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6 <bookinfo>
9 <authorgroup>
10 <author>
13 <affiliation>
14 <address>
16 </address>
17 </affiliation>
18 </author>
19 </authorgroup>
21 <copyright>
24 </copyright>
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>
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>
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
47 </para>
49 <para>
50 For more details see the file COPYING in the source
51 distribution of Linux.
52 </para>
53 </legalnotice>
54 </bookinfo>
56 <toc></toc>
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>
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>
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>
95 static struct video_device my_radio
96 {
97 "My radio",
98 VID_TYPE_TUNER,
99 radio_open.
100 radio_close,
101 NULL, /* no read */
102 NULL, /* no write */
103 NULL, /* no poll */
104 radio_ioctl,
105 NULL, /* no special init function */
106 NULL /* no private data */
107 };
110 </programlisting>
111 <para>
112 This declares our video4linux device driver interface. The VID_TYPE_ value
113 defines what kind of an interface we are, and defines basic capabilities.
114 </para>
115 <para>
116 The only defined value relevant for a radio card is VID_TYPE_TUNER which
117 indicates that the device can be tuned. Clearly our radio is going to have some
118 way to change channel so it is tuneable.
119 </para>
120 <para>
121 We declare an open and close routine, but we do not need read or write,
122 which are used to read and write video data to or from the card itself. As
123 we have no read or write there is no poll function.
124 </para>
125 <para>
126 The private initialise function is run when the device is registered. In
127 this driver we've already done all the work needed. The final pointer is a
128 private data pointer that can be used by the device driver to attach and
129 retrieve private data structures. We set this field "priv" to NULL for
130 the moment.
131 </para>
132 <para>
133 Having the structure defined is all very well but we now need to register it
134 with the kernel.
135 </para>
136 <programlisting>
139 static int io = 0x320;
141 int __init myradio_init(struct video_init *v)
142 {
143 if(!request_region(io, MY_IO_SIZE, "myradio"))
144 {
145 printk(KERN_ERR
146 "myradio: port 0x%03X is in use.\n", io);
147 return -EBUSY;
148 }
151 release_region(io, MY_IO_SIZE);
152 return -EINVAL;
153 }
154 return 0;
155 }
157 </programlisting>
158 <para>
159 The first stage of the initialisation, as is normally the case, is to check
160 that the I/O space we are about to fiddle with doesn't belong to some other
161 driver. If it is we leave well alone. If the user gives the address of the
162 wrong device then we will spot this. These policies will generally avoid
163 crashing the machine.
164 </para>
165 <para>
166 Now we ask the Video4Linux layer to register the device for us. We hand it
167 our carefully designed video_device structure and also tell it which group
168 of devices we want it registered with. In this case VFL_TYPE_RADIO.
169 </para>
170 <para>
171 The types available are
172 </para>
175 <tbody>
176 <row>
179 Radio devices are assigned in this block. As with all of these
180 selections the actual number assignment is done by the video layer
181 accordijng to what is free.</entry>
182 </row><row>
184 Video capture devices and also -- counter-intuitively for the name --
185 hardware video playback devices such as MPEG2 cards.</entry>
186 </row><row>
188 The VBI devices capture the hidden lines on a television picture
189 that carry further information like closed caption data, teletext
190 (primarily in Europe) and now Intercast and the ATVEC internet
191 television encodings.</entry>
192 </row><row>
194 VTX is 'Videotext' also known as 'Teletext'. This is a system for
195 sending numbered, 40x25, mostly textual page images over the hidden
196 lines. Unlike the /dev/vbi interfaces, this is for 'smart' decoder
197 chips. (The use of the word smart here has to be taken in context,
198 the smartest teletext chips are fairly dumb pieces of technology).
199 </entry>
200 </row>
201 </tbody>
202 </tgroup>
203 </table>
204 <para>
205 We are most definitely a radio.
206 </para>
207 <para>
208 Finally we allocate our I/O space so that nobody treads on us and return 0
209 to signify general happiness with the state of the universe.
210 </para>
211 </sect1>
215 <para>
216 The functions we declared in our video_device are mostly very simple.
217 Firstly we can drop in what is basically standard code for open and close.
218 </para>
219 <programlisting>
222 static int users = 0;
224 static int radio_open(struct video_device *dev, int flags)
225 {
226 if(users)
227 return -EBUSY;
228 users++;
229 return 0;
230 }
232 </programlisting>
233 <para>
234 At open time we need to do nothing but check if someone else is also using
235 the radio card. If nobody is using it we make a note that we are using it,
236 then we ensure that nobody unloads our driver on us.
237 </para>
238 <programlisting>
241 static int radio_close(struct video_device *dev)
242 {
243 users--;
244 }
246 </programlisting>
247 <para>
248 At close time we simply need to reduce the user count and allow the module
249 to become unloadable.
250 </para>
251 <para>
252 If you are sharp you will have noticed neither the open nor the close
253 routines attempt to reset or change the radio settings. This is intentional.
254 It allows an application to set up the radio and exit. It avoids a user
255 having to leave an application running all the time just to listen to the
256 radio.
257 </para>
258 </sect1>
261 <para>
262 This leaves the ioctl routine, without which the driver will not be
263 terribly useful to anyone.
