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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/DocBook/videobook.tmpl
downloadconfigs-1da177e4c3f41524e886b7f1b8a0c1fc7321cac2.tar.gz
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+<?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,
+ VID_HARDWARE_MYRADIO,
+ 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>
+ The VID_HARDWARE_ types are unique to each device. Numbers are assigned by
+ <email>alan@redhat.com</email> when device drivers are going to be released. Until then you
+ can pull a suitably large number out of your hat and use it. 10000 should be
+ safe for a very long time even allowing for the huge number of vendors
+ making new and different radio cards at the moment.
+ </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(&amp;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"><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(stuct 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, &amp;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"><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(&amp;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(&amp;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"><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"><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"><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(&amp;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, &amp;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, &amp;freq,
+ sizeof(unsigned long))!=0)
+ return -EFAULT;
+ if(hardware_set_freq(freq)&lt;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(&amp;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. &amp;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"><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"><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"><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(&amp;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(&amp;v, arg, sizeof(v)))
+ return -EFAULT;
+ if(v.audio)
+ return -EINVAL;
+ current_volume = v/16384;
+ if(v.flags&amp;VIDEO_AUDIO_MUTE)
+ hardware_set_volume(0);
+ else
+ hardware_set_volume(current_volume);
+ current_muted = v.flags &amp;
+ 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(&amp;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>
+ <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,
+ VID_HARDWARE_MYCAMERA,
+ 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"><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(&amp;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(stuct video_device *dev, int flags)
+{
+ if(users)
+ return -EBUSY;
+ if(request_irq(irq, camera_irq, 0, "camera", dev)&lt;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 SA_SHIRQ 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(&amp;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, &amp;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(&amp;capture_wait, &amp;wait);
+
+ while(!capture_ready)
+ {
+ if(file->flags&amp;O_NDELAY)
+ {
+ remove_wait_queue(&amp;capture_wait, &amp;wait);
+ current->state = TASK_RUNNING;
+ return -EWOULDBLOCK;
+ }
+ if(signal_pending(current))
+ {
+ remove_wait_queue(&amp;capture_wait, &amp;wait);
+ current->state = TASK_RUNNING;
+ return -ERESTARTSYS;
+ }
+ schedule();
+ current->state = TASK_INTERRUPTIBLE;
+ }
+ remove_wait_queue(&amp;capture_wait, &amp;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&lt;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, &amp;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(&amp;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(&amp;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"><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"><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"><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"><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(&amp;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(&amp;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"><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(&amp;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, &amp;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(&amp;v, arg, sizeof(v)))
+ return -EFAULT;
+ if(v.width!=320 &amp;&amp; v.width!=640)
+ return -EINVAL;
+ if(v.height!=200 &amp;&amp; v.height!=240
+ &amp;&amp; v.height!=400
+ &amp;&amp; v.height !=480)
+ return -EINVAL;
+ memcpy(&amp;capture_fb, &amp;v, sizeof(v));
+ hardware_set_fb(&amp;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"><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"><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, &amp;capture_win,
+ sizeof(capture_win)))
+ return -EFAULT;
+ return 0;
+ }
+
+
+ case VIDIOCSWIN:
+ {
+ struct video_window v;
+ if(copy_from_user(&amp;v, arg, sizeof(v)))
+ return -EFAULT;
+ if(v.width &gt; 640 || v.height &gt; 480)
+ return -EINVAL;
+ if(v.width &lt; 16 || v.height &lt; 16)
+ return -EINVAL;
+ hardware_set_key(v.chromakey);
+ hardware_set_window(v);
+ memcpy(&amp;capture_win, &amp;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/videodev.c
+ </chapter>
+
+</book>