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1 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V3.1//EN"[]>
3 <book id="V4LGuide">
4 <bookinfo>
5 <title>Video4Linux Programming</title>
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>
19 <copyright>
20 <year>2000</year>
21 <holder>Alan Cox</holder>
22 </copyright>
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>
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>
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>
47 <para>
48 For more details see the file COPYING in the source
49 distribution of Linux.
50 </para>
51 </legalnotice>
52 </bookinfo>
54 <toc></toc>
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>
93 static 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 };
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>
145 static int io = 0x320;
147 int __init myradio_init(struct video_init *v)
148 {
149 if(check_region(io, MY_IO_SIZE))
150 {
151 printk(KERN_ERR
152 "myradio: port 0x%03X is in use.\n", io);
153 return -EBUSY;
154 }
156 if(video_device_register(&my_radio, VFL_TYPE_RADIO)==-1)
157 return -EINVAL;
158 request_region(io, MY_IO_SIZE, "myradio");
159 return 0;
160 }
162 </programlisting>
163 <para>
164 The first stage of the initialisation, as is normally the case, is to check
165 that the I/O space we are about to fiddle with doesn't belong to some other
166 driver. If it is we leave well alone. If the user gives the address of the
167 wrong device then we will spot this. These policies will generally avoid
168 crashing the machine.
169 </para>
170 <para>
171 Now we ask the Video4Linux layer to register the device for us. We hand it
172 our carefully designed video_device structure and also tell it which group
173 of devices we want it registered with. In this case VFL_TYPE_RADIO.
174 </para>
175 <para>
176 The types available are
177 </para>
178 <table frame=all><title>Device Types</title>
179 <tgroup cols=3 align=left>
180 <tbody>
181 <row>
182 <entry>VFL_TYPE_RADIO</><>/dev/radio{n}</><>
184 Radio devices are assigned in this block. As with all of these
185 selections the actual number assignment is done by the video layer
186 accordijng to what is free.</entry>
187 </row><row>
188 <entry>VFL_TYPE_GRABBER</><>/dev/video{n}</><>
189 Video capture devices and also -- counter-intuitively for the name --
190 hardware video playback devices such as MPEG2 cards.</entry>
191 </row><row>
192 <entry>VFL_TYPE_VBI</><>/dev/vbi{n}</><>
193 The VBI devices capture the hidden lines on a television picture
194 that carry further information like closed caption data, teletext
195 (primarily in Europe) and now Intercast and the ATVEC internet
196 television encodings.</entry>
197 </row><row>
198 <entry>VFL_TYPE_VTX</><>/dev/vtx[n}</><>
199 VTX is 'Videotext' also known as 'Teletext'. This is a system for
200 sending numbered, 40x25, mostly textual page images over the hidden
201 lines. Unlike the /dev/vbi interfaces, this is for 'smart' decoder
202 chips. (The use of the word smart here has to be taken in context,
203 the smartest teletext chips are fairly dumb pieces of technology).
204 </entry>
205 </row>
206 </tbody>
207 </tgroup>
208 </table>
209 <para>
210 We are most definitely a radio.
211 </para>
212 <para>
213 Finally we allocate our I/O space so that nobody treads on us and return 0
214 to signify general happiness with the state of the universe.
215 </para>
216 </sect1>
217 <sect1 id="openradio">
218 <title>Opening And Closing The Radio</title>
220 <para>
221 The functions we declared in our video_device are mostly very simple.
222 Firstly we can drop in what is basically standard code for open and close.
223 </para>
224 <programlisting>
227 static int users = 0;
229 static int radio_open(stuct video_device *dev, int flags)
230 {
231 if(users)
232 return -EBUSY;
233 users++;
234 MOD_INC_USE_COUNT;
235 return 0;
236 }
238 </programlisting>
239 <para>
240 At open time we need to do nothing but check if someone else is also using
241 the radio card. If nobody is using it we make a note that we are using it,
242 then we ensure that nobody unloads our driver on us.
243 </para>
244 <programlisting>
247 static int radio_close(struct video_device *dev)
248 {
249 users--;
250 MOD_DEC_USE_COUNT;
251 }
253 </programlisting>
254 <para>
255 At close time we simply need to reduce the user count and allow the module
256 to become unloadable.