264 </para>
265 <programlisting>
268 static int radio_ioctl(struct video_device *dev, unsigned int cmd, void *arg)
269 {
270 switch(cmd)
271 {
272 case VIDIOCGCAP:
273 {
274 struct video_capability v;
275 v.type = VID_TYPE_TUNER;
276 v.channels = 1;
277 v.audios = 1;
278 v.maxwidth = 0;
279 v.minwidth = 0;
280 v.maxheight = 0;
281 v.minheight = 0;
282 strcpy(v.name, "My Radio");
283 if(copy_to_user(arg, &v, sizeof(v)))
284 return -EFAULT;
285 return 0;
286 }
288 </programlisting>
289 <para>
290 VIDIOCGCAP is the first ioctl all video4linux devices must support. It
291 allows the applications to find out what sort of a card they have found and
292 to figure out what they want to do about it. The fields in the structure are
293 </para>
296 <tbody>
297 <row>
299 </row><row>
301 this card. It could even by zero for a card that has
302 no tuning capability. For our simple FM radio it is 1.
304 </row><row>
306 radio there is only one audio input.</entry>
307 </row><row>
309 images in. We set these to zero. Radios do not
310 capture pictures</entry>
311 </row><row>
313 capturing. For our radio we report 0.
314 </entry>
315 </row><row>
317 matches the field we filled in in the struct
318 video_device when registering.</entry>
319 </row>
320 </tbody>
321 </tgroup>
322 </table>
323 <para>
324 Having filled in the fields, we use copy_to_user to copy the structure into
325 the users buffer. If the copy fails we return an EFAULT to the application
326 so that it knows it tried to feed us garbage.
327 </para>
328 <para>
329 The next pair of ioctl operations select which tuner is to be used and let
330 the application find the tuner properties. We have only a single FM band
331 tuner in our example device.
332 </para>
333 <programlisting>
336 case VIDIOCGTUNER:
337 {
338 struct video_tuner v;
340 return -EFAULT;
341 if(v.tuner)
342 return -EINVAL;
345 v.flags = VIDEO_TUNER_LOW;
346 v.mode = VIDEO_MODE_AUTO;
347 v.signal = 0xFFFF;
348 strcpy(v.name, "FM");
350 return -EFAULT;
351 return 0;
352 }
354 </programlisting>
355 <para>
356 The VIDIOCGTUNER ioctl allows applications to query a tuner. The application
357 sets the tuner field to the tuner number it wishes to query. The query does
358 not change the tuner that is being used, it merely enquires about the tuner
359 in question.
360 </para>
361 <para>
362 We have exactly one tuner so after copying the user buffer to our temporary
363 structure we complain if they asked for a tuner other than tuner 0.
364 </para>
365 <para>
366 The video_tuner structure has the following fields
367 </para>
370 <tbody>
371 <row>
373 </row><row>
375 This is intended for the application.</entry>
376 </row><row>
379 </row>
380 <row>
383 </row><row>
385 a device cannot tell the signal strength it should
386 report 65535. Many simple cards contain only a
387 signal/no signal bit. Such cards will report either
390 </row><row>
392 The range of frequencies supported by the radio
393 or TV. It is scaled according to the VIDEO_TUNER_LOW
394 flag.</entry>
396 </row>
397 </tbody>
398 </tgroup>
399 </table>
403 <tbody>
404 <row>
406 </row><row>
408 </row><row>
410 </row><row>
414 </entry>
415 </row><row>
417 </row><row>
418 <entry>VIDEO_TUNER_STEREO_ON</entry><entry>The tuner is currently receiving a stereo signal</entry>
419 </row>
420 </tbody>
421 </tgroup>
422 </table>
426 <tbody>
427 <row>
429 </row><row>
431 </row><row>
433 </row><row>
435 TV format switching</entry>
436 </row>
437 </tbody>
438 </tgroup>
439 </table>
440 <para>
441 The settings for the radio card are thus fairly simple. We report that we
442 are a tuner called "FM" for FM radio. In order to get the best tuning
444 unlikely our card can do that resolution but it is a fair bet the card can
446 radio usage.
447 </para>
448 <para>
449 We report that the tuner automatically handles deciding what format it is
450 receiving - true enough as it only handles FM radio. Our example card is
451 also incapable of detecting stereo or signal strengths so it reports a
452 strength of 0xFFFF (maximum) and no stereo detected.
453 </para>
454 <para>
456 FM broadcast radio range. It is important to find out what the card is
457 actually capable of tuning. It is easy enough to simply use the FM broadcast
458 range. Unfortunately if you do this you will discover the FM broadcast
459 ranges in the USA, Europe and Japan are all subtly different and some users
460 cannot receive all the stations they wish.
461 </para>
462 <para>
463 The application also needs to be able to set the tuner it wishes to use. In
464 our case, with a single tuner this is rather simple to arrange.
465 </para>
466 <programlisting>
468 case VIDIOCSTUNER:
469 {
470 struct video_tuner v;
471 if(copy_from_user(&v, arg, sizeof(v)))
472 return -EFAULT;
473 if(v.tuner != 0)
474 return -EINVAL;
475 return 0;
476 }
478 </programlisting>
479 <para>
480 We copy the user supplied structure into kernel memory so we can examine it.
481 If the user has selected a tuner other than zero we reject the request. If
482 they wanted tuner 0 then, surprisingly enough, that is the current tuner already.
483 </para>
484 <para>
485 The next two ioctls we need to provide are to get and set the frequency of
486 the radio. These both use an unsigned long argument which is the frequency.
