radio is going to have some 119 way to change channel so it is tuneable. 120 </para> 121 <para> 122 The VID_HARDWARE_ types are unique to each device. Numbers are assigned by 123 <email>alan@redhat.com</email> when device drivers are going to be released. Until then you 124 can pull a suitably large number out of your hat and use it. 10000 should be 125 safe for a very long time even allowing for the huge number of vendors 126 making new and different radio cards at the moment. 127 </para> 128 <para> 129 We declare an open and close routine, but we do not need read or write, 130 which are used to read and write video data to or from the card itself. As 131 we have no read or write there is no poll function. 132 </para> 133 <para> 134 The private initialise function is run when the device is registered. In 135 this driver we've already done all the work needed. The final pointer is a 136 private data pointer that can be used by the device driver to attach and 137 retrieve private data structures. We set this field "priv" to NULL for 138 the moment. 139 </para> 140 <para> 141 Having the structure defined is all very well but we now need to register it 142 with the kernel. 143 </para> 144 <programlisting> 145 146 147static int io = 0x320; 148 149int __init myradio_init(struct video_init *v) 150{ 151 if(!request_region(io, MY_IO_SIZE, "myradio")) 152 { 153 printk(KERN_ERR 154 "myradio: port 0x%03X is in use.\n", io); 155 return -EBUSY; 156 } 157 158 if(video_device_register(&my_radio, VFL_TYPE_RADIO)==-1) { 159 release_region(io, MY_IO_SIZE); 160 return -EINVAL; 161 } 162 return 0; 163} 164 165 </programlisting> 166 <para> 167 The first stage of the initialisation, as is normally the case, is to check 168 that the I/O space we are about to fiddle with doesn't belong to some other 169 driver. If it is we leave well alone. If the user gives the address of the 170 wrong device then we will spot this. These policies will generally avoid 171 crashing the machine. 172 </para> 173 <para> 174 Now we ask the Video4Linux layer to register the device for us. We hand it 175 our carefully designed video_device structure and also tell it which group 176 of devices we want it registered with. In this case VFL_TYPE_RADIO. 177 </para> 178 <para> 179 The types available are 180 </para> 181 <table frame="all"><title>Device Types</title> 182 <tgroup cols="3" align="left"> 183 <tbody> 184 <row> 185 <entry>VFL_TYPE_RADIO</entry><entry>/dev/radio{n}</entry><entry> 186 187 Radio devices are assigned in this block. As with all of these 188 selections the actual number assignment is done by the video layer 189 accordijng to what is free.</entry> 190 </row><row> 191 <entry>VFL_TYPE_GRABBER</entry><entry>/dev/video{n}</entry><entry> 192 Video capture devices and also -- counter-intuitively for the name -- 193 hardware video playback devices such as MPEG2 cards.</entry> 194 </row><row> 195 <entry>VFL_TYPE_VBI</entry><entry>/dev/vbi{n}</entry><entry> 196 The VBI devices capture the hidden lines on a television picture 197 that carry further information like closed caption data, teletext 198 (primarily in Europe) and now Intercast and the ATVEC internet 199 television encodings.</entry> 200 </row><row> 201 <entry>VFL_TYPE_VTX</entry><entry>/dev/vtx[n}</entry><entry> 202 VTX is 'Videotext' also known as 'Teletext'. This is a system for 203 sending numbered, 40x25, mostly textual page images over the hidden 204 lines. Unlike the /dev/vbi interfaces, this is for 'smart' decoder 205 chips. (The use of the word smart here has to be taken in context, 206 the smartest teletext chips are fairly dumb pieces of technology). 207 </entry> 208 </row> 209 </tbody> 210 </tgroup> 211 </table> 212 <para> 213 We are most definitely a radio. 214 </para> 215 <para> 216 Finally we allocate our I/O space so that nobody treads on us and return 0 217 to signify general happiness with the state of the universe. 218 </para> 219 </sect1> 220 <sect1 id="openradio"> 221 <title>Opening And Closing The Radio</title> 222 223 <para> 224 The functions we declared in our video_device are mostly very simple. 