+++ /dev/null
-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
- "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
-
-<book id="V4LGuide">
- <bookinfo>
- <title>Video4Linux Programming</title>
-
- <authorgroup>
- <author>
- <firstname>Alan</firstname>
- <surname>Cox</surname>
- <affiliation>
- <address>
- <email>alan@redhat.com</email>
- </address>
- </affiliation>
- </author>
- </authorgroup>
-
- <copyright>
- <year>2000</year>
- <holder>Alan Cox</holder>
- </copyright>
-
- <legalnotice>
- <para>
- This documentation is free software; you can redistribute
- it and/or modify it under the terms of the GNU General Public
- License as published by the Free Software Foundation; either
- version 2 of the License, or (at your option) any later
- version.
- </para>
-
- <para>
- This program is distributed in the hope that it will be
- useful, but WITHOUT ANY WARRANTY; without even the implied
- warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
- See the GNU General Public License for more details.
- </para>
-
- <para>
- You should have received a copy of the GNU General Public
- License along with this program; if not, write to the Free
- Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
- MA 02111-1307 USA
- </para>
-
- <para>
- For more details see the file COPYING in the source
- distribution of Linux.
- </para>
- </legalnotice>
- </bookinfo>
-
-<toc></toc>
-
- <chapter id="intro">
- <title>Introduction</title>
- <para>
- Parts of this document first appeared in Linux Magazine under a
- ninety day exclusivity.
- </para>
- <para>
- Video4Linux is intended to provide a common programming interface
- for the many TV and capture cards now on the market, as well as
- parallel port and USB video cameras. Radio, teletext decoders and
- vertical blanking data interfaces are also provided.
- </para>
- </chapter>
- <chapter id="radio">
- <title>Radio Devices</title>
- <para>
- There are a wide variety of radio interfaces available for PC's, and these
- are generally very simple to program. The biggest problem with supporting
- such devices is normally extracting documentation from the vendor.
- </para>
- <para>
- The radio interface supports a simple set of control ioctls standardised
- across all radio and tv interfaces. It does not support read or write, which
- are used for video streams. The reason radio cards do not allow you to read
- the audio stream into an application is that without exception they provide
- a connection on to a soundcard. Soundcards can be used to read the radio
- data just fine.
- </para>
- <sect1 id="registerradio">
- <title>Registering Radio Devices</title>
- <para>
- The Video4linux core provides an interface for registering devices. The
- first step in writing our radio card driver is to register it.
- </para>
- <programlisting>
-
-
-static struct video_device my_radio
-{
- "My radio",
- VID_TYPE_TUNER,
- radio_open.
- radio_close,
- NULL, /* no read */
- NULL, /* no write */
- NULL, /* no poll */
- radio_ioctl,
- NULL, /* no special init function */
- NULL /* no private data */
-};
-
-
- </programlisting>
- <para>
- This declares our video4linux device driver interface. The VID_TYPE_ value
- defines what kind of an interface we are, and defines basic capabilities.
- </para>
- <para>
- The only defined value relevant for a radio card is VID_TYPE_TUNER which
- indicates that the device can be tuned. Clearly our radio is going to have some
- way to change channel so it is tuneable.
- </para>
- <para>
- We declare an open and close routine, but we do not need read or write,
- which are used to read and write video data to or from the card itself. As
- we have no read or write there is no poll function.
- </para>
- <para>
- The private initialise function is run when the device is registered. In
- this driver we've already done all the work needed. The final pointer is a
- private data pointer that can be used by the device driver to attach and
- retrieve private data structures. We set this field "priv" to NULL for
- the moment.
- </para>
- <para>
- Having the structure defined is all very well but we now need to register it
- with the kernel.
- </para>
- <programlisting>
-
-
-static int io = 0x320;
-
-int __init myradio_init(struct video_init *v)
-{
- if(!request_region(io, MY_IO_SIZE, "myradio"))
- {
- printk(KERN_ERR
- "myradio: port 0x%03X is in use.\n", io);
- return -EBUSY;
- }
-
- if(video_device_register(&my_radio, VFL_TYPE_RADIO)==-1) {
- release_region(io, MY_IO_SIZE);
- return -EINVAL;
- }
- return 0;
-}
-
- </programlisting>
- <para>
- The first stage of the initialisation, as is normally the case, is to check
- that the I/O space we are about to fiddle with doesn't belong to some other
- driver. If it is we leave well alone. If the user gives the address of the
- wrong device then we will spot this. These policies will generally avoid
- crashing the machine.
- </para>
- <para>
- Now we ask the Video4Linux layer to register the device for us. We hand it
- our carefully designed video_device structure and also tell it which group
- of devices we want it registered with. In this case VFL_TYPE_RADIO.
- </para>
- <para>
- The types available are
- </para>
- <table frame="all" id="Device_Types"><title>Device Types</title>
- <tgroup cols="3" align="left">
- <tbody>
- <row>
- <entry>VFL_TYPE_RADIO</entry><entry>/dev/radio{n}</entry><entry>
-
- Radio devices are assigned in this block. As with all of these
- selections the actual number assignment is done by the video layer
- accordijng to what is free.</entry>
- </row><row>
- <entry>VFL_TYPE_GRABBER</entry><entry>/dev/video{n}</entry><entry>
- Video capture devices and also -- counter-intuitively for the name --
- hardware video playback devices such as MPEG2 cards.</entry>
- </row><row>
- <entry>VFL_TYPE_VBI</entry><entry>/dev/vbi{n}</entry><entry>
- The VBI devices capture the hidden lines on a television picture
- that carry further information like closed caption data, teletext
- (primarily in Europe) and now Intercast and the ATVEC internet
- television encodings.</entry>
- </row><row>
- <entry>VFL_TYPE_VTX</entry><entry>/dev/vtx[n}</entry><entry>
- VTX is 'Videotext' also known as 'Teletext'. This is a system for
- sending numbered, 40x25, mostly textual page images over the hidden
- lines. Unlike the /dev/vbi interfaces, this is for 'smart' decoder
- chips. (The use of the word smart here has to be taken in context,
- the smartest teletext chips are fairly dumb pieces of technology).
- </entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <para>
- We are most definitely a radio.
- </para>
- <para>
- Finally we allocate our I/O space so that nobody treads on us and return 0
- to signify general happiness with the state of the universe.