487 The scale of the frequency depends on the VIDEO_TUNER_LOW flag as I
488 mentioned earlier on. Since we have VIDEO_TUNER_LOW set this will be in
490 </para>
491 <programlisting>
493 static unsigned long current_freq;
497 case VIDIOCGFREQ:
498 if(copy_to_user(arg, &current_freq,
499 sizeof(unsigned long))
500 return -EFAULT;
501 return 0;
503 </programlisting>
504 <para>
505 Querying the frequency in our case is relatively simple. Our radio card is
506 too dumb to let us query the signal strength so we remember our setting if
507 we know it. All we have to do is copy it to the user.
508 </para>
509 <programlisting>
512 case VIDIOCSFREQ:
513 {
514 u32 freq;
515 if(copy_from_user(arg, &freq,
516 sizeof(unsigned long))!=0)
517 return -EFAULT;
519 return -EINVAL;
520 current_freq = freq;
521 return 0;
522 }
524 </programlisting>
525 <para>
526 Setting the frequency is a little more complex. We begin by copying the
527 desired frequency into kernel space. Next we call a hardware specific routine
528 to set the radio up. This might be as simple as some scaling and a few
529 writes to an I/O port. For most radio cards it turns out a good deal more
530 complicated and may involve programming things like a phase locked loop on
531 the card. This is what documentation is for.
532 </para>
533 <para>
534 The final set of operations we need to provide for our radio are the
535 volume controls. Not all radio cards can even do volume control. After all
536 there is a perfectly good volume control on the sound card. We will assume
537 our radio card has a simple 4 step volume control.
538 </para>
539 <para>
540 There are two ioctls with audio we need to support
541 </para>
542 <programlisting>
544 static int current_volume=0;
546 case VIDIOCGAUDIO:
547 {
548 struct video_audio v;
549 if(copy_from_user(&v, arg, sizeof(v)))
550 return -EFAULT;
551 if(v.audio != 0)
552 return -EINVAL;
553 v.volume = 16384*current_volume;
554 v.step = 16384;
555 strcpy(v.name, "Radio");
556 v.mode = VIDEO_SOUND_MONO;
557 v.balance = 0;
558 v.base = 0;
559 v.treble = 0;
561 if(copy_to_user(arg. &v, sizeof(v)))
562 return -EFAULT;
563 return 0;
564 }
566 </programlisting>
567 <para>
568 Much like the tuner we start by copying the user structure into kernel
569 space. Again we check if the user has asked for a valid audio input. We have
570 only input 0 and we punt if they ask for another input.
571 </para>
572 <para>
573 Then we fill in the video_audio structure. This has the following format
574 </para>
577 <tbody>
578 <row>
580 </row><row>
582 </row><row>
584 </row><row>
586 </row><row>
588 </entry>
589 </row><row>
592 </row><row>
595 We report MONO because our card is too stupid to know if it is in
596 mono or stereo.
597 </entry>
598 </row><row>
600 middle.</entry>
601 </row><row>
603 used to help make it easy for applications to set
604 slider behaviour.</entry>
605 </row>
606 </tbody>
607 </tgroup>
608 </table>
612 <tbody>
613 <row>
615 could fake this in our driver but we
616 choose not to bother.</entry>
617 </row><row>
619 </row><row>
621 </row><row>
623 </row>
624 </tbody>
625 </tgroup>
626 </table>
630 <tbody>
631 <row>
633 </row><row>
635 </row><row>
637 </row><row>
639 </row>
640 </tbody>
641 </tgroup>
642 </table>
643 <para>
644 Having filled in the structure we copy it back to user space.
645 </para>
646 <para>
647 The VIDIOCSAUDIO ioctl allows the user to set the audio parameters in the
648 video_audio structure. The driver does its best to honour the request.
649 </para>
650 <programlisting>
652 case VIDIOCSAUDIO:
653 {
654 struct video_audio v;
655 if(copy_from_user(&v, arg, sizeof(v)))
656 return -EFAULT;
657 if(v.audio)
658 return -EINVAL;
659 current_volume = v/16384;
660 hardware_set_volume(current_volume);
661 return 0;
662 }
664 </programlisting>
665 <para>
666 In our case there is very little that the user can set. The volume is
667 basically the limit. Note that we could pretend to have a mute feature
668 by rewriting this to
669 </para>
670 <programlisting>
672 case VIDIOCSAUDIO:
673 {
674 struct video_audio v;
675 if(copy_from_user(&v, arg, sizeof(v)))
676 return -EFAULT;
677 if(v.audio)
678 return -EINVAL;
679 current_volume = v/16384;
680 if(v.flags&VIDEO_AUDIO_MUTE)
681 hardware_set_volume(0);
682 else
683 hardware_set_volume(current_volume);
684 current_muted = v.flags &
685 VIDEO_AUDIO_MUTE;
686 return 0;
687 }
689 </programlisting>
690 <para>
691 This with the corresponding changes to the VIDIOCGAUDIO code to report the
692 state of the mute flag we save and to report the card has a mute function,
693 will allow applications to use a mute facility with this card. It is
694 questionable whether this is a good idea however. User applications can already
695 fake this themselves and kernel space is precious.