225 Firstly we can drop in what is basically standard code for open and close. 226 </para> 227 <programlisting> 228 229 230static int users = 0; 231 232static int radio_open(struct video_device *dev, int flags) 233{ 234 if(users) 235 return -EBUSY; 236 users++; 237 return 0; 238} 239 240 </programlisting> 241 <para> 242 At open time we need to do nothing but check if someone else is also using 243 the radio card. If nobody is using it we make a note that we are using it, 244 then we ensure that nobody unloads our driver on us. 245 </para> 246 <programlisting> 247 248 249static int radio_close(struct video_device *dev) 250{ 251 users--; 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><entry>The device text name. This is intended for the user.</entry> 307 </row><row> 308 <entry>channels</entry><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><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><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><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><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><entry>The number of the tuner in question</entry> 381 </row><row> 382 <entry>char name[32]</entry><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</entry> 386 <entry>Tuner capability flags</entry> 387 </row> 388 <row> 389 <entry>u16 mode</entry><entry>The current reception mode</entry> 390 391 </row><row> 392 <entry>u16 signal</entry><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><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><entry>A PAL TV tuner</entry> 414 </row><row> 415 <entry>VIDEO_TUNER_NTSC</entry><entry>An NTSC (US) TV tuner</entry> 416 </row><row> 417 <entry>VIDEO_TUNER_SECAM</entry><entry>A SECAM (French) TV tuner</entry> 418 </row><row> 419 <entry>VIDEO_TUNER_LOW</entry><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><entry>The tuner can set its format</entry> 425 </row><row> 426 <entry>VIDEO_TUNER_STEREO_ON</entry><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><entry>PAL Format</entry> 437 </row><row> 438 <entry>VIDEO_MODE_NTSC</entry><entry>NTSC Format (USA)</entry> 439 </row><row> 440 <entry>VIDEO_MODE_SECAM</entry><entry>French Format</entry> 441 </row><row> 442 <entry>VIDEO_MODE_AUTO</entry><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><entry>The input the user wishes to query</entry> 588 </row><row> 589 <entry>volume</entry><entry>The volume setting on a scale of 0-65535</entry> 590 </row><row> 591 <entry>base</entry><entry>The base level on a scale of 0-65535</entry> 592 </row><row> 593 <entry>treble</entry><entry>The treble level on a scale of 0-65535</entry> 594 </row><row> 595 <entry>flags</entry><entry>The features this audio device supports 596 </entry> 597 </row><row> 598 <entry>name</entry><entry>A text name to display to the user. We picked 599 "Radio" as it explains things quite nicely.</entry> 600 </row><row> 601 <entry>mode</entry><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><entry>The stereo balance on a scale of 0-65535, 32768 is 608 middle.</entry> 609 </row><row> 610 <entry>step</entry><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.</entry> 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><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><entry>The input has a mute option</entry> 627 </row><row> 628 <entry>VIDEO_AUDIO_TREBLE</entry><entry>The input has a treble control</entry> 629 </row><row> 630 <entry>VIDEO_AUDIO_BASS</entry><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><entry>Mono sound</entry> 641 </row><row> 642 <entry>VIDEO_SOUND_STEREO</entry><entry>Stereo sound</entry> 643 </row><row> 644 <entry>VIDEO_SOUND_LANG1</entry><entry>Alternative language 1 (TV specific)</entry> 645 </row><row> 646 <entry>VIDEO_SOUND_LANG2</entry><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#endif 737 738MODULE_AUTHOR("Alan Cox"); 739MODULE_DESCRIPTION("A driver for an imaginary radio card."); 740module_param(io, int, 0444); 741MODULE_PARM_DESC(io, "I/O address of the card."); 742 743static int __init init(void) 744{ 745 if(io==-1) 746 { 747 printk(KERN_ERR 748 "You must set an I/O address with io=0x???