257 </para>
258 <para>
259 If you are sharp you will have noticed neither the open nor the close
260 routines attempt to reset or change the radio settings. This is intentional.
261 It allows an application to set up the radio and exit. It avoids a user
262 having to leave an application running all the time just to listen to the
263 radio.
264 </para>
265 </sect1>
266 <sect1 id="ioctlradio">
267 <title>The Ioctl Interface</title>
268 <para>
269 This leaves the ioctl routine, without which the driver will not be
270 terribly useful to anyone.
271 </para>
272 <programlisting>
275 static int radio_ioctl(struct video_device *dev, unsigned int cmd, void *arg)
276 {
277 switch(cmd)
278 {
279 case VIDIOCGCAP:
280 {
281 struct video_capability v;
282 v.type = VID_TYPE_TUNER;
283 v.channels = 1;
284 v.audios = 1;
285 v.maxwidth = 0;
286 v.minwidth = 0;
287 v.maxheight = 0;
288 v.minheight = 0;
289 strcpy(v.name, "My Radio");
290 if(copy_to_user(arg, &v, sizeof(v)))
291 return -EFAULT;
292 return 0;
293 }
295 </programlisting>
296 <para>
297 VIDIOCGCAP is the first ioctl all video4linux devices must support. It
298 allows the applications to find out what sort of a card they have found and
299 to figure out what they want to do about it. The fields in the structure are
300 </para>
301 <table frame=all><title>struct video_capability fields</title>
302 <tgroup cols=2 align=left>
303 <tbody>
304 <row>
305 <entry>name</><>The device text name. This is intended for the user.</>
306 </row><row>
307 <entry>channels</><>The number of different channels you can tune on
308 this card. It could even by zero for a card that has
309 no tuning capability. For our simple FM radio it is 1.
310 An AM/FM radio would report 2.</entry>
311 </row><row>
312 <entry>audios</><>The number of audio inputs on this device. For our
313 radio there is only one audio input.</entry>
314 </row><row>
315 <entry>minwidth,minheight</><>The smallest size the card is capable of capturing
316 images in. We set these to zero. Radios do not
317 capture pictures</entry>
318 </row><row>
319 <entry>maxwidth,maxheight</><>The largest image size the card is capable of
320 capturing. For our radio we report 0.
321 </entry>
322 </row><row>
323 <entry>type</><>This reports the capabilities of the device, and
324 matches the field we filled in in the struct
325 video_device when registering.</entry>
326 </row>
327 </tbody>
328 </tgroup>
329 </table>
330 <para>
331 Having filled in the fields, we use copy_to_user to copy the structure into
332 the users buffer. If the copy fails we return an EFAULT to the application
333 so that it knows it tried to feed us garbage.
334 </para>
335 <para>
336 The next pair of ioctl operations select which tuner is to be used and let
337 the application find the tuner properties. We have only a single FM band
338 tuner in our example device.
339 </para>
340 <programlisting>
343 case VIDIOCGTUNER:
344 {
345 struct video_tuner v;
346 if(copy_from_user(&v, arg, sizeof(v))!=0)
347 return -EFAULT;
348 if(v.tuner)
349 return -EINVAL;
350 v.rangelow=(87*16000);
351 v.rangehigh=(108*16000);
352 v.flags = VIDEO_TUNER_LOW;
353 v.mode = VIDEO_MODE_AUTO;
354 v.signal = 0xFFFF;
355 strcpy(v.name, "FM");
356 if(copy_to_user(&v, arg, sizeof(v))!=0)
357 return -EFAULT;
358 return 0;
359 }
361 </programlisting>
362 <para>
363 The VIDIOCGTUNER ioctl allows applications to query a tuner. The application
364 sets the tuner field to the tuner number it wishes to query. The query does
365 not change the tuner that is being used, it merely enquires about the tuner
366 in question.
367 </para>
368 <para>
369 We have exactly one tuner so after copying the user buffer to our temporary
370 structure we complain if they asked for a tuner other than tuner 0.
