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| author | Linus Torvalds <torvalds@linux-foundation.org> | 2008-10-16 12:18:16 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2008-10-16 12:18:16 -0700 |
| commit | d1b5726358ccebcf614fc9f97cdf3354178dcb80 (patch) | |
| tree | d7bbd8e24bd312d9c2c18fc18f12f6a8ce9b084f /Documentation/DocBook | |
| parent | c472273f863c80b87e53356256c5466df24328f0 (diff) | |
| parent | 656e6c0050fd63ce42c55a6cb454a9b4b2f9ccf7 (diff) | |
| download | linux-d1b5726358ccebcf614fc9f97cdf3354178dcb80.tar.gz linux-d1b5726358ccebcf614fc9f97cdf3354178dcb80.tar.bz2 linux-d1b5726358ccebcf614fc9f97cdf3354178dcb80.zip | |
Merge branch 'docs' of git://git.lwn.net/linux-2.6
* 'docs' of git://git.lwn.net/linux-2.6:
Document panic_on_unrecovered_nmi sysctl
Add a reference to paper to SubmittingPatches
Add kerneldoc documentation for new printk format extensions
Remove videobook.tmpl
doc: Test-by?
Add the development process document
Documentation/block/data-integrity.txt: Fix section numbers
Diffstat (limited to 'Documentation/DocBook')
| -rw-r--r-- | Documentation/DocBook/Makefile | 2 | ||||
| -rw-r--r-- | Documentation/DocBook/videobook.tmpl | 1654 |
2 files changed, 1 insertions, 1655 deletions
diff --git a/Documentation/DocBook/Makefile b/Documentation/DocBook/Makefile index 1615350b7b53..fabc06466b93 100644 --- a/Documentation/DocBook/Makefile +++ b/Documentation/DocBook/Makefile @@ -6,7 +6,7 @@ # To add a new book the only step required is to add the book to the # list of DOCBOOKS. -DOCBOOKS := wanbook.xml z8530book.xml mcabook.xml videobook.xml \ +DOCBOOKS := wanbook.xml z8530book.xml mcabook.xml \ kernel-hacking.xml kernel-locking.xml deviceiobook.xml \ procfs-guide.xml writing_usb_driver.xml networking.xml \ kernel-api.xml filesystems.xml lsm.xml usb.xml kgdb.xml \ diff --git a/Documentation/DocBook/videobook.tmpl b/Documentation/DocBook/videobook.tmpl deleted file mode 100644 index 0bc25949b668..000000000000 --- a/Documentation/DocBook/videobook.tmpl +++ /dev/null @@ -1,1654 +0,0 @@ -<?xml version="1.0" encoding="UTF-8"?> -<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" - "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []> - -<book id="V4LGuide"> - <bookinfo> - <title>Video4Linux Programming</title> - - <authorgroup> - <author> - <firstname>Alan</firstname> - <surname>Cox</surname> - <affiliation> - <address> - <email>alan@redhat.com</email> - </address> - </affiliation> - </author> - </authorgroup> - - <copyright> - <year>2000</year> - <holder>Alan Cox</holder> - </copyright> - - <legalnotice> - <para> - This documentation is free software; you can redistribute - it and/or modify it under the terms of the GNU General Public - License as published by the Free Software Foundation; either - version 2 of the License, or (at your option) any later - version. - </para> - - <para> - This program is distributed in the hope that it will be - useful, but WITHOUT ANY WARRANTY; without even the implied - warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. - See the GNU General Public License for more details. - </para> - - <para> - You should have received a copy of the GNU General Public - License along with this program; if not, write to the Free - Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, - MA 02111-1307 USA - </para> - - <para> - For more details see the file COPYING in the source - distribution of Linux. - </para> - </legalnotice> - </bookinfo> - -<toc></toc> - - <chapter id="intro"> - <title>Introduction</title> - <para> - Parts of this document first appeared in Linux Magazine under a - ninety day exclusivity. - </para> - <para> - Video4Linux is intended to provide a common programming interface - for the many TV and capture cards now on the market, as well as - parallel port and USB video cameras. Radio, teletext decoders and - vertical blanking data interfaces are also provided. - </para> - </chapter> - <chapter id="radio"> - <title>Radio Devices</title> - <para> - There are a wide variety of radio interfaces available for PC's, and these - are generally very simple to program. The biggest problem with supporting - such devices is normally extracting documentation from the vendor. - </para> - <para> - The radio interface supports a simple set of control ioctls standardised - across all radio and tv interfaces. It does not support read or write, which - are used for video streams. The reason radio cards do not allow you to read - the audio stream into an application is that without exception they provide - a connection on to a soundcard. Soundcards can be used to read the radio - data just fine. - </para> - <sect1 id="registerradio"> - <title>Registering Radio Devices</title> - <para> - The Video4linux core provides an interface for registering devices. The - first step in writing our radio card driver is to register it. - </para> - <programlisting> - - -static struct video_device my_radio -{ - "My radio", - VID_TYPE_TUNER, - radio_open. - radio_close, - NULL, /* no read */ - NULL, /* no write */ - NULL, /* no poll */ - radio_ioctl, - NULL, /* no special init function */ - NULL /* no private data */ -}; - - - </programlisting> - <para> - This declares our video4linux device driver interface. The