\n"); 749 return -EINVAL; 750 } 751 return myradio_init(NULL); 752} 753 754static void __exit cleanup(void) 755{ 756 video_unregister_device(&my_radio); 757 release_region(io, MY_IO_SIZE); 758} 759 760module_init(init); 761module_exit(cleanup); 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: you can still set it using "my_radio.irq" if this file is called <filename>my_radio.c</filename>. 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, and can be read by anyone from sysfs. 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><entry>We support image capture</entry> 872</row><row> 873<entry>VID_TYPE_TELETEXT</entry><entry>A teletext capture device (vbi{n])</entry> 874</row><row> 875<entry>VID_TYPE_OVERLAY</entry><entry>The image can be directly overlaid onto the 876 frame buffer</entry> 877</row><row> 878<entry>VID_TYPE_CHROMAKEY</entry><entry>Chromakey can be used to select which parts 879 of the image to display</entry> 880</row><row> 881<entry>VID_TYPE_CLIPPING</entry><entry>It is possible to give the board a list of 882 rectangles to draw around. </entry> 883</row><row> 884<entry>VID_TYPE_FRAMERAM</entry><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</entry> 888</row><row> 889<entry>VID_TYPE_SCALES</entry><entry>The image can be scaled to various sizes, 890 rather than being a single fixed size.</entry> 891</row><row> 892<entry>VID_TYPE_MONOCHROME</entry><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.</entry> 896</row><row> 897<entry>VID_TYPE_SUBCAPTURE</entry><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.</entry> 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(struct 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 return 0; 960} 961 962 963static int camera_close(struct video_device *dev) 964{ 965 users--; 966 free_irq(irq, dev); 967} 968 </programlisting> 969 <para> 970 The open and close routines are also quite similar. The only real change is 971 that we now request an interrupt for the camera device interrupt line. If we 972 cannot get the interrupt we report EBUSY to the application and give up. 973 </para> 974 </sect1> 975 <sect1 id="irqvid"> 976 <title>Interrupt Handling</title> 977 <para> 978 Our example handler is for an ISA bus device. If it was PCI you would be 979 able to share the interrupt and would have set IRQF_SHARED to indicate a 980 shared IRQ. We pass the device pointer as the interrupt routine argument. We 981 don't need to since we only support one card but doing this will make it 982 easier to upgrade the driver for multiple devices in the future. 983 </para> 984 <para> 985 Our interrupt routine needs to do little if we assume the card can simply 986 queue one frame to be read after it captures it. 987 </para> 988 <programlisting> 989 990 991static struct wait_queue *capture_wait; 992static int capture_ready = 0; 993 994static void camera_irq(int irq, void *dev_id, 995 struct pt_regs *regs) 996{ 997 capture_ready=1; 998 wake_up_interruptible(&capture_wait); 999} 1000 </programlisting> 1001 <para> 1002 The interrupt handler is nice and simple for this card as we are assuming 1003 the card is buffering the frame for us. This means we have little to do but 1004 wake up anybody interested. We also set a capture_ready flag, as we may 1005 capture a frame before an application needs it. In this case we need to know 1006 that a frame is ready. If we had to collect the frame on the interrupt life 1007 would be more complex. 1008 </para> 1009 <para> 1010 The two new routines we need to supply are camera_read which returns a 1011 frame, and camera_poll which waits for a frame to become ready. 1012 </para> 1013 <programlisting> 1014 1015 1016static int camera_poll(struct video_device *dev, 1017 struct file *file, struct poll_table *wait) 1018{ 1019 poll_wait(file, &capture_wait, wait); 1020 if(capture_read) 1021 return POLLIN|POLLRDNORM; 1022 return 0; 1023} 1024 1025 </programlisting> 1026 <para> 1027 Our wait queue for polling is the capture_wait queue. This will cause the 1028 task to be woken up by our camera_irq routine. We check capture_read to see 1029 if there is an image present and if so report that it is readable. 