- </para>
- </sect1>
- <sect1 id="openradio">
- <title>Opening And Closing The Radio</title>
-
- <para>
- The functions we declared in our video_device are mostly very simple.
- Firstly we can drop in what is basically standard code for open and close.
- </para>
- <programlisting>
-
-
-static int users = 0;
-
-static int radio_open(struct video_device *dev, int flags)
-{
- if(users)
- return -EBUSY;
- users++;
- return 0;
-}
-
- </programlisting>
- <para>
- At open time we need to do nothing but check if someone else is also using
- the radio card. If nobody is using it we make a note that we are using it,
- then we ensure that nobody unloads our driver on us.
- </para>
- <programlisting>
-
-
-static int radio_close(struct video_device *dev)
-{
- users--;
-}
-
- </programlisting>
- <para>
- At close time we simply need to reduce the user count and allow the module
- to become unloadable.
- </para>
- <para>
- If you are sharp you will have noticed neither the open nor the close
- routines attempt to reset or change the radio settings. This is intentional.
- It allows an application to set up the radio and exit. It avoids a user
- having to leave an application running all the time just to listen to the
- radio.
- </para>
- </sect1>
- <sect1 id="ioctlradio">
- <title>The Ioctl Interface</title>
- <para>
- This leaves the ioctl routine, without which the driver will not be
- terribly useful to anyone.
- </para>
- <programlisting>
-
-
-static int radio_ioctl(struct video_device *dev, unsigned int cmd, void *arg)
-{
- switch(cmd)
- {
- case VIDIOCGCAP:
- {
- struct video_capability v;
- v.type = VID_TYPE_TUNER;
- v.channels = 1;
- v.audios = 1;
- v.maxwidth = 0;
- v.minwidth = 0;
- v.maxheight = 0;
- v.minheight = 0;
- strcpy(v.name, "My Radio");
- if(copy_to_user(arg, &v, sizeof(v)))
- return -EFAULT;
- return 0;
- }
-
- </programlisting>
- <para>
- VIDIOCGCAP is the first ioctl all video4linux devices must support. It
- allows the applications to find out what sort of a card they have found and
- to figure out what they want to do about it. The fields in the structure are
- </para>
- <table frame="all" id="video_capability_fields"><title>struct video_capability fields</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>name</entry><entry>The device text name. This is intended for the user.</entry>
- </row><row>
- <entry>channels</entry><entry>The number of different channels you can tune on
- this card. It could even by zero for a card that has
- no tuning capability. For our simple FM radio it is 1.
- An AM/FM radio would report 2.</entry>
- </row><row>
- <entry>audios</entry><entry>The number of audio inputs on this device. For our
- radio there is only one audio input.</entry>
- </row><row>
- <entry>minwidth,minheight</entry><entry>The smallest size the card is capable of capturing
- images in. We set these to zero. Radios do not
- capture pictures</entry>
- </row><row>
- <entry>maxwidth,maxheight</entry><entry>The largest image size the card is capable of
- capturing. For our radio we report 0.
- </entry>
- </row><row>
- <entry>type</entry><entry>This reports the capabilities of the device, and
- matches the field we filled in in the struct
- video_device when registering.</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <para>
- Having filled in the fields, we use copy_to_user to copy the structure into
- the users buffer. If the copy fails we return an EFAULT to the application
- so that it knows it tried to feed us garbage.
- </para>
- <para>
- The next pair of ioctl operations select which tuner is to be used and let
- the application find the tuner properties. We have only a single FM band
- tuner in our example device.
- </para>
- <programlisting>
-
-
- case VIDIOCGTUNER:
- {
- struct video_tuner v;
- if(copy_from_user(&v, arg, sizeof(v))!=0)
- return -EFAULT;
- if(v.tuner)
- return -EINVAL;
- v.rangelow=(87*16000);
- v.rangehigh=(108*16000);
- v.flags = VIDEO_TUNER_LOW;
- v.mode = VIDEO_MODE_AUTO;
- v.signal = 0xFFFF;
- strcpy(v.name, "FM");
- if(copy_to_user(&v, arg, sizeof(v))!=0)
- return -EFAULT;
- return 0;
- }
-
- </programlisting>
- <para>
- The VIDIOCGTUNER ioctl allows applications to query a tuner. The application
- sets the tuner field to the tuner number it wishes to query. The query does
- not change the tuner that is being used, it merely enquires about the tuner
- in question.
- </para>
- <para>
- We have exactly one tuner so after copying the user buffer to our temporary
- structure we complain if they asked for a tuner other than tuner 0.
- </para>
- <para>
- The video_tuner structure has the following fields
- </para>
- <table frame="all" id="video_tuner_fields"><title>struct video_tuner fields</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>int tuner</entry><entry>The number of the tuner in question</entry>
- </row><row>
- <entry>char name[32]</entry><entry>A text description of this tuner. "FM" will do fine.
- This is intended for the application.</entry>
- </row><row>
- <entry>u32 flags</entry>
- <entry>Tuner capability flags</entry>
- </row>
- <row>
- <entry>u16 mode</entry><entry>The current reception mode</entry>
-
- </row><row>
- <entry>u16 signal</entry><entry>The signal strength scaled between 0 and 65535. If
- a device cannot tell the signal strength it should
- report 65535. Many simple cards contain only a
- signal/no signal bit. Such cards will report either
- 0 or 65535.</entry>
-
- </row><row>
- <entry>u32 rangelow, rangehigh</entry><entry>
- The range of frequencies supported by the radio
- or TV. It is scaled according to the VIDEO_TUNER_LOW
- flag.</entry>
-
- </row>
- </tbody>
- </tgroup>
- </table>
-
- <table frame="all" id="video_tuner_flags"><title>struct video_tuner flags</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>VIDEO_TUNER_PAL</entry><entry>A PAL TV tuner</entry>
- </row><row>
- <entry>VIDEO_TUNER_NTSC</entry><entry>An NTSC (US) TV tuner</entry>
- </row><row>
- <entry>VIDEO_TUNER_SECAM</entry><entry>A SECAM (French) TV tuner</entry>
- </row><row>
- <entry>VIDEO_TUNER_LOW</entry><entry>
- The tuner frequency is scaled in 1/16th of a KHz
- steps. If not it is in 1/16th of a MHz steps
- </entry>
- </row><row>
- <entry>VIDEO_TUNER_NORM</entry><entry>The tuner can set its format</entry>
- </row><row>
- <entry>VIDEO_TUNER_STEREO_ON</entry><entry>The tuner is currently receiving a stereo signal</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
-
- <table frame="all" id="video_tuner_modes"><title>struct video_tuner modes</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>VIDEO_MODE_PAL</entry><entry>PAL Format</entry>
- </row><row>
- <entry>VIDEO_MODE_NTSC</entry><entry>NTSC Format (USA)</entry>
- </row><row>
- <entry>VIDEO_MODE_SECAM</entry><entry>French Format</entry>
- </row><row>
- <entry>VIDEO_MODE_AUTO</entry><entry>A device that does not need to do
- TV format switching</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <para>
- The settings for the radio card are thus fairly simple. We report that we
- are a tuner called "FM" for FM radio. In order to get the best tuning
- resolution we report VIDEO_TUNER_LOW and select tuning to 1/16th of KHz. Its
- unlikely our card can do that resolution but it is a fair bet the card can
- do better than 1/16th of a MHz. VIDEO_TUNER_LOW is appropriate to almost all
- radio usage.