696 </para>
697 <para>
698 We now have a working radio ioctl handler. So we just wrap up the function
699 </para>
700 <programlisting>
703 }
704 return -ENOIOCTLCMD;
705 }
707 </programlisting>
708 <para>
709 and pass the Video4Linux layer back an error so that it knows we did not
710 understand the request we got passed.
711 </para>
712 </sect1>
715 <para>
716 Finally we add in the usual module wrapping and the driver is done.
717 </para>
718 <programlisting>
720 #ifndef MODULE
722 static int io = 0x300;
724 #else
726 static int io = -1;
728 #endif
730 MODULE_AUTHOR("Alan Cox");
731 MODULE_DESCRIPTION("A driver for an imaginary radio card.");
732 module_param(io, int, 0444);
733 MODULE_PARM_DESC(io, "I/O address of the card.");
735 static int __init init(void)
736 {
737 if(io==-1)
738 {
739 printk(KERN_ERR
740 "You must set an I/O address with io=0x???\n");
741 return -EINVAL;
742 }
743 return myradio_init(NULL);
744 }
746 static void __exit cleanup(void)
747 {
748 video_unregister_device(&my_radio);
749 release_region(io, MY_IO_SIZE);
750 }
752 module_init(init);
753 module_exit(cleanup);
755 </programlisting>
756 <para>
757 In this example we set the IO base by default if the driver is compiled into
758 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
759 user sets the parameter. We set io to a nonsense port (-1) so that we can
760 tell if the user supplied an io parameter or not.
761 </para>
762 <para>
763 We use MODULE_ defines to give an author for the card driver and a
764 description. We also use them to declare that io is an integer and it is the
765 address of the card, and can be read by anyone from sysfs.
766 </para>
767 <para>
768 The clean-up routine unregisters the video_device we registered, and frees
769 up the I/O space. Note that the unregister takes the actual video_device
770 structure as its argument. Unlike the file operations structure which can be
771 shared by all instances of a device a video_device structure as an actual
772 instance of the device. If you are registering multiple radio devices you
773 need to fill in one structure per device (most likely by setting up a
774 template and copying it to each of the actual device structures).
775 </para>
776 </sect1>
777 </chapter>
782 <para>
783 The video capture devices share the same interfaces as radio devices. In
784 order to explain the video capture interface I will use the example of a
785 camera that has no tuners or audio input. This keeps the example relatively
786 clean. To get both combine the two driver examples.
787 </para>
788 <para>
789 Video capture devices divide into four categories. A little technology
790 backgrounder. Full motion video even at television resolution (which is
791 actually fairly low) is pretty resource-intensive. You are continually
792 passing megabytes of data every second from the capture card to the display.
793 several alternative approaches have emerged because copying this through the
794 processor and the user program is a particularly bad idea .
795 </para>
796 <para>
797 The first is to add the television image onto the video output directly.
798 This is also how some 3D cards work. These basic cards can generally drop the
799 video into any chosen rectangle of the display. Cards like this, which
800 include most mpeg1 cards that used the feature connector, aren't very
801 friendly in a windowing environment. They don't understand windows or
802 clipping. The video window is always on the top of the display.
803 </para>
804 <para>
805 Chroma keying is a technique used by cards to get around this. It is an old
806 television mixing trick where you mark all the areas you wish to replace
807 with a single clear colour that isn't used in the image - TV people use an
808 incredibly bright blue while computing people often use a particularly
809 virulent purple. Bright blue occurs on the desktop. Anyone with virulent
810 purple windows has another problem besides their TV overlay.
811 </para>
812 <para>
813 The third approach is to copy the data from the capture card to the video
814 card, but to do it directly across the PCI bus. This relieves the processor
815 from doing the work but does require some smartness on the part of the video
816 capture chip, as well as a suitable video card. Programming this kind of
817 card and more so debugging it can be extremely tricky. There are some quite
818 complicated interactions with the display and you may also have to cope with
819 various chipset bugs that show up when PCI cards start talking to each
820 other.
821 </para>
822 <para>
823 To keep our example fairly simple we will assume a card that supports
824 overlaying a flat rectangular image onto the frame buffer output, and which
825 can also capture stuff into processor memory.
826 </para>
827 </sect1>
830 <para>
831 This time we need to add more functions for our camera device.
832 </para>
833 <programlisting>
834 static struct video_device my_camera
835 {
836 "My Camera",
837 VID_TYPE_OVERLAY|VID_TYPE_SCALES|\
838 VID_TYPE_CAPTURE|VID_TYPE_CHROMAKEY,
839 camera_open.
840 camera_close,
841 camera_read, /* no read */
842 NULL, /* no write */
843 camera_poll, /* no poll */
844 camera_ioctl,
845 NULL, /* no special init function */
846 NULL /* no private data */
847 };
848 </programlisting>
849 <para>
850 We need a read() function which is used for capturing data from
851 the card, and we need a poll function so that a driver can wait for the next
852 frame to be captured.