1030 </para> 1031 </sect1> 1032 <sect1 id="rdvid"> 1033 <title>Reading The Video Image</title> 1034 <programlisting> 1035 1036 1037static long camera_read(struct video_device *dev, char *buf, 1038 unsigned long count) 1039{ 1040 struct wait_queue wait = { current, NULL }; 1041 u8 *ptr; 1042 int len; 1043 int i; 1044 1045 add_wait_queue(&capture_wait, &wait); 1046 1047 while(!capture_ready) 1048 { 1049 if(file->flags&O_NDELAY) 1050 { 1051 remove_wait_queue(&capture_wait, &wait); 1052 current->state = TASK_RUNNING; 1053 return -EWOULDBLOCK; 1054 } 1055 if(signal_pending(current)) 1056 { 1057 remove_wait_queue(&capture_wait, &wait); 1058 current->state = TASK_RUNNING; 1059 return -ERESTARTSYS; 1060 } 1061 schedule(); 1062 current->state = TASK_INTERRUPTIBLE; 1063 } 1064 remove_wait_queue(&capture_wait, &wait); 1065 current->state = TASK_RUNNING; 1066 1067 </programlisting> 1068 <para> 1069 The first thing we have to do is to ensure that the application waits until 1070 the next frame is ready. The code here is almost identical to the mouse code 1071 we used earlier in this chapter. It is one of the common building blocks of 1072 Linux device driver code and probably one which you will find occurs in any 1073 drivers you write. 1074 </para> 1075 <para> 1076 We wait for a frame to be ready, or for a signal to interrupt our waiting. If a 1077 signal occurs we need to return from the system call so that the signal can 1078 be sent to the application itself. We also check to see if the user actually 1079 wanted to avoid waiting - ie if they are using non-blocking I/O and have other things 1080 to get on with. 1081 </para> 1082 <para> 1083 Next we copy the data from the card to the user application. This is rarely 1084 as easy as our example makes out. We will add capture_w, and capture_h here 1085 to hold the width and height of the captured image. We assume the card only 1086 supports 24bit RGB for now. 1087 </para> 1088 <programlisting> 1089 1090 1091 1092 capture_ready = 0; 1093 1094 ptr=(u8 *)buf; 1095 len = capture_w * 3 * capture_h; /* 24bit RGB */ 1096 1097 if(len>count) 1098 len=count; /* Doesn't all fit */ 1099 1100 for(i=0; i<len; i++) 1101 { 1102 put_user(inb(io+IMAGE_DATA), ptr); 1103 ptr++; 1104 } 1105 1106 hardware_restart_capture(); 1107 1108 return i; 1109} 1110 1111 </programlisting> 1112 <para> 1113 For a real hardware device you would try to avoid the loop with put_user(). 1114 Each call to put_user() has a time overhead checking whether the accesses to user 1115 space are allowed. It would be better to read a line into a temporary buffer 1116 then copy this to user space in one go. 1117 </para> 1118 <para> 1119 Having captured the image and put it into user space we can kick the card to 1120 get the next frame acquired. 1121 </para> 1122 </sect1> 1123 <sect1 id="iocvid"> 1124 <title>Video Ioctl Handling</title> 1125 <para> 1126 As with the radio driver the major control interface is via the ioctl() 1127 function. Video capture devices support the same tuner calls as a radio 1128 device and also support additional calls to control how the video functions 1129 are handled. In this simple example the card has no tuners to avoid making 1130 the code complex. 1131 </para> 1132 <programlisting> 1133 1134 1135 1136static int camera_ioctl(struct video_device *dev, unsigned int cmd, void *arg) 1137{ 1138 switch(cmd) 1139 { 1140 case VIDIOCGCAP: 1141 { 1142 struct video_capability v; 1143 v.type = VID_TYPE_CAPTURE|\ 1144 VID_TYPE_CHROMAKEY|\ 1145 VID_TYPE_SCALES|\ 1146 VID_TYPE_OVERLAY; 1147 v.channels = 1; 1148 v.audios = 0; 1149 v.maxwidth = 640; 1150 v.minwidth = 16; 1151 v.maxheight = 480; 1152 v.minheight = 16; 1153 strcpy(v.name, "My Camera"); 1154 if(copy_to_user(arg, &v, sizeof(v))) 1155 return -EFAULT; 1156 return 0; 1157 } 1158 1159 1160 </programlisting> 1161 <para> 1162 The first ioctl we must support and which all video capture and radio 1163 devices are required to support is VIDIOCGCAP. This behaves exactly the same 1164 as with a radio device. This time, however, we report the extra capabilities 1165 we outlined earlier on when defining our video_dev structure. 