371 </para>
372 <para>
373 The video_tuner structure has the following fields
374 </para>
375 <table frame=all><title>struct video_tuner fields</title>
376 <tgroup cols=2 align=left>
377 <tbody>
378 <row>
379 <entry>int tuner</><entry>The number of the tuner in question</entry>
380 </row><row>
381 <entry>char name[32]</><entry>A text description of this tuner. "FM" will do fine.
382 This is intended for the application.</entry>
383 </row><row>
384 <entry>u32 flags</>
385 <entry>Tuner capability flags</entry>
386 </row>
387 <row>
388 <entry>u16 mode</><entry>The current reception mode</entry>
390 </row><row>
391 <entry>u16 signal</><entry>The signal strength scaled between 0 and 65535. If
392 a device cannot tell the signal strength it should
393 report 65535. Many simple cards contain only a
394 signal/no signal bit. Such cards will report either
395 0 or 65535.</entry>
397 </row><row>
398 <entry>u32 rangelow, rangehigh</><entry>
399 The range of frequencies supported by the radio
400 or TV. It is scaled according to the VIDEO_TUNER_LOW
401 flag.</entry>
403 </row>
404 </tbody>
405 </tgroup>
406 </table>
408 <table frame=all><title>struct video_tuner flags</title>
409 <tgroup cols=2 align=left>
410 <tbody>
411 <row>
412 <entry>VIDEO_TUNER_PAL</><entry>A PAL TV tuner</entry>
413 </row><row>
414 <entry>VIDEO_TUNER_NTSC</><entry>An NTSC (US) TV tuner</entry>
415 </row><row>
416 <entry>VIDEO_TUNER_SECAM</><entry>A SECAM (French) TV tuner</entry>
417 </row><row>
418 <entry>VIDEO_TUNER_LOW</><>
419 The tuner frequency is scaled in 1/16th of a KHz
420 steps. If not it is in 1/16th of a MHz steps
421 </entry>
422 </row><row>
423 <entry>VIDEO_TUNER_NORM</><entry>The tuner can set its format</entry>
424 </row><row>
425 <entry>VIDEO_TUNER_STEREO_ON</><entry>The tuner is currently receiving a stereo signal</entry>
426 </row>
427 </tbody>
428 </tgroup>
429 </table>
431 <table frame=all><title>struct video_tuner modes</title>
432 <tgroup cols=2 align=left>
433 <tbody>
434 <row>
435 <entry>VIDEO_MODE_PAL</><>PAL Format</entry>
436 </row><row>
437 <entry>VIDEO_MODE_NTSC</><>NTSC Format (USA)</entry>
438 </row><row>
439 <entry>VIDEO_MODE_SECAM</><>French Format</entry>
440 </row><row>
441 <entry>VIDEO_MODE_AUTO</><>A device that does not need to do
442 TV format switching</entry>
443 </row>
444 </tbody>
445 </tgroup>
446 </table>
447 <para>
448 The settings for the radio card are thus fairly simple. We report that we
449 are a tuner called "FM" for FM radio. In order to get the best tuning
450 resolution we report VIDEO_TUNER_LOW and select tuning to 1/16th of KHz. Its
451 unlikely our card can do that resolution but it is a fair bet the card can
452 do better than 1/16th of a MHz. VIDEO_TUNER_LOW is appropriate to almost all
453 radio usage.
454 </para>
455 <para>
456 We report that the tuner automatically handles deciding what format it is
457 receiving - true enough as it only handles FM radio. Our example card is
458 also incapable of detecting stereo or signal strengths so it reports a
459 strength of 0xFFFF (maximum) and no stereo detected.
460 </para>
461 <para>
462 To finish off we set the range that can be tuned to be 87-108Mhz, the normal
463 FM broadcast radio range. It is important to find out what the card is
464 actually capable of tuning. It is easy enough to simply use the FM broadcast
465 range. Unfortunately if you do this you will discover the FM broadcast
466 ranges in the USA, Europe and Japan are all subtly different and some users
467 cannot receive all the stations they wish.
468 </para>
469 <para>
470 The application also needs to be able to set the tuner it wishes to use. In
471 our case, with a single tuner this is rather simple to arrange.