853 </para>
854 <para>
855 We use the extra video capability flags that did not apply to the
856 radio interface. The video related flags are
857 </para>
860 <tbody>
861 <row>
863 </row><row>
865 </row><row>
867 frame buffer</entry>
868 </row><row>
870 of the image to display</entry>
871 </row><row>
873 rectangles to draw around. </entry>
874 </row><row>
876 and actually changes it. Applications need
877 to know this so they can clean up after the
878 card</entry>
879 </row><row>
881 rather than being a single fixed size.</entry>
882 </row><row>
884 complete answer to the question since a mono
885 camera on a colour capture card will still
886 produce mono output.</entry>
887 </row><row>
889 view to be captured. This enables
890 applications to avoid copying all of a large
891 image into memory when only some section is
892 relevant.</entry>
893 </row>
894 </tbody>
895 </tgroup>
896 </table>
897 <para>
898 We set VID_TYPE_CAPTURE so that we are seen as a capture card,
899 VID_TYPE_CHROMAKEY so the application knows it is time to draw in virulent
900 purple, and VID_TYPE_SCALES because we can be resized.
901 </para>
902 <para>
903 Our setup is fairly similar. This time we also want an interrupt line
904 for the 'frame captured' signal. Not all cards have this so some of them
905 cannot handle poll().
906 </para>
907 <programlisting>
910 static int io = 0x320;
911 static int irq = 11;
913 int __init mycamera_init(struct video_init *v)
914 {
915 if(!request_region(io, MY_IO_SIZE, "mycamera"))
916 {
917 printk(KERN_ERR
918 "mycamera: port 0x%03X is in use.\n", io);
919 return -EBUSY;
920 }
922 if(video_device_register(&my_camera,
923 VFL_TYPE_GRABBER)==-1) {
924 release_region(io, MY_IO_SIZE);
925 return -EINVAL;
926 }
927 return 0;
928 }
930 </programlisting>
931 <para>
932 This is little changed from the needs of the radio card. We specify
933 VFL_TYPE_GRABBER this time as we want to be allocated a /dev/video name.
934 </para>
935 </sect1>
938 <programlisting>
941 static int users = 0;
943 static int camera_open(struct video_device *dev, int flags)
944 {
945 if(users)
946 return -EBUSY;
948 return -EBUSY;
949 users++;
950 return 0;
951 }
954 static int camera_close(struct video_device *dev)
955 {
956 users--;
957 free_irq(irq, dev);
958 }
959 </programlisting>
960 <para>
961 The open and close routines are also quite similar. The only real change is
962 that we now request an interrupt for the camera device interrupt line. If we
963 cannot get the interrupt we report EBUSY to the application and give up.
964 </para>
965 </sect1>
968 <para>
969 Our example handler is for an ISA bus device. If it was PCI you would be
970 able to share the interrupt and would have set IRQF_SHARED to indicate a
971 shared IRQ. We pass the device pointer as the interrupt routine argument. We
972 don't need to since we only support one card but doing this will make it
973 easier to upgrade the driver for multiple devices in the future.
974 </para>
975 <para>
976 Our interrupt routine needs to do little if we assume the card can simply
977 queue one frame to be read after it captures it.
978 </para>
979 <programlisting>
982 static struct wait_queue *capture_wait;
983 static int capture_ready = 0;
985 static void camera_irq(int irq, void *dev_id,
986 struct pt_regs *regs)
987 {
988 capture_ready=1;
989 wake_up_interruptible(&capture_wait);
990 }
991 </programlisting>
992 <para>
993 The interrupt handler is nice and simple for this card as we are assuming
994 the card is buffering the frame for us. This means we have little to do but
995 wake up anybody interested. We also set a capture_ready flag, as we may
996 capture a frame before an application needs it. In this case we need to know
997 that a frame is ready. If we had to collect the frame on the interrupt life
998 would be more complex.
999 </para>
1000 <para>
1001 The two new routines we need to supply are camera_read which returns a
1002 frame, and camera_poll which waits for a frame to become ready.
1003 </para>
1004 <programlisting>
1007 static int camera_poll(struct video_device *dev,
1008 struct file *file, struct poll_table *wait)
1009 {
1010 poll_wait(file, &capture_wait, wait);
1011 if(capture_read)
1012 return POLLIN|POLLRDNORM;
1013 return 0;
1014 }
1016 </programlisting>
1017 <para>
1018 Our wait queue for polling is the capture_wait queue. This will cause the
1019 task to be woken up by our camera_irq routine. We check capture_read to see
1020 if there is an image present and if so report that it is readable.
1021 </para>
1022 </sect1>
1025 <programlisting>
1028 static long camera_read(struct video_device *dev, char *buf,
1029 unsigned long count)
1030 {
1031 struct wait_queue wait = { current, NULL };
1032 u8 *ptr;
1033 int len;
1034 int i;
1038 while(!capture_ready)
1039 {
1041 {
1043 current->state = TASK_RUNNING;
1044 return -EWOULDBLOCK;
1045 }
1046 if(signal_pending(current))
1047 {
1049 current->state = TASK_RUNNING;
1050 return -ERESTARTSYS;
1051 }
1052 schedule();
1053 current->state = TASK_INTERRUPTIBLE;
1054 }
1056 current->state = TASK_RUNNING;
1058 </programlisting>
1059 <para>
1060 The first thing we have to do is to ensure that the application waits until
1061 the next frame is ready. The code here is almost identical to the mouse code
1062 we used earlier in this chapter. It is one of the common building blocks of
1063 Linux device driver code and probably one which you will find occurs in any
1064 drivers you write.
1065 </para>
1066 <para>
1067 We wait for a frame to be ready, or for a signal to interrupt our waiting. If a
1068 signal occurs we need to return from the system call so that the signal can
1069 be sent to the application itself. We also check to see if the user actually
1070 wanted to avoid waiting - ie if they are using non-blocking I/O and have other things
1071 to get on with.