VID_TYPE_ value - defines what kind of an interface we are, and defines basic capabilities. - </para> - <para> - The only defined value relevant for a radio card is VID_TYPE_TUNER which - indicates that the device can be tuned. Clearly our radio is going to have some - way to change channel so it is tuneable. - </para> - <para> - We declare an open and close routine, but we do not need read or write, - which are used to read and write video data to or from the card itself. As - we have no read or write there is no poll function. - </para> - <para> - The private initialise function is run when the device is registered. In - this driver we've already done all the work needed. The final pointer is a - private data pointer that can be used by the device driver to attach and - retrieve private data structures. We set this field "priv" to NULL for - the moment. - </para> - <para> - Having the structure defined is all very well but we now need to register it - with the kernel. - </para> - <programlisting> - - -static int io = 0x320; - -int __init myradio_init(struct video_init *v) -{ - if(!request_region(io, MY_IO_SIZE, "myradio")) - { - printk(KERN_ERR - "myradio: port 0x%03X is in use.\n", io); - return -EBUSY; - } - - if(video_device_register(&my_radio, VFL_TYPE_RADIO)==-1) { - release_region(io, MY_IO_SIZE); - return -EINVAL; - } - return 0; -} - - </programlisting> - <para> - The first stage of the initialisation, as is normally the case, is to check - that the I/O space we are about to fiddle with doesn't belong to some other - driver. If it is we leave well alone. If the user gives the address of the - wrong device then we will spot this. These policies will generally avoid - crashing the machine. - </para> - <para> - Now we ask the Video4Linux layer to register the device for us. We hand it - our carefully designed video_device structure and also tell it which group - of devices we want it registered with. In this case VFL_TYPE_RADIO. - </para> - <para> - The types available are - </para> - <table frame="all" id="Device_Types"><title>Device Types</title> - <tgroup cols="3" align="left"> - <tbody> - <row> - <entry>VFL_TYPE_RADIO</entry><entry>/dev/radio{n}</entry><entry> - - Radio devices are assigned in this block. As with all of these - selections the actual number assignment is done by the video layer - accordijng to what is free.</entry> - </row><row> - <entry>VFL_TYPE_GRABBER</entry><entry>/dev/video{n}</entry><entry> - Video capture devices and also -- counter-intuitively for the name -- - hardware video playback devices such as MPEG2 cards.</entry> - </row><row> - <entry>VFL_TYPE_VBI</entry><entry>/dev/vbi{n}</entry><entry> - The VBI devices capture the hidden lines on a television picture - that carry further information like closed caption data, teletext - (primarily in Europe) and now Intercast and the ATVEC internet - television encodings.</entry> - </row><row> - <entry>VFL_TYPE_VTX</entry><entry>/dev/vtx[n}</entry><entry> - VTX is 'Videotext' also known as 'Teletext'. This is a system for - sending numbered, 40x25, mostly textual page images over the hidden - lines. Unlike the /dev/vbi interfaces, this is for 'smart' decoder - chips. (The use of the word smart here has to be taken in context, - the smartest teletext chips are fairly dumb pieces of technology). - </entry> - </row> - </tbody> - </tgroup> - </table> - <para> - We are most definitely a radio. - </para> - <para> - Finally we allocate our I/O space so that nobody treads on us and return 0 - to signify general happiness with the state of the universe. - </para> - </sect1> - <sect1 id="openradio"> - <title>Opening And Closing The Radio</title> - - <para> - The functions we declared in our video_device are mostly very simple. - Firstly we can drop in what is basically standard code for open and close. - </para> - <programlisting> - - -static int users = 0; - -static int radio_open(struct video_device *dev, int flags) -{ - if(users) - return -EBUSY; - users++; - return 0; -} - - </programlisting> - <para> - At open time we need to do nothing but check if someone else is also using - the radio card. If nobody is using it we make a note that we are using it, - then we ensure that nobody unloads our driver on us. - </para> - <programlisting> - - -static int radio_close(struct video_device *dev) -{ - users--; -} - - </programlisting> - <para> - At close time we simply need to reduce the user count and allow the module - to become unloadable. - </para> - <para> - If you are sharp you will have noticed neither the open nor the close - routines attempt to reset or change the radio settings. This is intentional. - It allows an application to set up the radio and exit. It avoids a user - having to leave an application running all the time just to listen to the - radio. - </para> - </sect1> - <sect1 id="ioctlradio"> - <title>The Ioctl Interface</title> - <para> - This leaves the ioctl routine, without which the driver will not be - terribly useful to anyone. - </para> - <programlisting> - - -static int radio_ioctl(struct video_device *dev, unsigned int cmd, void *arg) -{ - switch(cmd) - { - case VIDIOCGCAP: - { - struct video_capability v; - v.type = VID_TYPE_TUNER; - v.channels = 1; - v.audios = 1; - v.maxwidth = 0; - v.minwidth = 0; - v.maxheight = 0; - v.minheight = 0; - strcpy(v.name, "My Radio"); - if(copy_to_user(arg, &v, sizeof(v))) - return -EFAULT; - return 0; - } - - </programlisting> - <para> - VIDIOCGCAP is the first ioctl all video4linux devices must support. It - allows the applications to find out what sort of a card they have found and - to figure out what they want to do about it. The fields in the structure are - </para> - <table frame="all" id="video_capability_fields"><title>struct video_capability fields</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>name</entry><entry>The device text name. This is intended for the user.