472 </para>
473 <programlisting>
475 case VIDIOCSTUNER:
476 {
477 struct video_tuner v;
478 if(copy_from_user(&v, arg, sizeof(v)))
479 return -EFAULT;
480 if(v.tuner != 0)
481 return -EINVAL;
482 return 0;
483 }
485 </programlisting>
486 <para>
487 We copy the user supplied structure into kernel memory so we can examine it.
488 If the user has selected a tuner other than zero we reject the request. If
489 they wanted tuner 0 then, suprisingly enough, that is the current tuner already.
490 </para>
491 <para>
492 The next two ioctls we need to provide are to get and set the frequency of
493 the radio. These both use an unsigned long argument which is the frequency.
494 The scale of the frequency depends on the VIDEO_TUNER_LOW flag as I
495 mentioned earlier on. Since we have VIDEO_TUNER_LOW set this will be in
496 1/16ths of a KHz.
497 </para>
498 <programlisting>
500 static unsigned long current_freq;
504 case VIDIOCGFREQ:
505 if(copy_to_user(arg, &current_freq,
506 sizeof(unsigned long))
507 return -EFAULT;
508 return 0;
510 </programlisting>
511 <para>
512 Querying the frequency in our case is relatively simple. Our radio card is
513 too dumb to let us query the signal strength so we remember our setting if
514 we know it. All we have to do is copy it to the user.
515 </para>
516 <programlisting>
519 case VIDIOCSFREQ:
520 {
521 u32 freq;
522 if(copy_from_user(arg, &freq,
523 sizeof(unsigned long))!=0)
524 return -EFAULT;
525 if(hardware_set_freq(freq)<0)
526 return -EINVAL;
527 current_freq = freq;
528 return 0;
529 }
531 </programlisting>
532 <para>
533 Setting the frequency is a little more complex. We begin by copying the
534 desired frequency into kernel space. Next we call a hardware specific routine
535 to set the radio up. This might be as simple as some scaling and a few
536 writes to an I/O port. For most radio cards it turns out a good deal more
537 complicated and may involve programming things like a phase locked loop on
538 the card. This is what documentation is for.
539 </para>
540 <para>
541 The final set of operations we need to provide for our radio are the
542 volume controls. Not all radio cards can even do volume control. After all
543 there is a perfectly good volume control on the sound card. We will assume
544 our radio card has a simple 4 step volume control.
545 </para>
546 <para>
547 There are two ioctls with audio we need to support
548 </para>
549 <programlisting>
551 static int current_volume=0;
553 case VIDIOCGAUDIO:
554 {
555 struct video_audio v;
556 if(copy_from_user(&v, arg, sizeof(v)))
557 return -EFAULT;
558 if(v.audio != 0)
559 return -EINVAL;
560 v.volume = 16384*current_volume;
561 v.step = 16384;
562 strcpy(v.name, "Radio");
563 v.mode = VIDEO_SOUND_MONO;
564 v.balance = 0;
565 v.base = 0;
566 v.treble = 0;
568 if(copy_to_user(arg. &v, sizeof(v)))
569 return -EFAULT;
570 return 0;
571 }
573 </programlisting>
574 <para>
575 Much like the tuner we start by copying the user structure into kernel
576 space. Again we check if the user has asked for a valid audio input. We have
577 only input 0 and we punt if they ask for another input.
578 </para>
579 <para>
580 Then we fill in the video_audio structure. This has the following format
581 </para>
582 <table frame=all><title>struct video_audio fields</title>
583 <tgroup cols=2 align=left>
584 <tbody>
585 <row>
586 <entry>audio</><>The input the user wishes to query</>
587 </row><row>
588 <entry>volume</><>The volume setting on a scale of 0-65535</>
589 </row><row>
590 <entry>base</><>The base level on a scale of 0-65535</>
591 </row><row>
592 <entry>treble</><>The treble level on a scale of 0-65535</>
593 </row><row>
594 <entry>flags</><>The features this audio device supports
595 </entry>
596 </row><row>
597 <entry>name</><>A text name to display to the user. We picked
598 "Radio" as it explains things quite nicely.</>
599 </row><row>
600 <entry>mode</><>The current reception mode for the audio
602 We report MONO because our card is too stupid to know if it is in
603 mono or stereo.