- </para>
- <para>
- We report that the tuner automatically handles deciding what format it is
- receiving - true enough as it only handles FM radio. Our example card is
- also incapable of detecting stereo or signal strengths so it reports a
- strength of 0xFFFF (maximum) and no stereo detected.
- </para>
- <para>
- To finish off we set the range that can be tuned to be 87-108Mhz, the normal
- FM broadcast radio range. It is important to find out what the card is
- actually capable of tuning. It is easy enough to simply use the FM broadcast
- range. Unfortunately if you do this you will discover the FM broadcast
- ranges in the USA, Europe and Japan are all subtly different and some users
- cannot receive all the stations they wish.
- </para>
- <para>
- The application also needs to be able to set the tuner it wishes to use. In
- our case, with a single tuner this is rather simple to arrange.
- </para>
- <programlisting>
-
- case VIDIOCSTUNER:
- {
- struct video_tuner v;
- if(copy_from_user(&v, arg, sizeof(v)))
- return -EFAULT;
- if(v.tuner != 0)
- return -EINVAL;
- return 0;
- }
-
- </programlisting>
- <para>
- We copy the user supplied structure into kernel memory so we can examine it.
- If the user has selected a tuner other than zero we reject the request. If
- they wanted tuner 0 then, surprisingly enough, that is the current tuner already.
- </para>
- <para>
- The next two ioctls we need to provide are to get and set the frequency of
- the radio. These both use an unsigned long argument which is the frequency.
- The scale of the frequency depends on the VIDEO_TUNER_LOW flag as I
- mentioned earlier on. Since we have VIDEO_TUNER_LOW set this will be in
- 1/16ths of a KHz.
- </para>
- <programlisting>
-
-static unsigned long current_freq;
-
-
-
- case VIDIOCGFREQ:
- if(copy_to_user(arg, &current_freq,
- sizeof(unsigned long))
- return -EFAULT;
- return 0;
-
- </programlisting>
- <para>
- Querying the frequency in our case is relatively simple. Our radio card is
- too dumb to let us query the signal strength so we remember our setting if
- we know it. All we have to do is copy it to the user.
- </para>
- <programlisting>
-
-
- case VIDIOCSFREQ:
- {
- u32 freq;
- if(copy_from_user(arg, &freq,
- sizeof(unsigned long))!=0)
- return -EFAULT;
- if(hardware_set_freq(freq)<0)
- return -EINVAL;
- current_freq = freq;
- return 0;
- }
-
- </programlisting>
- <para>
- Setting the frequency is a little more complex. We begin by copying the
- desired frequency into kernel space. Next we call a hardware specific routine
- to set the radio up. This might be as simple as some scaling and a few
- writes to an I/O port. For most radio cards it turns out a good deal more
- complicated and may involve programming things like a phase locked loop on
- the card. This is what documentation is for.
- </para>
- <para>
- The final set of operations we need to provide for our radio are the
- volume controls. Not all radio cards can even do volume control. After all
- there is a perfectly good volume control on the sound card. We will assume
- our radio card has a simple 4 step volume control.
- </para>
- <para>
- There are two ioctls with audio we need to support
- </para>
- <programlisting>
-
-static int current_volume=0;
-
- case VIDIOCGAUDIO:
- {
- struct video_audio v;
- if(copy_from_user(&v, arg, sizeof(v)))
- return -EFAULT;
- if(v.audio != 0)
- return -EINVAL;
- v.volume = 16384*current_volume;
- v.step = 16384;
- strcpy(v.name, "Radio");
- v.mode = VIDEO_SOUND_MONO;
- v.balance = 0;
- v.base = 0;
- v.treble = 0;
-
- if(copy_to_user(arg. &v, sizeof(v)))
- return -EFAULT;
- return 0;
- }
-
- </programlisting>
- <para>
- Much like the tuner we start by copying the user structure into kernel
- space. Again we check if the user has asked for a valid audio input. We have
- only input 0 and we punt if they ask for another input.
- </para>
- <para>
- Then we fill in the video_audio structure. This has the following format
- </para>
- <table frame="all" id="video_audio_fields"><title>struct video_audio fields</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>audio</entry><entry>The input the user wishes to query</entry>
- </row><row>
- <entry>volume</entry><entry>The volume setting on a scale of 0-65535</entry>
- </row><row>
- <entry>base</entry><entry>The base level on a scale of 0-65535</entry>
- </row><row>
- <entry>treble</entry><entry>The treble level on a scale of 0-65535</entry>
- </row><row>
- <entry>flags</entry><entry>The features this audio device supports
- </entry>
- </row><row>
- <entry>name</entry><entry>A text name to display to the user. We picked
- "Radio" as it explains things quite nicely.</entry>
- </row><row>
- <entry>mode</entry><entry>The current reception mode for the audio
-
- We report MONO because our card is too stupid to know if it is in
- mono or stereo.