</entry> - </row><row> - <entry>channels</entry><entry>The number of different channels you can tune on - this card. It could even by zero for a card that has - no tuning capability. For our simple FM radio it is 1. - An AM/FM radio would report 2.</entry> - </row><row> - <entry>audios</entry><entry>The number of audio inputs on this device. For our - radio there is only one audio input.</entry> - </row><row> - <entry>minwidth,minheight</entry><entry>The smallest size the card is capable of capturing - images in. We set these to zero. Radios do not - capture pictures</entry> - </row><row> - <entry>maxwidth,maxheight</entry><entry>The largest image size the card is capable of - capturing. For our radio we report 0. - </entry> - </row><row> - <entry>type</entry><entry>This reports the capabilities of the device, and - matches the field we filled in in the struct - video_device when registering.</entry> - </row> - </tbody> - </tgroup> - </table> - <para> - Having filled in the fields, we use copy_to_user to copy the structure into - the users buffer. If the copy fails we return an EFAULT to the application - so that it knows it tried to feed us garbage. - </para> - <para> - The next pair of ioctl operations select which tuner is to be used and let - the application find the tuner properties. We have only a single FM band - tuner in our example device. - </para> - <programlisting> - - - case VIDIOCGTUNER: - { - struct video_tuner v; - if(copy_from_user(&v, arg, sizeof(v))!=0) - return -EFAULT; - if(v.tuner) - return -EINVAL; - v.rangelow=(87*16000); - v.rangehigh=(108*16000); - v.flags = VIDEO_TUNER_LOW; - v.mode = VIDEO_MODE_AUTO; - v.signal = 0xFFFF; - strcpy(v.name, "FM"); - if(copy_to_user(&v, arg, sizeof(v))!=0) - return -EFAULT; - return 0; - } - - </programlisting> - <para> - The VIDIOCGTUNER ioctl allows applications to query a tuner. The application - sets the tuner field to the tuner number it wishes to query. The query does - not change the tuner that is being used, it merely enquires about the tuner - in question. - </para> - <para> - We have exactly one tuner so after copying the user buffer to our temporary - structure we complain if they asked for a tuner other than tuner 0. - </para> - <para> - The video_tuner structure has the following fields - </para> - <table frame="all" id="video_tuner_fields"><title>struct video_tuner fields</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>int tuner</entry><entry>The number of the tuner in question</entry> - </row><row> - <entry>char name[32]</entry><entry>A text description of this tuner. "FM" will do fine. - This is intended for the application.</entry> - </row><row> - <entry>u32 flags</entry> - <entry>Tuner capability flags</entry> - </row> - <row> - <entry>u16 mode</entry><entry>The current reception mode</entry> - - </row><row> - <entry>u16 signal</entry><entry>The signal strength scaled between 0 and 65535. If - a device cannot tell the signal strength it should - report 65535. Many simple cards contain only a - signal/no signal bit. Such cards will report either - 0 or 65535.</entry> - - </row><row> - <entry>u32 rangelow, rangehigh</entry><entry> - The range of frequencies supported by the radio - or TV. It is scaled according to the VIDEO_TUNER_LOW - flag.</entry> - - </row> - </tbody> - </tgroup> - </table> - - <table frame="all" id="video_tuner_flags"><title>struct video_tuner flags</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>VIDEO_TUNER_PAL</entry><entry>A PAL TV tuner</entry> - </row><row> - <entry>VIDEO_TUNER_NTSC</entry><entry>An NTSC (US) TV tuner</entry> - </row><row> - <entry>VIDEO_TUNER_SECAM</entry><entry>A SECAM (French) TV tuner</entry> - </row><row> - <entry>VIDEO_TUNER_LOW</entry><entry> - The tuner frequency is scaled in 1/16th of a KHz - steps. If not it is in 1/16th of a MHz steps - </entry> - </row><row> - <entry>VIDEO_TUNER_NORM</entry><entry>The tuner can set its format</entry> - </row><row> - <entry>VIDEO_TUNER_STEREO_ON</entry><entry>The tuner is currently receiving a stereo signal</entry> - </row> - </tbody> - </tgroup> - </table> - - <table frame="all" id="video_tuner_modes"><title>struct video_tuner modes</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>VIDEO_MODE_PAL</entry><entry>PAL Format</entry> - </row><row> - <entry>VIDEO_MODE_NTSC</entry><entry>NTSC Format (USA)</entry> - </row><row> - <entry>VIDEO_MODE_SECAM</entry><entry>French Format</entry> - </row><row> - <entry>VIDEO_MODE_AUTO</entry><entry>A device that does not need to do - TV format switching</entry> - </row> - </tbody> - </tgroup> - </table> - <para> - The