1072 </para>
1073 <para>
1074 Next we copy the data from the card to the user application. This is rarely
1075 as easy as our example makes out. We will add capture_w, and capture_h here
1076 to hold the width and height of the captured image. We assume the card only
1077 supports 24bit RGB for now.
1078 </para>
1079 <programlisting>
1083 capture_ready = 0;
1085 ptr=(u8 *)buf;
1088 if(len>count)
1089 len=count; /* Doesn't all fit */
1092 {
1093 put_user(inb(io+IMAGE_DATA), ptr);
1094 ptr++;
1095 }
1097 hardware_restart_capture();
1099 return i;
1100 }
1102 </programlisting>
1103 <para>
1104 For a real hardware device you would try to avoid the loop with put_user().
1105 Each call to put_user() has a time overhead checking whether the accesses to user
1106 space are allowed. It would be better to read a line into a temporary buffer
1107 then copy this to user space in one go.
1108 </para>
1109 <para>
1110 Having captured the image and put it into user space we can kick the card to
1111 get the next frame acquired.
1112 </para>
1113 </sect1>
1116 <para>
1117 As with the radio driver the major control interface is via the ioctl()
1118 function. Video capture devices support the same tuner calls as a radio
1119 device and also support additional calls to control how the video functions
1120 are handled. In this simple example the card has no tuners to avoid making
1121 the code complex.
1122 </para>
1123 <programlisting>
1127 static int camera_ioctl(struct video_device *dev, unsigned int cmd, void *arg)
1128 {
1129 switch(cmd)
1130 {
1131 case VIDIOCGCAP:
1132 {
1133 struct video_capability v;
1134 v.type = VID_TYPE_CAPTURE|\
1135 VID_TYPE_CHROMAKEY|\
1136 VID_TYPE_SCALES|\
1137 VID_TYPE_OVERLAY;
1138 v.channels = 1;
1139 v.audios = 0;
1140 v.maxwidth = 640;
1141 v.minwidth = 16;
1142 v.maxheight = 480;
1143 v.minheight = 16;
1144 strcpy(v.name, "My Camera");
1145 if(copy_to_user(arg, &v, sizeof(v)))
1146 return -EFAULT;
1147 return 0;
1148 }
1151 </programlisting>
1152 <para>
1153 The first ioctl we must support and which all video capture and radio
1154 devices are required to support is VIDIOCGCAP. This behaves exactly the same
1155 as with a radio device. This time, however, we report the extra capabilities
1156 we outlined earlier on when defining our video_dev structure.
1157 </para>
1158 <para>
1159 We now set the video flags saying that we support overlay, capture,
1160 scaling and chromakey. We also report size limits - our smallest image is
1162 </para>
1163 <para>
1164 To keep things simple we report no audio and no tuning capabilities at all.
1165 </para>
1166 <programlisting>
1168 case VIDIOCGCHAN:
1169 {
1170 struct video_channel v;
1171 if(copy_from_user(&v, arg, sizeof(v)))
1172 return -EFAULT;
1173 if(v.channel != 0)
1174 return -EINVAL;
1175 v.flags = 0;
1176 v.tuners = 0;
1177 v.type = VIDEO_TYPE_CAMERA;
1178 v.norm = VIDEO_MODE_AUTO;
1179 strcpy(v.name, "Camera Input");break;
1180 if(copy_to_user(&v, arg, sizeof(v)))
1181 return -EFAULT;
1182 return 0;
1183 }
1186 </programlisting>
1187 <para>
1188 This follows what is very much the standard way an ioctl handler looks
1189 in Linux. We copy the data into a kernel space variable and we check that the
1190 request is valid (in this case that the input is 0). Finally we copy the
1191 camera info back to the user.
1192 </para>
1193 <para>
1194 The VIDIOCGCHAN ioctl allows a user to ask about video channels (that is
1195 inputs to the video card). Our example card has a single camera input. The
1196 fields in the structure are
1197 </para>
1200 <tbody>
1201 <row>
1204 </row><row>
1206 to describe the port to the user.
1207 Appropriate names are therefore things like
1209 </row><row>
1211 </row><row>
1213 </row><row>
1215 if relevant for this channel.
1216 </entry>
1217 </row>
1218 </tbody>
1219 </tgroup>
1220 </table>
1223 <tbody>
1224 <row>
1226 </row><row>
1228 </row>
1229 </tbody>
1230 </tgroup>
1231 </table>
1234 <tbody>
1235 <row>
1237 </row><row>
1239 </row><row>
1241 </row>
1242 </tbody>
1243 </tgroup>
1244 </table>
1247 <tbody>
1248 <row>
1250 </row><row>
1252 </row><row>
1254 </row><row>
1256 matter</entry>
1257 </row>
1258 </tbody>
1259 </tgroup>
1260 </table>
1261 <para>
1262 The corresponding VIDIOCSCHAN ioctl allows a user to change channel and to
1263 request the norm is changed - for example to switch between a PAL or an NTSC
1264 format camera.