604 </entry>
605 </row><row>
606 <entry>balance</><>The stereo balance on a scale of 0-65535, 32768 is
607 middle.</>
608 </row><row>
609 <entry>step</><>The step by which the volume control jumps. This is
610 used to help make it easy for applications to set
611 slider behaviour.</>
612 </row>
613 </tbody>
614 </tgroup>
615 </table>
617 <table frame=all><title>struct video_audio flags</title>
618 <tgroup cols=2 align=left>
619 <tbody>
620 <row>
621 <entry>VIDEO_AUDIO_MUTE</><>The audio is currently muted. We
622 could fake this in our driver but we
623 choose not to bother.</entry>
624 </row><row>
625 <entry>VIDEO_AUDIO_MUTABLE</><>The input has a mute option</entry>
626 </row><row>
627 <entry>VIDEO_AUDIO_TREBLE</><>The input has a treble control</entry>
628 </row><row>
629 <entry>VIDEO_AUDIO_BASS</><>The input has a base control</entry>
630 </row>
631 </tbody>
632 </tgroup>
633 </table>
635 <table frame=all><title>struct video_audio modes</title>
636 <tgroup cols=2 align=left>
637 <tbody>
638 <row>
639 <entry>VIDEO_SOUND_MONO</><>Mono sound</entry>
640 </row><row>
641 <entry>VIDEO_SOUND_STEREO</><>Stereo sound</entry>
642 </row><row>
643 <entry>VIDEO_SOUND_LANG1</><>Alternative language 1 (TV specific)</entry>
644 </row><row>
645 <entry>VIDEO_SOUND_LANG2</><>Alternative language 2 (TV specific)</entry>
646 </row>
647 </tbody>
648 </tgroup>
649 </table>
650 <para>
651 Having filled in the structure we copy it back to user space.
652 </para>
653 <para>
654 The VIDIOCSAUDIO ioctl allows the user to set the audio parameters in the
655 video_audio stucture. The driver does its best to honour the request.
656 </para>
657 <programlisting>
659 case VIDIOCSAUDIO:
660 {
661 struct video_audio v;
662 if(copy_from_user(&v, arg, sizeof(v)))
663 return -EFAULT;
664 if(v.audio)
665 return -EINVAL;
666 current_volume = v/16384;
667 hardware_set_volume(current_volume);
668 return 0;
669 }
671 </programlisting>
672 <para>
673 In our case there is very little that the user can set. The volume is
674 basically the limit. Note that we could pretend to have a mute feature
675 by rewriting this to
676 </para>
677 <programlisting>
679 case VIDIOCSAUDIO:
680 {
681 struct video_audio v;
682 if(copy_from_user(&v, arg, sizeof(v)))
683 return -EFAULT;
684 if(v.audio)
685 return -EINVAL;
686 current_volume = v/16384;
687 if(v.flags&VIDEO_AUDIO_MUTE)
688 hardware_set_volume(0);
689 else
690 hardware_set_volume(current_volume);
691 current_muted = v.flags &
692 VIDEO_AUDIO_MUTE;
693 return 0;
694 }
696 </programlisting>
697 <para>
698 This with the corresponding changes to the VIDIOCGAUDIO code to report the
699 state of the mute flag we save and to report the card has a mute function,
700 will allow applications to use a mute facility with this card. It is
701 questionable whether this is a good idea however. User applications can already
702 fake this themselves and kernel space is precious.
703 </para>
704 <para>
705 We now have a working radio ioctl handler. So we just wrap up the function
706 </para>
707 <programlisting>
710 }
711 return -ENOIOCTLCMD;
712 }
714 </programlisting>
715 <para>
716 and pass the Video4Linux layer back an error so that it knows we did not
717 understand the request we got passed.