settings for the radio card are thus fairly simple. We report that we - are a tuner called "FM" for FM radio. In order to get the best tuning - resolution we report VIDEO_TUNER_LOW and select tuning to 1/16th of KHz. Its - unlikely our card can do that resolution but it is a fair bet the card can - do better than 1/16th of a MHz. VIDEO_TUNER_LOW is appropriate to almost all - radio usage. - </para> - <para> - We report that the tuner automatically handles deciding what format it is - receiving - true enough as it only handles FM radio. Our example card is - also incapable of detecting stereo or signal strengths so it reports a - strength of 0xFFFF (maximum) and no stereo detected. - </para> - <para> - To finish off we set the range that can be tuned to be 87-108Mhz, the normal - FM broadcast radio range. It is important to find out what the card is - actually capable of tuning. It is easy enough to simply use the FM broadcast - range. Unfortunately if you do this you will discover the FM broadcast - ranges in the USA, Europe and Japan are all subtly different and some users - cannot receive all the stations they wish. - </para> - <para> - The application also needs to be able to set the tuner it wishes to use. In - our case, with a single tuner this is rather simple to arrange. - </para> - <programlisting> - - case VIDIOCSTUNER: - { - struct video_tuner v; - if(copy_from_user(&v, arg, sizeof(v))) - return -EFAULT; - if(v.tuner != 0) - return -EINVAL; - return 0; - } - - </programlisting> - <para> - We copy the user supplied structure into kernel memory so we can examine it. - If the user has selected a tuner other than zero we reject the request. If - they wanted tuner 0 then, surprisingly enough, that is the current tuner already. - </para> - <para> - The next two ioctls we need to provide are to get and set the frequency of - the radio. These both use an unsigned long argument which is the frequency. - The scale of the frequency depends on the VIDEO_TUNER_LOW flag as I - mentioned earlier on. Since we have VIDEO_TUNER_LOW set this will be in - 1/16ths of a KHz. - </para> - <programlisting> - -static unsigned long current_freq; - - - - case VIDIOCGFREQ: - if(copy_to_user(arg, &current_freq, - sizeof(unsigned long)) - return -EFAULT; - return 0; - - </programlisting> - <para> - Querying the frequency in our case is relatively simple. Our radio card is - too dumb to let us query the signal strength so we remember our setting if - we know it. All we have to do is copy it to the user. - </para> - <programlisting> - - - case VIDIOCSFREQ: - { - u32 freq; - if(copy_from_user(arg, &freq, - sizeof(unsigned long))!=0) - return -EFAULT; - if(hardware_set_freq(freq)<0) - return -EINVAL; - current_freq = freq; - return 0; - } - - </programlisting> - <para> - Setting the frequency is a little more complex. We begin by copying the - desired frequency into kernel space. Next we call a hardware specific routine - to set the radio up. This might be as simple as some scaling and a few - writes to an I/O port. For most radio cards it turns out a good deal more - complicated and may involve programming things like a phase locked loop on - the card. This is what documentation is for. - </para> - <para> - The final set of operations we need to provide for our radio are the - volume controls. Not all radio cards can even do volume control. After all - there is a perfectly good volume control on the sound card. We will assume - our radio card has a simple 4 step volume control. - </para> - <para> - There are two ioctls with audio we need to support - </para> - <programlisting> - -static int current_volume=0; - - case VIDIOCGAUDIO: - { - struct video_audio v; - if(copy_from_user(&v, arg, sizeof(v))) - return -EFAULT; - if(v.audio != 0) - return -EINVAL; - v.volume = 16384*current_volume; - v.step = 16384; - strcpy(v.name, "Radio"); - v.mode = VIDEO_SOUND_MONO; - v.balance = 0; - v.base = 0; - v.treble = 0; - - if(copy_to_user(arg. &v, sizeof(v))) - return -EFAULT; - return 0; - } - - </programlisting> - <para> - Much like the tuner we start by copying the user structure into kernel - space. Again we check if the user has asked for a valid audio input. We have - only input 0 and we punt if they ask for another input. - </para> - <para> - Then we fill in the video_audio structure. This has the following format - </para> - <table frame="all" id="video_audio_fields"><title>struct video_audio fields</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>audio</entry><entry>The input the user wishes to query</entry> - </row><row> - <entry>volume</entry><entry>The volume setting on a scale of 0-65535</entry> - </row><row> - <entry>base</entry><entry>The base level on a scale of 0-65535</entry> - </row><row> - <entry>treble</entry><entry>The treble level on a scale of 0-65535</entry> - </row><row> - <entry>flags</entry><entry>The features this audio device supports - </entry> - </row><row> - <entry>name</entry><entry>A text name to display to the user. We picked - "Radio" as it explains things quite nicely.</entry> - </row><row> - <entry>mode</entry><entry>The current reception mode for the audio - - We report MONO because our card is too stupid to know if it is in - mono or stereo. - </entry> - </row><row> - <entry>balance</entry><entry>The stereo balance on a scale of 0-65535, 32768 is - middle.