1265 </para>
1266 <programlisting>
1269 case VIDIOCSCHAN:
1270 {
1271 struct video_channel v;
1272 if(copy_from_user(&v, arg, sizeof(v)))
1273 return -EFAULT;
1274 if(v.channel != 0)
1275 return -EINVAL;
1276 if(v.norm != VIDEO_MODE_AUTO)
1277 return -EINVAL;
1278 return 0;
1279 }
1282 </programlisting>
1283 <para>
1284 The implementation of this call in our driver is remarkably easy. Because we
1285 are assuming fixed format hardware we need only check that the user has not
1286 tried to change anything.
1287 </para>
1288 <para>
1289 The user also needs to be able to configure and adjust the picture they are
1290 seeing. This is much like adjusting a television set. A user application
1291 also needs to know the palette being used so that it knows how to display
1292 the image that has been captured. The VIDIOCGPICT and VIDIOCSPICT ioctl
1293 calls provide this information.
1294 </para>
1295 <programlisting>
1298 case VIDIOCGPICT
1299 {
1300 struct video_picture v;
1301 v.brightness = hardware_brightness();
1302 v.hue = hardware_hue();
1303 v.colour = hardware_saturation();
1304 v.contrast = hardware_brightness();
1305 /* Not settable */
1306 v.whiteness = 32768;
1308 v.palette = VIDEO_PALETTE_RGB24;
1309 if(copy_to_user(&v, arg,
1310 sizeof(v)))
1311 return -EFAULT;
1312 return 0;
1313 }
1316 </programlisting>
1317 <para>
1318 The brightness, hue, color, and contrast provide the picture controls that
1319 are akin to a conventional television. Whiteness provides additional
1321 and have 32768 as the mid point setting. The scaling means that applications
1322 do not have to worry about the capability range of the hardware but can let
1323 it make a best effort attempt.
1324 </para>
1325 <para>
1326 Our depth is 24, as this is in bits. We will be returning RGB24 format. This
1327 has one byte of red, then one of green, then one of blue. This then repeats
1328 for every other pixel in the image. The other common formats the interface
1329 defines are
1330 </para>
1333 <tbody>
1334 <row>
1336 like</entry>
1337 </row><row>
1340 </row><row>
1342 each occupy five bits.</entry>
1343 </row>
1344 </tbody>
1345 </tgroup>
1346 </table>
1347 <para>
1348 Additional modes are support for YUV capture formats. These are common for
1349 TV and video conferencing applications.
1350 </para>
1351 <para>
1352 The VIDIOCSPICT ioctl allows a user to set some of the picture parameters.
1353 Exactly which ones are supported depends heavily on the card itself. It is
1354 possible to support many modes and effects in software. In general doing
1355 this in the kernel is a bad idea. Video capture is a performance-sensitive
1356 application and the programs can often do better if they aren't being
1357 'helped' by an overkeen driver writer. Thus for our device we will report
1358 RGB24 only and refuse to allow a change.
1359 </para>
1360 <programlisting>
1363 case VIDIOCSPICT:
1364 {
1365 struct video_picture v;
1366 if(copy_from_user(&v, arg, sizeof(v)))
1367 return -EFAULT;
1368 if(v.depth!=24 ||
1369 v.palette != VIDEO_PALETTE_RGB24)
1370 return -EINVAL;
1371 set_hardware_brightness(v.brightness);
1372 set_hardware_hue(v.hue);
1373 set_hardware_saturation(v.colour);
1374 set_hardware_brightness(v.contrast);
1375 return 0;
1376 }
1379 </programlisting>
1380 <para>
1381 We check the user has not tried to change the palette or the depth. We do
1382 not want to carry out some of the changes and then return an error. This may
1383 confuse the application which will be assuming no change occurred.
1384 </para>
1385 <para>
1386 In much the same way as you need to be able to set the picture controls to
1387 get the right capture images, many cards need to know what they are
1388 displaying onto when generating overlay output. In some cases getting this
1389 wrong even makes a nasty mess or may crash the computer. For that reason
1390 the VIDIOCSBUF ioctl used to set up the frame buffer information may well
1391 only be usable by root.
1392 </para>
1393 <para>
1394 We will assume our card is one of the old ISA devices with feature connector
1395 and only supports a couple of standard video modes. Very common for older
1396 cards although the PCI devices are way smarter than this.
1397 </para>
1398 <programlisting>
1401 static struct video_buffer capture_fb;
1403 case VIDIOCGFBUF:
1404 {
1405 if(copy_to_user(arg, &capture_fb,
1406 sizeof(capture_fb)))
1407 return -EFAULT;
1408 return 0;
1410 }
1413 </programlisting>
1414 <para>
1415 We keep the frame buffer information in the format the ioctl uses. This
1416 makes it nice and easy to work with in the ioctl calls.
1417 </para>
1418 <programlisting>
1420 case VIDIOCSFBUF:
1421 {
1422 struct video_buffer v;
1424 if(!capable(CAP_SYS_ADMIN))
1425 return -EPERM;
1427 if(copy_from_user(&v, arg, sizeof(v)))
1428 return -EFAULT;
1430 return -EINVAL;
1434 return -EINVAL;
1436 hardware_set_fb(&v);
1437 return 0;
1438 }
1442 </programlisting>
1443 <para>
1444 The capable() function checks a user has the required capability. The Linux
1445 operating system has a set of about 30 capabilities indicating privileged
1446 access to services. The default set up gives the superuser (uid 0) all of
1447 them and nobody else has any.