718 </para>
719 </sect1>
720 <sect1 id="modradio">
721 <title>Module Wrapper</title>
722 <para>
723 Finally we add in the usual module wrapping and the driver is done.
724 </para>
725 <programlisting>
727 #ifndef MODULE
729 static int io = 0x300;
731 #else
733 static int io = -1;
736 MODULE_AUTHOR("Alan Cox");
737 MODULE_DESCRIPTION("A driver for an imaginary radio card.");
738 MODULE_PARM(io, "i");
739 MODULE_PARM_DESC(io, "I/O address of the card.");
741 EXPORT_NO_SYMBOLS;
743 int init_module(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 }
754 void cleanup_module(void)
755 {
756 video_unregister_device(&my_radio);
757 release_region(io, MY_IO_SIZE);
758 }
760 #endif
762 </programlisting>
763 <para>
764 In this example we set the IO base by default if the driver is compiled into
765 the kernel where you cannot pass a parameter. For the module we require the
766 user sets the parameter. We set io to a nonsense port (-1) so that we can
767 tell if the user supplied an io parameter or not.
768 </para>
769 <para>
770 We use MODULE_ defines to give an author for the card driver and a
771 description. We also use them to declare that io is an integer and it is the
772 address of the card.
773 </para>
774 <para>
775 The clean-up routine unregisters the video_device we registered, and frees
776 up the I/O space. Note that the unregister takes the actual video_device
777 structure as its argument. Unlike the file operations structure which can be
778 shared by all instances of a device a video_device structure as an actual
779 instance of the device. If you are registering multiple radio devices you
780 need to fill in one structure per device (most likely by setting up a
781 template and copying it to each of the actual device structures).
782 </para>
783 </sect1>
784 </chapter>
785 <chapter>
786 <title>Video Capture Devices</title>
787 <sect1 id="introvid">
788 <title>Video Capture Device Types</title>
789 <para>
790 The video capture devices share the same interfaces as radio devices. In
791 order to explain the video capture interface I will use the example of a
792 camera that has no tuners or audio input. This keeps the example relatively
793 clean. To get both combine the two driver examples.
794 </para>
795 <para>
796 Video capture devices divide into four categories. A little technology
797 backgrounder. Full motion video even at television resolution (which is
798 actually fairly low) is pretty resource-intensive. You are continually
799 passing megabytes of data every second from the capture card to the display.
800 several alternative approaches have emerged because copying this through the
801 processor and the user program is a particularly bad idea .
802 </para>
803 <para>
804 The first is to add the television image onto the video output directly.
805 This is also how some 3D cards work. These basic cards can generally drop the
806 video into any chosen rectangle of the display. Cards like this, which
807 include most mpeg1 cards that used the feature connector, aren't very
808 friendly in a windowing environment. They don't understand windows or
809 clipping. The video window is always on the top of the display.
810 </para>
811 <para>
812 Chroma keying is a technique used by cards to get around this. It is an old
813 television mixing trick where you mark all the areas you wish to replace
814 with a single clear colour that isn't used in the image - TV people use an
815 incredibly bright blue while computing people often use a paticularly
816 virulent purple. Bright blue occurs on the desktop. Anyone with virulent
817 purple windows has another problem besides their TV overlay.
818 </para>
819 <para>
820 The third approach is to copy the data from the capture card to the video
821 card, but to do it directly across the PCI bus. This relieves the processor
822 from doing the work but does require some smartness on the part of the video
823 capture chip, as well as a suitable video card. Programming this kind of
824 card and more so debugging it can be extremely tricky. There are some quite
825 complicated interactions with the display and you may also have to cope with
826 various chipset bugs that show up when PCI cards start talking to each
827 other.
828 </para>
829 <para>
830 To keep our example fairly simple we will assume a card that supports
831 overlaying a flat rectangular image onto the frame buffer output, and which
832 can also capture stuff into processor memory.
833 </para>
834 </sect1>
835 <sect1 id="regvid">
836 <title>Registering Video Capture Devices</title>
837 <para>
838 This time we need to add more functions for our camera device.
839 </para>
840 <programlisting>
841 static struct video_device my_camera
842 {
843 "My Camera",
844 VID_TYPE_OVERLAY|VID_TYPE_SCALES|\
845 VID_TYPE_CAPTURE|VID_TYPE_CHROMAKEY,
846 VID_HARDWARE_MYCAMERA,
847 camera_open.