</entry> - </row><row> - <entry>step</entry><entry>The step by which the volume control jumps. This is - used to help make it easy for applications to set - slider behaviour.</entry> - </row> - </tbody> - </tgroup> - </table> - - <table frame="all" id="video_audio_flags"><title>struct video_audio flags</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>VIDEO_AUDIO_MUTE</entry><entry>The audio is currently muted. We - could fake this in our driver but we - choose not to bother.</entry> - </row><row> - <entry>VIDEO_AUDIO_MUTABLE</entry><entry>The input has a mute option</entry> - </row><row> - <entry>VIDEO_AUDIO_TREBLE</entry><entry>The input has a treble control</entry> - </row><row> - <entry>VIDEO_AUDIO_BASS</entry><entry>The input has a base control</entry> - </row> - </tbody> - </tgroup> - </table> - - <table frame="all" id="video_audio_modes"><title>struct video_audio modes</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>VIDEO_SOUND_MONO</entry><entry>Mono sound</entry> - </row><row> - <entry>VIDEO_SOUND_STEREO</entry><entry>Stereo sound</entry> - </row><row> - <entry>VIDEO_SOUND_LANG1</entry><entry>Alternative language 1 (TV specific)</entry> - </row><row> - <entry>VIDEO_SOUND_LANG2</entry><entry>Alternative language 2 (TV specific)</entry> - </row> - </tbody> - </tgroup> - </table> - <para> - Having filled in the structure we copy it back to user space. - </para> - <para> - The VIDIOCSAUDIO ioctl allows the user to set the audio parameters in the - video_audio structure. The driver does its best to honour the request. - </para> - <programlisting> - - case VIDIOCSAUDIO: - { - struct video_audio v; - if(copy_from_user(&v, arg, sizeof(v))) - return -EFAULT; - if(v.audio) - return -EINVAL; - current_volume = v/16384; - hardware_set_volume(current_volume); - return 0; - } - - </programlisting> - <para> - In our case there is very little that the user can set. The volume is - basically the limit. Note that we could pretend to have a mute feature - by rewriting this to - </para> - <programlisting> - - case VIDIOCSAUDIO: - { - struct video_audio v; - if(copy_from_user(&v, arg, sizeof(v))) - return -EFAULT; - if(v.audio) - return -EINVAL; - current_volume = v/16384; - if(v.flags&VIDEO_AUDIO_MUTE) - hardware_set_volume(0); - else - hardware_set_volume(current_volume); - current_muted = v.flags & - VIDEO_AUDIO_MUTE; - return 0; - } - - </programlisting> - <para> - This with the corresponding changes to the VIDIOCGAUDIO code to report the - state of the mute flag we save and to report the card has a mute function, - will allow applications to use a mute facility with this card. It is - questionable whether this is a good idea however. User applications can already - fake this themselves and kernel space is precious. - </para> - <para> - We now have a working radio ioctl handler. So we just wrap up the function - </para> - <programlisting> - - - } - return -ENOIOCTLCMD; -} - - </programlisting> - <para> - and pass the Video4Linux layer back an error so that it knows we did not - understand the request we got passed. - </para> - </sect1> - <sect1 id="modradio"> - <title>Module Wrapper</title> - <para> - Finally we add in the usual module wrapping and the driver is done. - </para> - <programlisting> - -#ifndef MODULE - -static int io = 0x300; - -#else - -static int io = -1; - -#endif - -MODULE_AUTHOR("Alan Cox"); -MODULE_DESCRIPTION("A driver for an imaginary radio card."); -module_param(io, int, 0444); -MODULE_PARM_DESC(io, "I/O address of the card."); - -static int __init init(void) -{ - if(io==-1) - { - printk(KERN_ERR - "You must set an I/O address with io=0x???\n"); - return -EINVAL; - } - return myradio_init(NULL); -} - -static void __exit cleanup(void) -{ - video_unregister_device(&my_radio); - release_region(io, MY_IO_SIZE); -} - -module_init(init); -module_exit(cleanup); - - </programlisting> - <para> - In this example we set the IO base by default if the driver is compiled into - the kernel: you can still set it using "my_radio.irq" if this file is called <filename>my_radio.c</filename>. For the module we require the - user sets the parameter. We set io to a nonsense port (-1) so that we can - tell if the user supplied an io parameter or not. - </para> - <para> - We use MODULE_ defines to give an author for the card driver and a - description. We also use them to declare that io is an integer and it is the - address of the card, and can be read by anyone from sysfs. - </para> - <para> - The clean-up routine unregisters the video_device we registered, and frees - up the I/O space. Note that the unregister takes the actual video_device - structure as its argument. Unlike the file operations structure which can be - shared by all instances of a device a video_device structure as an actual - instance of the device. If you are registering multiple radio devices you - need to fill in one structure per device (most likely by setting up a - template and copying it to each of the actual device structures). - </para> - </sect1> - </chapter> - <chapter id="Video_Capture_Devices"> - <title>Video Capture Devices</title> - <sect1 