- </entry>
- </row><row>
- <entry>balance</entry><entry>The stereo balance on a scale of 0-65535, 32768 is
- middle.</entry>
- </row><row>
- <entry>step</entry><entry>The step by which the volume control jumps. This is
- used to help make it easy for applications to set
- slider behaviour.</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
-
- <table frame="all" id="video_audio_flags"><title>struct video_audio flags</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>VIDEO_AUDIO_MUTE</entry><entry>The audio is currently muted. We
- could fake this in our driver but we
- choose not to bother.</entry>
- </row><row>
- <entry>VIDEO_AUDIO_MUTABLE</entry><entry>The input has a mute option</entry>
- </row><row>
- <entry>VIDEO_AUDIO_TREBLE</entry><entry>The input has a treble control</entry>
- </row><row>
- <entry>VIDEO_AUDIO_BASS</entry><entry>The input has a base control</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
-
- <table frame="all" id="video_audio_modes"><title>struct video_audio modes</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>VIDEO_SOUND_MONO</entry><entry>Mono sound</entry>
- </row><row>
- <entry>VIDEO_SOUND_STEREO</entry><entry>Stereo sound</entry>
- </row><row>
- <entry>VIDEO_SOUND_LANG1</entry><entry>Alternative language 1 (TV specific)</entry>
- </row><row>
- <entry>VIDEO_SOUND_LANG2</entry><entry>Alternative language 2 (TV specific)</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <para>
- Having filled in the structure we copy it back to user space.
- </para>
- <para>
- The VIDIOCSAUDIO ioctl allows the user to set the audio parameters in the
- video_audio structure. The driver does its best to honour the request.
- </para>
- <programlisting>
-
- case VIDIOCSAUDIO:
- {
- struct video_audio v;
- if(copy_from_user(&v, arg, sizeof(v)))
- return -EFAULT;
- if(v.audio)
- return -EINVAL;
- current_volume = v/16384;
- hardware_set_volume(current_volume);
- return 0;
- }
-
- </programlisting>
- <para>
- In our case there is very little that the user can set. The volume is
- basically the limit. Note that we could pretend to have a mute feature
- by rewriting this to
- </para>
- <programlisting>
-
- case VIDIOCSAUDIO:
- {
- struct video_audio v;
- if(copy_from_user(&v, arg, sizeof(v)))
- return -EFAULT;
- if(v.audio)
- return -EINVAL;
- current_volume = v/16384;
- if(v.flags&VIDEO_AUDIO_MUTE)
- hardware_set_volume(0);
- else
- hardware_set_volume(current_volume);
- current_muted = v.flags &
- VIDEO_AUDIO_MUTE;
- return 0;
- }
-
- </programlisting>
- <para>
- This with the corresponding changes to the VIDIOCGAUDIO code to report the
- state of the mute flag we save and to report the card has a mute function,
- will allow applications to use a mute facility with this card. It is
- questionable whether this is a good idea however. User applications can already
- fake this themselves and kernel space is precious.
- </para>
- <para>
- We now have a working radio ioctl handler. So we just wrap up the function
- </para>
- <programlisting>
-
-
- }
- return -ENOIOCTLCMD;
-}
-
- </programlisting>
- <para>
- and pass the Video4Linux layer back an error so that it knows we did not
- understand the request we got passed.
- </para>
- </sect1>
- <sect1 id="modradio">
- <title>Module Wrapper</title>
- <para>
- Finally we add in the usual module wrapping and the driver is done.
- </para>
- <programlisting>
-
-#ifndef MODULE
-
-static int io = 0x300;
-
-#else
-
-static int io = -1;
-
-#endif
-
-MODULE_AUTHOR("Alan Cox");
-MODULE_DESCRIPTION("A driver for an imaginary radio card.");
-module_param(io, int, 0444);
-MODULE_PARM_DESC(io, "I/O address of the card.");
-
-static int __init init(void)
-{
- if(io==-1)
- {
- printk(KERN_ERR
- "You must set an I/O address with io=0x???\n");
- return -EINVAL;
- }
- return myradio_init(NULL);
-}
-
-static void __exit cleanup(void)
-{
- video_unregister_device(&my_radio);
- release_region(io, MY_IO_SIZE);
-}
-
-module_init(init);
-module_exit(cleanup);
-
- </programlisting>
- <para>
- In this example we set the IO base by default if the driver is compiled into
- 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
- user sets the parameter. We set io to a nonsense port (-1) so that we can
- tell if the user supplied an io parameter or not.
- </para>
- <para>
- We use MODULE_ defines to give an author for the card driver and a
- description. We also use them to declare that io is an integer and it is the
- address of the card, and can be read by anyone from sysfs.
- </para>
- <para>
- The clean-up routine unregisters the video_device we registered, and frees
- up the I/O space. Note that the unregister takes the actual video_device
- structure as its argument. Unlike the file operations structure which can be
- shared by all instances of a device a video_device structure as an actual
- instance of the device. If you are registering multiple radio devices you
- need to fill in one structure per device (most likely by setting up a
- template and copying it to each of the actual device structures).
- </para>
- </sect1>
- </chapter>
- <chapter id="Video_Capture_Devices">
- <title>Video Capture Devices</title>
- <sect1 id="introvid">
- <title>Video Capture Device Types</title>
- <para>
- The video capture devices share the same interfaces as radio devices. In
- order to explain the video capture interface I will use the example of a
- camera that has no tuners or audio input. This keeps the example relatively
- clean. To get both combine the two driver examples.
- </para>
- <para>
- Video capture devices divide into four categories. A little technology
- backgrounder. Full motion video even at television resolution (which is
- actually fairly low) is pretty resource-intensive. You are continually
- passing megabytes of data every second from the capture card to the display.
- several alternative approaches have emerged because copying this through the
- processor and the user program is a particularly bad idea .
- </para>
- <para>
- The first is to add the television image onto the video output directly.
- This is also how some 3D cards work. These basic cards can generally drop the
- video into any chosen rectangle of the display. Cards like this, which
- include most mpeg1 cards that used the feature connector, aren't very
- friendly in a windowing environment. They don't understand windows or
- clipping. The video window is always on the top of the display.
- </para>
- <para>
- Chroma keying is a technique used by cards to get around this. It is an old
- television mixing trick where you mark all the areas you wish to replace
- with a single clear colour that isn't used in the image - TV people use an
- incredibly bright blue while computing people often use a particularly
- virulent purple. Bright blue occurs on the desktop. Anyone with virulent
- purple windows has another problem besides their TV overlay.