1166 </para> 1167 <para> 1168 We now set the video flags saying that we support overlay, capture, 1169 scaling and chromakey. We also report size limits - our smallest image is 1170 16x16 pixels, our largest is 640x480. 1171 </para> 1172 <para> 1173 To keep things simple we report no audio and no tuning capabilities at all. 1174 </para> 1175 <programlisting> 1176 1177 case VIDIOCGCHAN: 1178 { 1179 struct video_channel v; 1180 if(copy_from_user(&v, arg, sizeof(v))) 1181 return -EFAULT; 1182 if(v.channel != 0) 1183 return -EINVAL; 1184 v.flags = 0; 1185 v.tuners = 0; 1186 v.type = VIDEO_TYPE_CAMERA; 1187 v.norm = VIDEO_MODE_AUTO; 1188 strcpy(v.name, "Camera Input");break; 1189 if(copy_to_user(&v, arg, sizeof(v))) 1190 return -EFAULT; 1191 return 0; 1192 } 1193 1194 1195 </programlisting> 1196 <para> 1197 This follows what is very much the standard way an ioctl handler looks 1198 in Linux. We copy the data into a kernel space variable and we check that the 1199 request is valid (in this case that the input is 0). Finally we copy the 1200 camera info back to the user. 1201 </para> 1202 <para> 1203 The VIDIOCGCHAN ioctl allows a user to ask about video channels (that is 1204 inputs to the video card). Our example card has a single camera input. The 1205 fields in the structure are 1206 </para> 1207 <table frame="all"><title>struct video_channel fields</title> 1208 <tgroup cols="2" align="left"> 1209 <tbody> 1210 <row> 1211 1212 <entry>channel</entry><entry>The channel number we are selecting</entry> 1213 </row><row> 1214 <entry>name</entry><entry>The name for this channel. This is intended 1215 to describe the port to the user. 1216 Appropriate names are therefore things like 1217 "Camera" "SCART input"</entry> 1218 </row><row> 1219 <entry>flags</entry><entry>Channel properties</entry> 1220 </row><row> 1221 <entry>type</entry><entry>Input type</entry> 1222 </row><row> 1223 <entry>norm</entry><entry>The current television encoding being used 1224 if relevant for this channel. 1225 </entry> 1226 </row> 1227 </tbody> 1228 </tgroup> 1229 </table> 1230 <table frame="all"><title>struct video_channel flags</title> 1231 <tgroup cols="2" align="left"> 1232 <tbody> 1233 <row> 1234 <entry>VIDEO_VC_TUNER</entry><entry>Channel has a tuner.</entry> 1235 </row><row> 1236 <entry>VIDEO_VC_AUDIO</entry><entry>Channel has audio.</entry> 1237 </row> 1238 </tbody> 1239 </tgroup> 1240 </table> 1241 <table frame="all"><title>struct video_channel types</title> 1242 <tgroup cols="2" align="left"> 1243 <tbody> 1244 <row> 1245 <entry>VIDEO_TYPE_TV</entry><entry>Television input.</entry> 1246 </row><row> 1247 <entry>VIDEO_TYPE_CAMERA</entry><entry>Fixed camera input.</entry> 1248 </row><row> 1249 <entry>0</entry><entry>Type is unknown.</entry> 1250 </row> 1251 </tbody> 1252 </tgroup> 1253 </table> 1254 <table frame="all"><title>struct video_channel norms</title> 1255 <tgroup cols="2" align="left"> 1256 <tbody> 1257 <row> 1258 <entry>VIDEO_MODE_PAL</entry><entry>PAL encoded Television</entry> 1259 </row><row> 1260 <entry>VIDEO_MODE_NTSC</entry><entry>NTSC (US) encoded Television</entry> 1261 </row><row> 1262 <entry>VIDEO_MODE_SECAM</entry><entry>SECAM (French) Television </entry> 1263 </row><row> 1264 <entry>VIDEO_MODE_AUTO</entry><entry>Automatic switching, or format does not 1265 matter</entry> 1266 </row> 1267 </tbody> 1268 </tgroup> 1269 </table> 1270 <para> 1271 The corresponding VIDIOCSCHAN ioctl allows a user to change channel and to 1272 request the norm is changed - for example to switch between a PAL or an NTSC 1273 format camera. 1274 </para> 1275 <programlisting> 1276 1277 1278 case VIDIOCSCHAN: 1279 { 1280 struct video_channel v; 1281 if(copy_from_user(&v, arg, sizeof(v))) 1282 return -EFAULT; 1283 if(v.channel != 0) 1284 return -EINVAL; 1285 if(v.norm != VIDEO_MODE_AUTO) 1286 return -EINVAL; 1287 return 0; 1288 } 1289 1290 1291 </programlisting> 1292 <para> 1293 The implementation of this call in our driver is remarkably easy. Because we 1294 are assuming fixed format hardware we need only check that the user has not 1295 tried to change anything. 