id="introvid"> - <title>Video Capture Device Types</title> - <para> - The video capture devices share the same interfaces as radio devices. In - order to explain the video capture interface I will use the example of a - camera that has no tuners or audio input. This keeps the example relatively - clean. To get both combine the two driver examples. - </para> - <para> - Video capture devices divide into four categories. A little technology - backgrounder. Full motion video even at television resolution (which is - actually fairly low) is pretty resource-intensive. You are continually - passing megabytes of data every second from the capture card to the display. - several alternative approaches have emerged because copying this through the - processor and the user program is a particularly bad idea . - </para> - <para> - The first is to add the television image onto the video output directly. - This is also how some 3D cards work. These basic cards can generally drop the - video into any chosen rectangle of the display. Cards like this, which - include most mpeg1 cards that used the feature connector, aren't very - friendly in a windowing environment. They don't understand windows or - clipping. The video window is always on the top of the display. - </para> - <para> - Chroma keying is a technique used by cards to get around this. It is an old - television mixing trick where you mark all the areas you wish to replace - with a single clear colour that isn't used in the image - TV people use an - incredibly bright blue while computing people often use a particularly - virulent purple. Bright blue occurs on the desktop. Anyone with virulent - purple windows has another problem besides their TV overlay. - </para> - <para> - The third approach is to copy the data from the capture card to the video - card, but to do it directly across the PCI bus. This relieves the processor - from doing the work but does require some smartness on the part of the video - capture chip, as well as a suitable video card. Programming this kind of - card and more so debugging it can be extremely tricky. There are some quite - complicated interactions with the display and you may also have to cope with - various chipset bugs that show up when PCI cards start talking to each - other. - </para> - <para> - To keep our example fairly simple we will assume a card that supports - overlaying a flat rectangular image onto the frame buffer output, and which - can also capture stuff into processor memory. - </para> - </sect1> - <sect1 id="regvid"> - <title>Registering Video Capture Devices</title> - <para> - This time we need to add more functions for our camera device. - </para> - <programlisting> -static struct video_device my_camera -{ - "My Camera", - VID_TYPE_OVERLAY|VID_TYPE_SCALES|\ - VID_TYPE_CAPTURE|VID_TYPE_CHROMAKEY, - camera_open. - camera_close, - camera_read, /* no read */ - NULL, /* no write */ - camera_poll, /* no poll */ - camera_ioctl, - NULL, /* no special init function */ - NULL /* no private data */ -}; - </programlisting> - <para> - We need a read() function which is used for capturing data from - the card, and we need a poll function so that a driver can wait for the next - frame to be captured. - </para> - <para> - We use the extra video capability flags that did not apply to the - radio interface. The video related flags are - </para> - <table frame="all" id="Capture_Capabilities"><title>Capture Capabilities</title> - <tgroup cols="2" align="left"> - <tbody> - <row> -<entry>VID_TYPE_CAPTURE</entry><entry>We support image capture</entry> -</row><row> -<entry>VID_TYPE_TELETEXT</entry><entry>A teletext capture device (vbi{n])</entry> -</row><row> -<entry>VID_TYPE_OVERLAY</entry><entry>The image can be directly overlaid onto the - frame buffer</entry> -</row><row> -<entry>VID_TYPE_CHROMAKEY</entry><entry>Chromakey can be used to select which parts - of the image to display</entry> -</row><row> -<entry>VID_TYPE_CLIPPING</entry><entry>It is possible to give the board a list of - rectangles to draw around. </entry> -</row><row> -<entry>VID_TYPE_FRAMERAM</entry><entry>The video capture goes into the video memory - and actually changes it. Applications need - to know this so they can clean up after the - card</entry> -</row><row> -<entry>VID_TYPE_SCALES</entry><entry>The image can be scaled to various sizes, - rather than being a single fixed size.</entry> -</row><row> -<entry>VID_TYPE_MONOCHROME</entry><entry>The capture will be monochrome. This isn't a - complete answer to the question since a mono - camera on a colour capture card will still - produce mono output.</entry> -</row><row> -<entry>VID_TYPE_SUBCAPTURE</entry><entry>The card allows only part of its field of - view to be captured. This enables - applications to avoid copying all of a large - image into memory when only some section is - relevant.</entry> - </row> - </tbody> - </tgroup> - </table> - <para> - We set VID_TYPE_CAPTURE so that we are seen as a capture card, - VID_TYPE_CHROMAKEY so the application knows it is time to draw in virulent - purple, and VID_TYPE_SCALES because we can be resized. - </para> - <para> - Our setup is fairly similar. This time we also want an interrupt line - for the 'frame captured' signal. Not all cards have this so some of them - cannot handle poll(). - </para> - <programlisting> - - -static int io = 0x320; -static int irq = 11; - -int __init mycamera_init(struct video_init *v) -{ - if(!request_region(io, MY_IO_SIZE, "mycamera")) - { - printk(KERN_ERR - "mycamera: port 0x%03X is in use.