- </para>
- <para>
- The third approach is to copy the data from the capture card to the video
- card, but to do it directly across the PCI bus. This relieves the processor
- from doing the work but does require some smartness on the part of the video
- capture chip, as well as a suitable video card. Programming this kind of
- card and more so debugging it can be extremely tricky. There are some quite
- complicated interactions with the display and you may also have to cope with
- various chipset bugs that show up when PCI cards start talking to each
- other.
- </para>
- <para>
- To keep our example fairly simple we will assume a card that supports
- overlaying a flat rectangular image onto the frame buffer output, and which
- can also capture stuff into processor memory.
- </para>
- </sect1>
- <sect1 id="regvid">
- <title>Registering Video Capture Devices</title>
- <para>
- This time we need to add more functions for our camera device.
- </para>
- <programlisting>
-static struct video_device my_camera
-{
- "My Camera",
- VID_TYPE_OVERLAY|VID_TYPE_SCALES|\
- VID_TYPE_CAPTURE|VID_TYPE_CHROMAKEY,
- camera_open.
- camera_close,
- camera_read, /* no read */
- NULL, /* no write */
- camera_poll, /* no poll */
- camera_ioctl,
- NULL, /* no special init function */
- NULL /* no private data */
-};
- </programlisting>
- <para>
- We need a read() function which is used for capturing data from
- the card, and we need a poll function so that a driver can wait for the next
- frame to be captured.
- </para>
- <para>
- We use the extra video capability flags that did not apply to the
- radio interface. The video related flags are
- </para>
- <table frame="all" id="Capture_Capabilities"><title>Capture Capabilities</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
-<entry>VID_TYPE_CAPTURE</entry><entry>We support image capture</entry>
-</row><row>
-<entry>VID_TYPE_TELETEXT</entry><entry>A teletext capture device (vbi{n])</entry>
-</row><row>
-<entry>VID_TYPE_OVERLAY</entry><entry>The image can be directly overlaid onto the
- frame buffer</entry>
-</row><row>
-<entry>VID_TYPE_CHROMAKEY</entry><entry>Chromakey can be used to select which parts
- of the image to display</entry>
-</row><row>
-<entry>VID_TYPE_CLIPPING</entry><entry>It is possible to give the board a list of
- rectangles to draw around. </entry>
-</row><row>
-<entry>VID_TYPE_FRAMERAM</entry><entry>The video capture goes into the video memory
- and actually changes it. Applications need
- to know this so they can clean up after the
- card</entry>
-</row><row>
-<entry>VID_TYPE_SCALES</entry><entry>The image can be scaled to various sizes,
- rather than being a single fixed size.</entry>
-</row><row>
-<entry>VID_TYPE_MONOCHROME</entry><entry>The capture will be monochrome. This isn't a
- complete answer to the question since a mono
- camera on a colour capture card will still
- produce mono output.</entry>
-</row><row>
-<entry>VID_TYPE_SUBCAPTURE</entry><entry>The card allows only part of its field of
- view to be captured. This enables
- applications to avoid copying all of a large
- image into memory when only some section is
- relevant.</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <para>
- We set VID_TYPE_CAPTURE so that we are seen as a capture card,
- VID_TYPE_CHROMAKEY so the application knows it is time to draw in virulent
- purple, and VID_TYPE_SCALES because we can be resized.
- </para>
- <para>
- Our setup is fairly similar. This time we also want an interrupt line
- for the 'frame captured' signal. Not all cards have this so some of them
- cannot handle poll().
- </para>
- <programlisting>
-
-
-static int io = 0x320;
-static int irq = 11;
-
-int __init mycamera_init(struct video_init *v)
-{
- if(!request_region(io, MY_IO_SIZE, "mycamera"))
- {
- printk(KERN_ERR
- "mycamera: port 0x%03X is in use.\n", io);
- return -EBUSY;
- }
-
- if(video_device_register(&my_camera,
- VFL_TYPE_GRABBER)==-1) {
- release_region(io, MY_IO_SIZE);
- return -EINVAL;
- }
- return 0;
-}
-
- </programlisting>
- <para>
- This is little changed from the needs of the radio card. We specify
- VFL_TYPE_GRABBER this time as we want to be allocated a /dev/video name.
- </para>
- </sect1>
- <sect1 id="opvid">
- <title>Opening And Closing The Capture Device</title>
- <programlisting>
-
-
-static int users = 0;
-
-static int camera_open(struct video_device *dev, int flags)
-{
- if(users)
- return -EBUSY;
- if(request_irq(irq, camera_irq, 0, "camera", dev)<0)
- return -EBUSY;
- users++;
- return 0;
-}
-
-
-static int camera_close(struct video_device *dev)
-{
- users--;
- free_irq(irq, dev);
-}
- </programlisting>
- <para>
- The open and close routines are also quite similar. The only real change is
- that we now request an interrupt for the camera device interrupt line. If we
- cannot get the interrupt we report EBUSY to the application and give up.
- </para>
- </sect1>
- <sect1 id="irqvid">
- <title>Interrupt Handling</title>
- <para>
- Our example handler is for an ISA bus device. If it was PCI you would be
- able to share the interrupt and would have set IRQF_SHARED to indicate a
- shared IRQ. We pass the device pointer as the interrupt routine argument. We
- don't need to since we only support one card but doing this will make it
- easier to upgrade the driver for multiple devices in the future.
- </para>
- <para>
- Our interrupt routine needs to do little if we assume the card can simply
- queue one frame to be read after it captures it.
- </para>
- <programlisting>
-
-
-static struct wait_queue *capture_wait;
-static int capture_ready = 0;
-
-static void camera_irq(int irq, void *dev_id,
- struct pt_regs *regs)
-{
- capture_ready=1;
- wake_up_interruptible(&capture_wait);
-}
- </programlisting>
- <para>
- The interrupt handler is nice and simple for this card as we are assuming
- the card is buffering the frame for us. This means we have little to do but
- wake up anybody interested. We also set a capture_ready flag, as we may
- capture a frame before an application needs it. In this case we need to know
- that a frame is ready. If we had to collect the frame on the interrupt life
- would be more complex.
- </para>
- <para>
- The two new routines we need to supply are camera_read which returns a
- frame, and camera_poll which waits for a frame to become ready.
