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!

1 files changed, 1663 insertions, 0 deletions

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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>