848 camera_close,
849 camera_read, /* no read */
850 NULL, /* no write */
851 camera_poll, /* no poll */
852 camera_ioctl,
853 NULL, /* no special init function */
854 NULL /* no private data */
855 };
856 </programlisting>
857 <para>
858 We need a read() function which is used for capturing data from
859 the card, and we need a poll function so that a driver can wait for the next
860 frame to be captured.
861 </para>
862 <para>
863 We use the extra video capability flags that did not apply to the
864 radio interface. The video related flags are
865 </para>
866 <table frame=all><title>Capture Capabilities</title>
867 <tgroup cols=2 align=left>
868 <tbody>
869 <row>
870 <entry>VID_TYPE_CAPTURE</><>We support image capture</>
871 </row><row>
872 <entry>VID_TYPE_TELETEXT</><>A teletext capture device (vbi{n])</>
873 </row><row>
874 <entry>VID_TYPE_OVERLAY</><>The image can be directly overlaid onto the
875 frame buffer</>
876 </row><row>
877 <entry>VID_TYPE_CHROMAKEY</><>Chromakey can be used to select which parts
878 of the image to display</>
879 </row><row>
880 <entry>VID_TYPE_CLIPPING</><>It is possible to give the board a list of
881 rectangles to draw around. </>
882 </row><row>
883 <entry>VID_TYPE_FRAMERAM</><>The video capture goes into the video memory
884 and actually changes it. Applications need
885 to know this so they can clean up after the
886 card</>
887 </row><row>
888 <entry>VID_TYPE_SCALES</><>The image can be scaled to various sizes,
889 rather than being a single fixed size.</>
890 </row><row>
891 <entry>VID_TYPE_MONOCHROME</><>The capture will be monochrome. This isn't a
892 complete answer to the question since a mono
893 camera on a colour capture card will still
894 produce mono output.</>
895 </row><row>
896 <entry>VID_TYPE_SUBCAPTURE</><>The card allows only part of its field of
897 view to be captured. This enables
898 applications to avoid copying all of a large
899 image into memory when only some section is
900 relevant.</>
901 </row>
902 </tbody>
903 </tgroup>
904 </table>
905 <para>
906 We set VID_TYPE_CAPTURE so that we are seen as a capture card,
907 VID_TYPE_CHROMAKEY so the application knows it is time to draw in virulent
908 purple, and VID_TYPE_SCALES because we can be resized.
909 </para>
910 <para>
911 Our setup is fairly similar. This time we also want an interrupt line
912 for the 'frame captured' signal. Not all cards have this so some of them
913 cannot handle poll().
914 </para>
915 <programlisting>
918 static int io = 0x320;
919 static int irq = 11;
921 int __init mycamera_init(struct video_init *v)
922 {
923 if(check_region(io, MY_IO_SIZE))
924 {
925 printk(KERN_ERR
926 "mycamera: port 0x%03X is in use.\n", io);
927 return -EBUSY;
928 }
930 if(video_device_register(&my_camera,
931 VFL_TYPE_GRABBER)==-1)
932 return -EINVAL;
933 request_region(io, MY_IO_SIZE, "mycamera");
934 return 0;