1296 </para> 1297 <para> 1298 The user also needs to be able to configure and adjust the picture they are 1299 seeing. This is much like adjusting a television set. A user application 1300 also needs to know the palette being used so that it knows how to display 1301 the image that has been captured. The VIDIOCGPICT and VIDIOCSPICT ioctl 1302 calls provide this information. 1303 </para> 1304 <programlisting> 1305 1306 1307 case VIDIOCGPICT 1308 { 1309 struct video_picture v; 1310 v.brightness = hardware_brightness(); 1311 v.hue = hardware_hue(); 1312 v.colour = hardware_saturation(); 1313 v.contrast = hardware_brightness(); 1314 /* Not settable */ 1315 v.whiteness = 32768; 1316 v.depth = 24; /* 24bit */ 1317 v.palette = VIDEO_PALETTE_RGB24; 1318 if(copy_to_user(&v, arg, 1319 sizeof(v))) 1320 return -EFAULT; 1321 return 0; 1322 } 1323 1324 1325 </programlisting> 1326 <para> 1327 The brightness, hue, color, and contrast provide the picture controls that 1328 are akin to a conventional television. Whiteness provides additional 1329 control for greyscale images. All of these values are scaled between 0-65535 1330 and have 32768 as the mid point setting. The scaling means that applications 1331 do not have to worry about the capability range of the hardware but can let 1332 it make a best effort attempt. 1333 </para> 1334 <para> 1335 Our depth is 24, as this is in bits. We will be returning RGB24 format. This 1336 has one byte of red, then one of green, then one of blue. This then repeats 1337 for every other pixel in the image. The other common formats the interface 1338 defines are 1339 </para> 1340 <table frame="all"><title>Framebuffer Encodings</title> 1341 <tgroup cols="2" align="left"> 1342 <tbody> 1343 <row> 1344 <entry>GREY</entry><entry>Linear greyscale. This is for simple cameras and the 1345 like</entry> 1346 </row><row> 1347 <entry>RGB565</entry><entry>The top 5 bits hold 32 red levels, the next six bits 1348 hold green and the low 5 bits hold blue. </entry> 1349 </row><row> 1350 <entry>RGB555</entry><entry>The top bit is clear. The red green and blue levels 1351 each occupy five bits.</entry> 1352 </row> 1353 </tbody> 1354 </tgroup> 1355 </table> 1356 <para> 1357 Additional modes are support for YUV capture formats. These are common for 1358 TV and video conferencing applications. 1359 </para> 1360 <para> 1361 The VIDIOCSPICT ioctl allows a user to set some of the picture parameters. 1362 Exactly which ones are supported depends heavily on the card itself. It is 1363 possible to support many modes and effects in software. In general doing 1364 this in the kernel is a bad idea. Video capture is a performance-sensitive 1365 application and the programs can often do better if they aren't being 1366 'helped' by an overkeen driver writer. Thus for our device we will report 1367 RGB24 only and refuse to allow a change. 1368 </para> 1369 <programlisting> 1370 1371 1372 case VIDIOCSPICT: 1373 { 1374 struct video_picture v; 1375 if(copy_from_user(&v, arg, sizeof(v))) 1376 return -EFAULT; 1377 if(v.depth!=24 || 1378 v.palette != VIDEO_PALETTE_RGB24) 1379 return -EINVAL; 1380 set_hardware_brightness(v.brightness); 1381 set_hardware_hue(v.hue); 1382 set_hardware_saturation(v.colour); 1383 set_hardware_brightness(v.contrast); 1384 return 0; 1385 } 1386 1387 1388 </programlisting> 1389 <para> 1390 We check the user has not tried to change the palette or the depth. We do 1391 not want to carry out some of the changes and then return an error. This may 1392 confuse the application which will be assuming no change occurred. 1393 </para> 1394 <para> 1395 In much the same way as you need to be able to set the picture controls to 1396 get the right capture images, many cards need to know what they are 1397 displaying onto when generating overlay output. In some cases getting this 1398 wrong even makes a nasty mess or may crash the computer. For that reason 1399 the VIDIOCSBUF ioctl used to set up the frame buffer information may well 1400 only be usable by root. 1401 </para> 1402 <para> 1403 We will assume our card is one of the old ISA devices with feature connector 1404 and only supports a couple of standard video modes. Very common for older 1405 cards although the PCI devices are way smarter than this. 1406 </para> 1407 <programlisting> 1408 1409 1410static struct video_buffer capture_fb; 1411 1412 case VIDIOCGFBUF: 1413 { 1414 if(copy_to_user(arg, &capture_fb, 1415 sizeof(capture_fb))) 1416 return -EFAULT; 1417 return 0; 1418 1419 } 1420 1421 1422 </programlisting> 1423 <para> 1424 We keep the frame buffer information in the format the ioctl uses. This 1425 makes it nice and easy to work with in the ioctl calls. 