1448 </para>
1449 <para>
1450 We check that the user has the SYS_ADMIN capability, that is they are
1451 allowed to operate as the machine administrator. We don't want anyone but
1452 the administrator making a mess of the display.
1453 </para>
1454 <para>
1456 EGA or VGA depths). If the mode is not a standard video mode we reject it as
1457 not supported by our card. If the mode is acceptable we save it so that
1458 VIDIOCFBUF will give the right answer next time it is called. The
1459 hardware_set_fb() function is some undescribed card specific function to
1460 program the card for the desired mode.
1461 </para>
1462 <para>
1463 Before the driver can display an overlay window it needs to know where the
1464 window should be placed, and also how large it should be. If the card
1465 supports clipping it needs to know which rectangles to omit from the
1466 display. The video_window structure is used to describe the way the image
1467 should be displayed.
1468 </para>
1471 <tbody>
1472 <row>
1474 may use a smaller size if this size is not available</entry>
1475 </row><row>
1477 size if this size is not available.</entry>
1478 </row><row>
1480 is in pixels relative to the left hand edge of the
1481 picture. Not all cards can display images aligned on
1482 any pixel boundary. If the position is unsuitable
1483 the card adjusts the image right and reduces the
1484 width.</entry>
1485 </row><row>
1487 is counted in pixels relative to the top edge of the
1488 picture. As with the width if the card cannot
1489 display starting on this line it will adjust the
1490 values.</entry>
1491 </row><row>
1493 chromakey colour if chroma keying is being used. </entry>
1494 </row><row>
1496 over.</entry>
1497 </row><row>
1499 </row>
1500 </tbody>
1501 </tgroup>
1502 </table>
1503 <para>
1504 Each clip is a struct video_clip which has the following fields
1505 </para>
1508 <tbody>
1509 <row>
1511 </row><row>
1513 </row><row>
1515 </row>
1516 </tbody>
1517 </tgroup>
1518 </table>
1519 <para>
1520 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.
1521 This may well mean it has to leave alone. small areas the application wished to be
1522 drawn.
1523 </para>
1524 <para>
1525 Our example card uses chromakey so does not have to address most of the
1526 clipping. We will add a video_window structure to our global variables to
1527 remember our parameters, as we did with the frame buffer.
1528 </para>
1529 <programlisting>
1532 case VIDIOCGWIN:
1533 {
1534 if(copy_to_user(arg, &capture_win,
1535 sizeof(capture_win)))
1536 return -EFAULT;
1537 return 0;
1538 }
1541 case VIDIOCSWIN:
1542 {
1543 struct video_window v;
1544 if(copy_from_user(&v, arg, sizeof(v)))
1545 return -EFAULT;
1547 return -EINVAL;
1549 return -EINVAL;
1550 hardware_set_key(v.chromakey);
1551 hardware_set_window(v);
1553 capture_w = v.width;
1554 capture_h = v.height;
1555 return 0;
1556 }
1559 </programlisting>
1560 <para>
1561 Because we are using Chromakey our setup is fairly simple. Mostly we have to
1562 check the values are sane and load them into the capture card.
1563 </para>
1564 <para>
1565 With all the setup done we can now turn on the actual capture/overlay. This
1566 is done with the VIDIOCCAPTURE ioctl. This takes a single integer argument
1568 </para>
1569 <programlisting>
1572 case VIDIOCCAPTURE:
1573 {
1574 int v;
1575 if(get_user(v, (int *)arg))
1576 return -EFAULT;
1577 if(v==0)
1578 hardware_capture_off();
1579 else
1580 {
1581 if(capture_fb.width == 0
1582 || capture_w == 0)
1583 return -EINVAL;
1584 hardware_capture_on();
1585 }
1586 return 0;
1587 }
1590 </programlisting>
1591 <para>
1592 We grab the flag from user space and either enable or disable according to
1593 its value. There is one small corner case we have to consider here. Suppose
1594 that the capture was requested before the video window or the frame buffer
1595 had been set up. In those cases there will be unconfigured fields in our
1596 card data, as well as unconfigured hardware settings. We check for this case and
1597 return an error if the frame buffer or the capture window width is zero.
1598 </para>
1599 <programlisting>
1602 default:
1603 return -ENOIOCTLCMD;
1604 }
1605 }
1606 </programlisting>
1607 <para>
1609 We don't need to support any other ioctls, so if we get this far, it is time
1610 to tell the video layer that we don't now what the user is talking about.
1611 </para>
1612 </sect1>
1615 <para>
1616 The Video4Linux layer supports additional features, including a high
1617 performance mmap() based capture mode and capturing part of the image.
1618 These features are out of the scope of the book. You should however have enough
1619 example code to implement most simple video4linux devices for radio and TV
1620 cards.
1621 </para>
1622 </sect1>
1623 </chapter>
1626 <para>
1627 <variablelist>
1629 <listitem>
1630 <para>
1631 The driver assumes multiple opens should not be allowed. A driver
1632 can work around this but not cleanly.
1633 </para>
1634 </listitem></varlistentry>
1637 <listitem>
1638 <para>
1639 The existing API poorly reflects compression capable devices. There
1640 are plans afoot to merge V4L, V4L2 and some other ideas into a
1641 better interface.
1642 </para>
1643 </listitem></varlistentry>
1644 </variablelist>
1646 </para>
1647 </chapter>
1651 !Edrivers/media/video/videodev.c
1652 </chapter>
1654 </book>