- </para>
- <programlisting>
-
-
-static int camera_poll(struct video_device *dev,
- struct file *file, struct poll_table *wait)
-{
- poll_wait(file, &capture_wait, wait);
- if(capture_read)
- return POLLIN|POLLRDNORM;
- return 0;
-}
-
- </programlisting>
- <para>
- Our wait queue for polling is the capture_wait queue. This will cause the
- task to be woken up by our camera_irq routine. We check capture_read to see
- if there is an image present and if so report that it is readable.
- </para>
- </sect1>
- <sect1 id="rdvid">
- <title>Reading The Video Image</title>
- <programlisting>
-
-
-static long camera_read(struct video_device *dev, char *buf,
- unsigned long count)
-{
- struct wait_queue wait = { current, NULL };
- u8 *ptr;
- int len;
- int i;
-
- add_wait_queue(&capture_wait, &wait);
-
- while(!capture_ready)
- {
- if(file->flags&O_NDELAY)
- {
- remove_wait_queue(&capture_wait, &wait);
- current->state = TASK_RUNNING;
- return -EWOULDBLOCK;
- }
- if(signal_pending(current))
- {
- remove_wait_queue(&capture_wait, &wait);
- current->state = TASK_RUNNING;
- return -ERESTARTSYS;
- }
- schedule();
- current->state = TASK_INTERRUPTIBLE;
- }
- remove_wait_queue(&capture_wait, &wait);
- current->state = TASK_RUNNING;
-
- </programlisting>
- <para>
- The first thing we have to do is to ensure that the application waits until
- the next frame is ready. The code here is almost identical to the mouse code
- we used earlier in this chapter. It is one of the common building blocks of
- Linux device driver code and probably one which you will find occurs in any
- drivers you write.
- </para>
- <para>
- We wait for a frame to be ready, or for a signal to interrupt our waiting. If a
- signal occurs we need to return from the system call so that the signal can
- be sent to the application itself. We also check to see if the user actually
- wanted to avoid waiting - ie if they are using non-blocking I/O and have other things
- to get on with.
- </para>
- <para>
- Next we copy the data from the card to the user application. This is rarely
- as easy as our example makes out. We will add capture_w, and capture_h here
- to hold the width and height of the captured image. We assume the card only
- supports 24bit RGB for now.
- </para>
- <programlisting>
-
-
-
- capture_ready = 0;
-
- ptr=(u8 *)buf;
- len = capture_w * 3 * capture_h; /* 24bit RGB */
-
- if(len>count)
- len=count; /* Doesn't all fit */
-
- for(i=0; i<len; i++)
- {
- put_user(inb(io+IMAGE_DATA), ptr);
- ptr++;
- }
-
- hardware_restart_capture();
-
- return i;
-}
-
- </programlisting>
- <para>
- For a real hardware device you would try to avoid the loop with put_user().
- Each call to put_user() has a time overhead checking whether the accesses to user
- space are allowed. It would be better to read a line into a temporary buffer
- then copy this to user space in one go.
- </para>
- <para>
- Having captured the image and put it into user space we can kick the card to
- get the next frame acquired.
- </para>
- </sect1>
- <sect1 id="iocvid">
- <title>Video Ioctl Handling</title>
- <para>
- As with the radio driver the major control interface is via the ioctl()
- function. Video capture devices support the same tuner calls as a radio
- device and also support additional calls to control how the video functions
- are handled. In this simple example the card has no tuners to avoid making
- the code complex.
- </para>
- <programlisting>
-
-
-
-static int camera_ioctl(struct video_device *dev, unsigned int cmd, void *arg)
-{
- switch(cmd)
- {
- case VIDIOCGCAP:
- {
- struct video_capability v;
- v.type = VID_TYPE_CAPTURE|\
- VID_TYPE_CHROMAKEY|\
- VID_TYPE_SCALES|\
- VID_TYPE_OVERLAY;
- v.channels = 1;
- v.audios = 0;
- v.maxwidth = 640;
- v.minwidth = 16;
- v.maxheight = 480;
- v.minheight = 16;
- strcpy(v.name, "My Camera");
- if(copy_to_user(arg, &v, sizeof(v)))
- return -EFAULT;
- return 0;
- }
-
-
- </programlisting>
- <para>
- The first ioctl we must support and which all video capture and radio
- devices are required to support is VIDIOCGCAP. This behaves exactly the same
- as with a radio device. This time, however, we report the extra capabilities
- we outlined earlier on when defining our video_dev structure.
- </para>
- <para>
- We now set the video flags saying that we support overlay, capture,
- scaling and chromakey. We also report size limits - our smallest image is
- 16x16 pixels, our largest is 640x480.
- </para>
- <para>
- To keep things simple we report no audio and no tuning capabilities at all.
- </para>
- <programlisting>
-
- case VIDIOCGCHAN:
- {
- struct video_channel v;
- if(copy_from_user(&v, arg, sizeof(v)))
- return -EFAULT;
- if(v.channel != 0)
- return -EINVAL;
- v.flags = 0;
- v.tuners = 0;
- v.type = VIDEO_TYPE_CAMERA;
- v.norm = VIDEO_MODE_AUTO;
- strcpy(v.name, "Camera Input");break;
- if(copy_to_user(&v, arg, sizeof(v)))
- return -EFAULT;
- return 0;
- }
-
-
- </programlisting>
- <para>
- This follows what is very much the standard way an ioctl handler looks
- in Linux. We copy the data into a kernel space variable and we check that the
- request is valid (in this case that the input is 0). Finally we copy the
- camera info back to the user.
- </para>
- <para>
- The VIDIOCGCHAN ioctl allows a user to ask about video channels (that is
- inputs to the video card). Our example card has a single camera input. The
- fields in the structure are
- </para>
- <table frame="all" id="video_channel_fields"><title>struct video_channel fields</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
-
- <entry>channel</entry><entry>The channel number we are selecting</entry>
- </row><row>
- <entry>name</entry><entry>The name for this channel. This is intended
- to describe the port to the user.
- Appropriate names are therefore things like
- "Camera" "SCART input"</entry>
- </row><row>
- <entry>flags</entry><entry>Channel properties</entry>
- </row><row>
- <entry>type</entry><entry>Input type</entry>
- </row><row>
- <entry>norm</entry><entry>The current television encoding being used
- if relevant for this channel.