1426 </para> 1427 <programlisting> 1428 1429 case VIDIOCSFBUF: 1430 { 1431 struct video_buffer v; 1432 1433 if(!capable(CAP_SYS_ADMIN)) 1434 return -EPERM; 1435 1436 if(copy_from_user(&v, arg, sizeof(v))) 1437 return -EFAULT; 1438 if(v.width!=320 && v.width!=640) 1439 return -EINVAL; 1440 if(v.height!=200 && v.height!=240 1441 && v.height!=400 1442 && v.height !=480) 1443 return -EINVAL; 1444 memcpy(&capture_fb, &v, sizeof(v)); 1445 hardware_set_fb(&v); 1446 return 0; 1447 } 1448 1449 1450 1451 </programlisting> 1452 <para> 1453 The capable() function checks a user has the required capability. The Linux 1454 operating system has a set of about 30 capabilities indicating privileged 1455 access to services. The default set up gives the superuser (uid 0) all of 1456 them and nobody else has any. 1457 </para> 1458 <para> 1459 We check that the user has the SYS_ADMIN capability, that is they are 1460 allowed to operate as the machine administrator. We don't want anyone but 1461 the administrator making a mess of the display. 1462 </para> 1463 <para> 1464 Next we check for standard PC video modes (320 or 640 wide with either 1465 EGA or VGA depths). If the mode is not a standard video mode we reject it as 1466 not supported by our card. If the mode is acceptable we save it so that 1467 VIDIOCFBUF will give the right answer next time it is called. The 1468 hardware_set_fb() function is some undescribed card specific function to 1469 program the card for the desired mode. 1470 </para> 1471 <para> 1472 Before the driver can display an overlay window it needs to know where the 1473 window should be placed, and also how large it should be. If the card 1474 supports clipping it needs to know which rectangles to omit from the 1475 display. The video_window structure is used to describe the way the image 1476 should be displayed. 1477 </para> 1478 <table frame="all"><title>struct video_window fields</title> 1479 <tgroup cols="2" align="left"> 1480 <tbody> 1481 <row> 1482 <entry>width</entry><entry>The width in pixels of the desired image. The card 1483 may use a smaller size if this size is not available</entry> 1484 </row><row> 1485 <entry>height</entry><entry>The height of the image. The card may use a smaller 1486 size if this size is not available.</entry> 1487 </row><row> 1488 <entry>x</entry><entry> The X position of the top left of the window. This 1489 is in pixels relative to the left hand edge of the 1490 picture. Not all cards can display images aligned on 1491 any pixel boundary. If the position is unsuitable 1492 the card adjusts the image right and reduces the 1493 width.</entry> 1494 </row><row> 1495 <entry>y</entry><entry> The Y position of the top left of the window. This 1496 is counted in pixels relative to the top edge of the 1497 picture. As with the width if the card cannot 1498 display starting on this line it will adjust the 1499 values.</entry> 1500 </row><row> 1501 <entry>chromakey</entry><entry>The colour (expressed in RGB32 format) for the 1502 chromakey colour if chroma keying is being used. </entry> 1503 </row><row> 1504 <entry>clips</entry><entry>An array of rectangles that must not be drawn 1505 over.</entry> 1506 </row><row> 1507 <entry>clipcount</entry><entry>The number of clips in this array.</entry> 1508 </row> 1509 </tbody> 1510 </tgroup> 1511 </table> 1512 <para> 1513 Each clip is a struct video_clip which has the following fields 1514 </para> 1515 <table frame="all"><title>video_clip fields</title> 1516 <tgroup cols="2" align="left"> 1517 <tbody> 1518 <row> 1519 <entry>x, y</entry><entry>Co-ordinates relative to the display</entry> 1520 </row><row> 1521 <entry>width, height</entry><entry>Width and height in pixels</entry> 1522 </row><row> 1523 <entry>next</entry><entry>A spare field for the application to use</entry> 1524 </row> 1525 </tbody> 1526 </tgroup> 1527 </table> 1528 <para> 1529 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. 