935 }
937 </programlisting>
938 <para>
939 This is little changed from the needs of the radio card. We specify
940 VFL_TYPE_GRABBER this time as we want to be allocated a /dev/video name.
941 </para>
942 </sect1>
943 <sect1 id="opvid">
944 <title>Opening And Closing The Capture Device</title>
945 <programlisting>
948 static int users = 0;
950 static int camera_open(stuct video_device *dev, int flags)
951 {
952 if(users)
953 return -EBUSY;
954 if(request_irq(irq, camera_irq, 0, "camera", dev)<0)
955 return -EBUSY;
956 users++;
957 MOD_INC_USE_COUNT;
958 return 0;
959 }
962 static int camera_close(struct video_device *dev)
963 {
964 users--;
965 free_irq(irq, dev);
966 MOD_DEC_USE_COUNT;
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 SA_SHIRQ 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>
991 static struct wait_queue *capture_wait;
992 static int capture_ready = 0;
994 static 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>
1016 static 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 }
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>
1037 static 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;
1045 add_wait_queue(&capture_wait, &wait);
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;
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>
1092 capture_ready = 0;
1094 ptr=(u8 *)buf;
1095 len = capture_w * 3 * capture_h; /* 24bit RGB */
1097 if(len>count)
1098 len=count; /* Doesn't all fit */
1100 for(i=0; i<len; i++)
1101 {
1102 put_user(inb(io+IMAGE_DATA), ptr);
1103 ptr++;
1104 }
1106 hardware_restart_capture();
1108 return i;
1109 }
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>
1136 static 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 }
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>
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 }
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>
1212 <entry>channel</><>The channel number we are selecting</entry>
1213 </row><row>
1214 <entry>name</><>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</><>Channel properties</entry>
1220 </row><row>
1221 <entry>type</><>Input type</entry>
1222 </row><row>
1223 <entry>norm</><>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</><>Channel has a tuner.</entry>
1235 </row><row>
1236 <entry>VIDEO_VC_AUDIO</><>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</><>Television input.</entry>
1246 </row><row>
1247 <entry>VIDEO_TYPE_CAMERA</><>Fixed camera input.</entry>
1248 </row><row>
1249 <entry>0</><>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</><>PAL encoded Television</entry>
1259 </row><row>
1260 <entry>VIDEO_MODE_NTSC</><>NTSC (US) encoded Television</entry>
1261 </row><row>
1262 <entry>VIDEO_MODE_SECAM</><>SECAM (French) Televison </entry>
1263 </row><row>
1264 <entry>VIDEO_MODE_AUTO</><>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 exaple to switch between a PAL or an NTSC
1273 format camera.
1274 </para>
1275 <programlisting>
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 }
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>
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 }
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 returing 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</><>Linear greyscale. This is for simple cameras and the
1345 like</>
1346 </row><row>
1347 <entry>RGB565</><>The top 5 bits hold 32 red levels, the next six bits
1348 hold green and the low 5 bits hold blue. </>
1349 </row><row>
1350 <entry>RGB555</><>The top bit is clear. The red green and blue levels
1351 each occupy five bits.</>
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>
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 }
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>
1410 static struct video_buffer capture_fb;
1412 case VIDIOCGFBUF:
1413 {
1414 if(copy_to_user(arg, &capture_fb,
1415 sizeof(capture_fb)))
1416 return -EFAULT;
1417 return 0;
1419 }
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>
1429 case VIDIOCSFBUF:
1430 {
1431 struct video_buffer v;
1433 if(!capable(CAP_SYS_ADMIN))
1434 return -EPERM;
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 }
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</><>The width in pixels of the desired image. The card
1483 may use a smaller size if this size is not available</>
1484 </row><row>
1485 <entry>height</><>The height of the image. The card may use a smaller
1486 size if this size is not available.</>
1487 </row><row>
1488 <entry>x</><> 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.</>
1494 </row><row>
1495 <entry>y</><> 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.</>
1500 </row><row>
1501 <entry>chromakey</><>The colour (expressed in RGB32 format) for the
1502 chromakey colour if chroma keying is being used. </>
1503 </row><row>
1504 <entry>clips</><>An array of rectangles that must not be drawn
1505 over.</>
1506 </row><row>
1507 <entry>clipcount</><>The number of clips in this array.</>
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</><>Co-ordinates relative to the display</>
1520 </row><row>
1521 <entry>width, height</><>Width and height in pixels</>
1522 </row><row>
1523 <entry>next</><>A spare field for the application to use</>
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>
1541 case VIDIOCGWIN:
1542 {
1543 if(copy_to_user(arg, &capture_win,
1544 sizeof(capture_win)))
1545 return -EFAULT;
1546 return 0;
1547 }
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 }
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>
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 }
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>
1611 default:
1612 return -ENOIOCTLCMD;
1613 }
1614 }
1615 </programlisting>
1616 <para>
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>
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>
1655 </para>
1656 </chapter>
1658 <chapter id="pubfunctions">
1659 <title>Public Functions Provided</title>
1660 !Edrivers/media/video/videodev.c
1661 </chapter>
1663 </book>