- </entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <table frame="all" id="video_channel_flags"><title>struct video_channel flags</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>VIDEO_VC_TUNER</entry><entry>Channel has a tuner.</entry>
- </row><row>
- <entry>VIDEO_VC_AUDIO</entry><entry>Channel has audio.</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <table frame="all" id="video_channel_types"><title>struct video_channel types</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>VIDEO_TYPE_TV</entry><entry>Television input.</entry>
- </row><row>
- <entry>VIDEO_TYPE_CAMERA</entry><entry>Fixed camera input.</entry>
- </row><row>
- <entry>0</entry><entry>Type is unknown.</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <table frame="all" id="video_channel_norms"><title>struct video_channel norms</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>VIDEO_MODE_PAL</entry><entry>PAL encoded Television</entry>
- </row><row>
- <entry>VIDEO_MODE_NTSC</entry><entry>NTSC (US) encoded Television</entry>
- </row><row>
- <entry>VIDEO_MODE_SECAM</entry><entry>SECAM (French) Television </entry>
- </row><row>
- <entry>VIDEO_MODE_AUTO</entry><entry>Automatic switching, or format does not
- matter</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <para>
- The corresponding VIDIOCSCHAN ioctl allows a user to change channel and to
- request the norm is changed - for example to switch between a PAL or an NTSC
- format camera.
- </para>
- <programlisting>
-
-
- case VIDIOCSCHAN:
- {
- struct video_channel v;
- if(copy_from_user(&v, arg, sizeof(v)))
- return -EFAULT;
- if(v.channel != 0)
- return -EINVAL;
- if(v.norm != VIDEO_MODE_AUTO)
- return -EINVAL;
- return 0;
- }
-
-
- </programlisting>
- <para>
- The implementation of this call in our driver is remarkably easy. Because we
- are assuming fixed format hardware we need only check that the user has not
- tried to change anything.
- </para>
- <para>
- The user also needs to be able to configure and adjust the picture they are
- seeing. This is much like adjusting a television set. A user application
- also needs to know the palette being used so that it knows how to display
- the image that has been captured. The VIDIOCGPICT and VIDIOCSPICT ioctl
- calls provide this information.
- </para>
- <programlisting>
-
-
- case VIDIOCGPICT
- {
- struct video_picture v;
- v.brightness = hardware_brightness();
- v.hue = hardware_hue();
- v.colour = hardware_saturation();
- v.contrast = hardware_brightness();
- /* Not settable */
- v.whiteness = 32768;
- v.depth = 24; /* 24bit */
- v.palette = VIDEO_PALETTE_RGB24;
- if(copy_to_user(&v, arg,
- sizeof(v)))
- return -EFAULT;
- return 0;
- }
-
-
- </programlisting>
- <para>
- The brightness, hue, color, and contrast provide the picture controls that
- are akin to a conventional television. Whiteness provides additional
- control for greyscale images. All of these values are scaled between 0-65535
- and have 32768 as the mid point setting. The scaling means that applications
- do not have to worry about the capability range of the hardware but can let
- it make a best effort attempt.
- </para>
- <para>
- Our depth is 24, as this is in bits. We will be returning RGB24 format. This
- has one byte of red, then one of green, then one of blue. This then repeats
- for every other pixel in the image. The other common formats the interface
- defines are
- </para>
- <table frame="all" id="Framebuffer_Encodings"><title>Framebuffer Encodings</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>GREY</entry><entry>Linear greyscale. This is for simple cameras and the
- like</entry>
- </row><row>
- <entry>RGB565</entry><entry>The top 5 bits hold 32 red levels, the next six bits
- hold green and the low 5 bits hold blue. </entry>
- </row><row>
- <entry>RGB555</entry><entry>The top bit is clear. The red green and blue levels
- each occupy five bits.</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <para>
- Additional modes are support for YUV capture formats. These are common for
- TV and video conferencing applications.
- </para>
- <para>
- The VIDIOCSPICT ioctl allows a user to set some of the picture parameters.
- Exactly which ones are supported depends heavily on the card itself. It is
- possible to support many modes and effects in software. In general doing
- this in the kernel is a bad idea. Video capture is a performance-sensitive
- application and the programs can often do better if they aren't being
- 'helped' by an overkeen driver writer. Thus for our device we will report
- RGB24 only and refuse to allow a change.
- </para>
- <programlisting>
-
-
- case VIDIOCSPICT:
- {
- struct video_picture v;
- if(copy_from_user(&v, arg, sizeof(v)))
- return -EFAULT;
- if(v.depth!=24 ||
- v.palette != VIDEO_PALETTE_RGB24)
- return -EINVAL;
- set_hardware_brightness(v.brightness);
- set_hardware_hue(v.hue);
- set_hardware_saturation(v.colour);
- set_hardware_brightness(v.contrast);
- return 0;
- }
-
-
- </programlisting>
- <para>
- We check the user has not tried to change the palette or the depth. We do
- not want to carry out some of the changes and then return an error. This may
- confuse the application which will be assuming no change occurred.
- </para>
- <para>
- In much the same way as you need to be able to set the picture controls to
- get the right capture images, many cards need to know what they are
- displaying onto when generating overlay output. In some cases getting this
- wrong even makes a nasty mess or may crash the computer. For that reason
- the VIDIOCSBUF ioctl used to set up the frame buffer information may well
- only be usable by root.
- </para>
- <para>
- We will assume our card is one of the old ISA devices with feature connector
- and only supports a couple of standard video modes. Very common for older
- cards although the PCI devices are way smarter than this.
- </para>
- <programlisting>
-
-
-static struct video_buffer capture_fb;
-
- case VIDIOCGFBUF:
- {
- if(copy_to_user(arg, &capture_fb,
- sizeof(capture_fb)))
- return -EFAULT;
- return 0;
-
- }
-
-
- </programlisting>
- <para>
- We keep the frame buffer information in the format the ioctl uses. This
- makes it nice and easy to work with in the ioctl calls.
- </para>
- <programlisting>
-
- case VIDIOCSFBUF:
- {
- struct video_buffer v;
-
- if(!capable(CAP_SYS_ADMIN))
- return -EPERM;
-
- if(copy_from_user(&v, arg, sizeof(v)))
- return -EFAULT;
- if(v.width!=320 && v.width!=640)
- return -EINVAL;
- if(v.height!=200 && v.height!=240
- && v.height!=400
- && v.height !=480)
- return -EINVAL;
- memcpy(&capture_fb, &v, sizeof(v));
- hardware_set_fb(&v);
- return 0;
- }
-
-
-
- </programlisting>
- <para>
- The capable() function checks a user has the required capability. The Linux
- operating system has a set of about 30 capabilities indicating privileged
- access to services. The default set up gives the superuser (uid 0) all of
- them and nobody else has any.