1530 This may well mean it has to leave alone. small areas the application wished to be 1531 drawn. 1532 </para> 1533 <para> 1534 Our example card uses chromakey so does not have to address most of the 1535 clipping. We will add a video_window structure to our global variables to 1536 remember our parameters, as we did with the frame buffer. 1537 </para> 1538 <programlisting> 1539 1540 1541 case VIDIOCGWIN: 1542 { 1543 if(copy_to_user(arg, &capture_win, 1544 sizeof(capture_win))) 1545 return -EFAULT; 1546 return 0; 1547 } 1548 1549 1550 case VIDIOCSWIN: 1551 { 1552 struct video_window v; 1553 if(copy_from_user(&v, arg, sizeof(v))) 1554 return -EFAULT; 1555 if(v.width > 640 || v.height > 480) 1556 return -EINVAL; 1557 if(v.width < 16 || v.height < 16) 1558 return -EINVAL; 1559 hardware_set_key(v.chromakey); 1560 hardware_set_window(v); 1561 memcpy(&capture_win, &v, sizeof(v)); 1562 capture_w = v.width; 1563 capture_h = v.height; 1564 return 0; 1565 } 1566 1567 1568 </programlisting> 1569 <para> 1570 Because we are using Chromakey our setup is fairly simple. Mostly we have to 1571 check the values are sane and load them into the capture card. 1572 </para> 1573 <para> 1574 With all the setup done we can now turn on the actual capture/overlay. This 1575 is done with the VIDIOCCAPTURE ioctl. This takes a single integer argument 1576 where 0 is on and 1 is off. 1577 </para> 1578 <programlisting> 1579 1580 1581 case VIDIOCCAPTURE: 1582 { 1583 int v; 1584 if(get_user(v, (int *)arg)) 1585 return -EFAULT; 1586 if(v==0) 1587 hardware_capture_off(); 1588 else 1589 { 1590 if(capture_fb.width == 0 1591 || capture_w == 0) 1592 return -EINVAL; 1593 hardware_capture_on(); 1594 } 1595 return 0; 1596 } 1597 1598 1599 </programlisting> 1600 <para> 1601 We grab the flag from user space and either enable or disable according to 1602 its value. There is one small corner case we have to consider here. Suppose 1603 that the capture was requested before the video window or the frame buffer 1604 had been set up. In those cases there will be unconfigured fields in our 1605 card data, as well as unconfigured hardware settings. We check for this case and 1606 return an error if the frame buffer or the capture window width is zero. 1607 </para> 1608 <programlisting> 1609 1610 1611 default: 1612 return -ENOIOCTLCMD; 1613 } 1614} 1615 </programlisting> 1616 <para> 1617 1618 We don't need to support any other ioctls, so if we get this far, it is time 1619 to tell the video layer that we don't now what the user is talking about. 1620 </para> 1621 </sect1> 1622 <sect1 id="endvid"> 1623 <title>Other Functionality</title> 1624 <para> 1625 The Video4Linux layer supports additional features, including a high 1626 performance mmap() based capture mode and capturing part of the image. 1627 These features are out of the scope of the book. You should however have enough 1628 example code to implement most simple video4linux devices for radio and TV 1629 cards. 1630 </para> 1631 </sect1> 1632 </chapter> 1633 <chapter id="bugs"> 1634 <title>Known Bugs And Assumptions</title> 1635 <para> 1636 <variablelist> 1637 <varlistentry><term>Multiple Opens</term> 1638 <listitem> 1639 <para> 1640 The driver assumes multiple opens should not be allowed. A driver 1641 can work around this but not cleanly. 1642 </para> 1643 </listitem></varlistentry> 1644 1645 <varlistentry><term>API Deficiencies</term> 1646 <listitem> 1647 <para> 1648 The existing API poorly reflects compression capable devices. There 1649 are plans afoot to merge V4L, V4L2 and some other ideas into a 1650 better interface. 1651 </para> 1652 </listitem></varlistentry> 1653 </variablelist> 1654 1655 </para> 1656 </chapter> 1657 1658 <chapter id="pubfunctions"> 1659 <title>Public Functions Provided</title> 1660!Edrivers/media/video/videodev.c 1661 </chapter> 1662 1663</book> 1664