- </para>
- <para>
- We check that the user has the SYS_ADMIN capability, that is they are
- allowed to operate as the machine administrator. We don't want anyone but
- the administrator making a mess of the display.
- </para>
- <para>
- Next we check for standard PC video modes (320 or 640 wide with either
- EGA or VGA depths). If the mode is not a standard video mode we reject it as
- not supported by our card. If the mode is acceptable we save it so that
- VIDIOCFBUF will give the right answer next time it is called. The
- hardware_set_fb() function is some undescribed card specific function to
- program the card for the desired mode.
- </para>
- <para>
- Before the driver can display an overlay window it needs to know where the
- window should be placed, and also how large it should be. If the card
- supports clipping it needs to know which rectangles to omit from the
- display. The video_window structure is used to describe the way the image
- should be displayed.
- </para>
- <table frame="all" id="video_window_fields"><title>struct video_window fields</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>width</entry><entry>The width in pixels of the desired image. The card
- may use a smaller size if this size is not available</entry>
- </row><row>
- <entry>height</entry><entry>The height of the image. The card may use a smaller
- size if this size is not available.</entry>
- </row><row>
- <entry>x</entry><entry> The X position of the top left of the window. This
- is in pixels relative to the left hand edge of the
- picture. Not all cards can display images aligned on
- any pixel boundary. If the position is unsuitable
- the card adjusts the image right and reduces the
- width.</entry>
- </row><row>
- <entry>y</entry><entry> The Y position of the top left of the window. This
- is counted in pixels relative to the top edge of the
- picture. As with the width if the card cannot
- display starting on this line it will adjust the
- values.</entry>
- </row><row>
- <entry>chromakey</entry><entry>The colour (expressed in RGB32 format) for the
- chromakey colour if chroma keying is being used. </entry>
- </row><row>
- <entry>clips</entry><entry>An array of rectangles that must not be drawn
- over.</entry>
- </row><row>
- <entry>clipcount</entry><entry>The number of clips in this array.</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <para>
- Each clip is a struct video_clip which has the following fields
- </para>
- <table frame="all" id="video_clip_fields"><title>video_clip fields</title>
- <tgroup cols="2" align="left">
- <tbody>
- <row>
- <entry>x, y</entry><entry>Co-ordinates relative to the display</entry>
- </row><row>
- <entry>width, height</entry><entry>Width and height in pixels</entry>
- </row><row>
- <entry>next</entry><entry>A spare field for the application to use</entry>
- </row>
- </tbody>
- </tgroup>
- </table>
- <para>
- 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.
- This may well mean it has to leave alone. small areas the application wished to be
- drawn.
- </para>
- <para>
- Our example card uses chromakey so does not have to address most of the
- clipping. We will add a video_window structure to our global variables to
- remember our parameters, as we did with the frame buffer.
- </para>
- <programlisting>
-
-
- case VIDIOCGWIN:
- {
- if(copy_to_user(arg, &capture_win,
- sizeof(capture_win)))
- return -EFAULT;
- return 0;
- }
-
-
- case VIDIOCSWIN:
- {
- struct video_window v;
- if(copy_from_user(&v, arg, sizeof(v)))
- return -EFAULT;
- if(v.width > 640 || v.height > 480)
- return -EINVAL;
- if(v.width < 16 || v.height < 16)
- return -EINVAL;
- hardware_set_key(v.chromakey);
- hardware_set_window(v);
- memcpy(&capture_win, &v, sizeof(v));
- capture_w = v.width;
- capture_h = v.height;
- return 0;
- }
-
-
- </programlisting>
- <para>
- Because we are using Chromakey our setup is fairly simple. Mostly we have to
- check the values are sane and load them into the capture card.
- </para>
- <para>
- With all the setup done we can now turn on the actual capture/overlay. This
- is done with the VIDIOCCAPTURE ioctl. This takes a single integer argument
- where 0 is on and 1 is off.
- </para>
- <programlisting>
-
-
- case VIDIOCCAPTURE:
- {
- int v;
- if(get_user(v, (int *)arg))
- return -EFAULT;
- if(v==0)
- hardware_capture_off();
- else
- {
- if(capture_fb.width == 0
- || capture_w == 0)
- return -EINVAL;
- hardware_capture_on();
- }
- return 0;
- }
-
-
- </programlisting>
- <para>
- We grab the flag from user space and either enable or disable according to
- its value. There is one small corner case we have to consider here. Suppose
- that the capture was requested before the video window or the frame buffer
- had been set up. In those cases there will be unconfigured fields in our
- card data, as well as unconfigured hardware settings. We check for this case and
- return an error if the frame buffer or the capture window width is zero.
- </para>
- <programlisting>
-
-
- default:
- return -ENOIOCTLCMD;
- }
-}
- </programlisting>
- <para>
-
- We don't need to support any other ioctls, so if we get this far, it is time
- to tell the video layer that we don't now what the user is talking about.
- </para>
- </sect1>
- <sect1 id="endvid">
- <title>Other Functionality</title>
- <para>
- The Video4Linux layer supports additional features, including a high
- performance mmap() based capture mode and capturing part of the image.
- These features are out of the scope of the book. You should however have enough
- example code to implement most simple video4linux devices for radio and TV
- cards.
- </para>
- </sect1>
- </chapter>
- <chapter id="bugs">
- <title>Known Bugs And Assumptions</title>
- <para>
- <variablelist>
- <varlistentry><term>Multiple Opens</term>
- <listitem>
- <para>
- The driver assumes multiple opens should not be allowed. A driver
- can work around this but not cleanly.
- </para>
- </listitem></varlistentry>
-
- <varlistentry><term>API Deficiencies</term>
- <listitem>
- <para>
- The existing API poorly reflects compression capable devices. There
- are plans afoot to merge V4L, V4L2 and some other ideas into a
- better interface.
- </para>
- </listitem></varlistentry>
- </variablelist>
-
- </para>
- </chapter>
-
- <chapter id="pubfunctions">
- <title>Public Functions Provided</title>
-!Edrivers/media/video/v4l2-dev.c
- </chapter>
-
-</book>
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