\n", io); - return -EBUSY; - } - - if(video_device_register(&my_camera, - VFL_TYPE_GRABBER)==-1) { - release_region(io, MY_IO_SIZE); - return -EINVAL; - } - return 0; -} - - </programlisting> - <para> - This is little changed from the needs of the radio card. We specify - VFL_TYPE_GRABBER this time as we want to be allocated a /dev/video name. - </para> - </sect1> - <sect1 id="opvid"> - <title>Opening And Closing The Capture Device</title> - <programlisting> - - -static int users = 0; - -static int camera_open(struct video_device *dev, int flags) -{ - if(users) - return -EBUSY; - if(request_irq(irq, camera_irq, 0, "camera", dev)<0) - return -EBUSY; - users++; - return 0; -} - - -static int camera_close(struct video_device *dev) -{ - users--; - free_irq(irq, dev); -} - </programlisting> - <para> - The open and close routines are also quite similar. The only real change is - that we now request an interrupt for the camera device interrupt line. If we - cannot get the interrupt we report EBUSY to the application and give up. - </para> - </sect1> - <sect1 id="irqvid"> - <title>Interrupt Handling</title> - <para> - Our example handler is for an ISA bus device. If it was PCI you would be - able to share the interrupt and would have set IRQF_SHARED to indicate a - shared IRQ. We pass the device pointer as the interrupt routine argument. We - don't need to since we only support one card but doing this will make it - easier to upgrade the driver for multiple devices in the future. - </para> - <para> - Our interrupt routine needs to do little if we assume the card can simply - queue one frame to be read after it captures it. - </para> - <programlisting> - - -static struct wait_queue *capture_wait; -static int capture_ready = 0; - -static void camera_irq(int irq, void *dev_id, - struct pt_regs *regs) -{ - capture_ready=1; - wake_up_interruptible(&capture_wait); -} - </programlisting> - <para> - The interrupt handler is nice and simple for this card as we are assuming - the card is buffering the frame for us. This means we have little to do but - wake up anybody interested. We also set a capture_ready flag, as we may - capture a frame before an application needs it. In this case we need to know - that a frame is ready. If we had to collect the frame on the interrupt life - would be more complex. - </para> - <para> - The two new routines we need to supply are camera_read which returns a - frame, and camera_poll which waits for a frame to become ready. - </para> - <programlisting> - - -static int camera_poll(struct video_device *dev, - struct file *file, struct poll_table *wait) -{ - poll_wait(file, &capture_wait, wait); - if(capture_read) - return POLLIN|POLLRDNORM; - return 0; -} - - </programlisting> - <para> - Our wait queue for polling is the capture_wait queue. This will cause the - task to be woken up by our camera_irq routine. We check capture_read to see - if there is an image present and if so report that it is readable. - </para> - </sect1> - <sect1 id="rdvid"> - <title>Reading The Video Image</title> - <programlisting> - - -static long camera_read(struct video_device *dev, char *buf, - unsigned long count) -{ - struct wait_queue wait = { current, NULL }; - u8 *ptr; - int len; - int i; - - add_wait_queue(&capture_wait, &wait); - - while(!capture_ready) - { - if(file->flags&O_NDELAY) - { - remove_wait_queue(&capture_wait, &wait); - current->state = TASK_RUNNING; - return -EWOULDBLOCK; - } - if(signal_pending(current)) - { - remove_wait_queue(&capture_wait, &wait); - current->state = TASK_RUNNING; - return -ERESTARTSYS; - } - schedule(); - current->state = TASK_INTERRUPTIBLE; - } - remove_wait_queue(&capture_wait, &wait); - current->state = TASK_RUNNING; - - </programlisting> - <para> - The first thing we have to do is to ensure that the application waits until - the next frame is ready. The code here is almost identical to the mouse code - we used earlier in this chapter. It is one of the common building blocks of - Linux device driver code and probably one which you will find occurs in any - drivers you write. - </para> - <para> - We wait for a frame to be ready, or for a signal to interrupt our waiting. If a - signal occurs we need to return from the system call so that the signal can - be sent to the application itself. We also check to see if the user actually - wanted to avoid waiting - ie if they are using non-blocking I/O and have other things - to get on with. - </para> - <para> - Next we copy the data from the card to the user application. This is rarely - as easy as our example makes out. We will add capture_w, and capture_h here - to hold the width and height of the captured image. We assume the card only - supports 24bit RGB for now. - </para> - <programlisting> - - - - capture_ready = 0; - - ptr=(u8 *)buf; - len = capture_w * 3 * capture_h; /* 24bit RGB */ - - if(len>count) - len=count; /* Doesn't all fit */ - - for(i=0; i<len; i++) - { - put_user(inb(io+IMAGE_DATA), ptr); - ptr++; - } - - hardware_restart_capture(); - - return i; -} - - </programlisting> - <para> - For a real hardware device you would try to avoid the loop with put_user(). - Each call to put_user() has a time overhead checking whether the accesses to user - space are allowed. It would be better to read a line into a temporary buffer - then copy this to user space in one go. - </para> - <para> - Having captured the image and put it into user space we can kick the card to - get the next frame acquired. - </para> - </sect1> - <sect1 id="iocvid"> - <title>Video Ioctl Handling</title> - <para> - As with the radio driver the major control interface is via the ioctl() - function. Video capture devices support the same tuner calls as a radio - device and also support additional calls to control how the video functions - are handled. In this simple example the card has no tuners to avoid making - the code complex. - </para> - <programlisting> - - - -static int camera_ioctl(struct video_device *dev, unsigned int cmd, void *arg) -{ - switch(cmd) - { - case VIDIOCGCAP: - { - struct video_capability v; - v.type = VID_TYPE_CAPTURE|\ - VID_TYPE_CHROMAKEY|\ - VID_TYPE_SCALES|\ - VID_TYPE_OVERLAY; - v.channels = 1; - v.audios = 0; - v.maxwidth = 640; - v.minwidth = 16; - v.maxheight = 480; - v.minheight = 16; - strcpy(v.name, "My Camera"); - if(copy_to_user(arg, &v, sizeof(v))) - return -EFAULT; - return 0; - } - - - </programlisting> - <para> - The first ioctl we must support and which all video capture and radio - devices are required to support is VIDIOCGCAP. This behaves exactly the same - as with a radio device. This time, however, we report the extra capabilities - we outlined earlier on when defining our video_dev structure. - </para> - <para> - We now set the video flags saying that we support overlay, capture, - scaling and chromakey. We also report size limits - our smallest image is - 16x16 pixels, our largest is 640x480. - </para> - <para> - To keep things simple we report no audio and no tuning capabilities at all. - </para> - <programlisting> - - case VIDIOCGCHAN: - { - struct video_channel v; - if(copy_from_user(&v, arg, sizeof(v))) - return -EFAULT; - if(v.channel != 0) - return -EINVAL; - v.flags = 0; - v.tuners = 0; - v.type = VIDEO_TYPE_CAMERA; - v.norm = VIDEO_MODE_AUTO; - strcpy(v.name, "Camera Input");break; - if(copy_to_user(&v, arg, sizeof(v))) - return -EFAULT; - return 0; - } - - - </programlisting> - <para> - This follows what is very much the standard way an ioctl handler looks - in Linux. We copy the data into a kernel space variable and we check that the - request is valid (in this case that the input is 0). Finally we copy the - camera info back to the user. - </para> - <para> - The VIDIOCGCHAN ioctl allows a user to ask about video channels (that is - inputs to the video card). Our example card has a single camera input. The - fields in the structure are - </para> - <table frame="all" id="video_channel_fields"><title>struct video_channel fields</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - - <entry>channel</entry><entry>The channel number we are selecting</entry> - </row><row> - <entry>name</entry><entry>The name for this channel. This is intended - to describe the port to the user. - Appropriate names are therefore things like - "Camera" "SCART input"</entry> - </row><row> - <entry>flags</entry><entry>Channel properties</entry> - </row><row> - <entry>type</entry><entry>Input type</entry> - </row><row> - <entry>norm</entry><entry>The current television encoding being used - if relevant for this channel. - </entry> - </row> - </tbody> - </tgroup> - </table> - <table frame="all" id="video_channel_flags"><title>struct video_channel flags</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>VIDEO_VC_TUNER</entry><entry>Channel has a tuner.</entry> - </row><row> - <entry>VIDEO_VC_AUDIO</entry><entry>Channel has audio.</entry> - </row> - </tbody> - </tgroup> - </table> - <table frame="all" id="video_channel_types"><title>struct video_channel types</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>VIDEO_TYPE_TV</entry><entry>Television input.</entry> - </row><row> - <entry>VIDEO_TYPE_CAMERA</entry><entry>Fixed camera input.</entry> - </row><row> - <entry>0</entry><entry>Type is unknown.</entry> - </row> - </tbody> - </tgroup> - </table> - <table frame="all" id="video_channel_norms"><title>struct video_channel norms</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>VIDEO_MODE_PAL</entry><entry>PAL encoded Television</entry> - </row><row> - <entry>VIDEO_MODE_NTSC</entry><entry>NTSC (US) encoded Television</entry> - </row><row> - <entry>VIDEO_MODE_SECAM</entry><entry>SECAM (French) Television </entry> - </row><row> - <entry>VIDEO_MODE_AUTO</entry><entry>Automatic switching, or format does not - matter</entry> - </row> - </tbody> - </tgroup> - </table> - <para> - The corresponding VIDIOCSCHAN ioctl allows a user to change channel and to - request the norm is changed - for example to switch between a PAL or an NTSC - format camera. - </para> - <programlisting> - - - case VIDIOCSCHAN: - { - struct video_channel v; - if(copy_from_user(&v, arg, sizeof(v))) - return -EFAULT; - if(v.channel != 0) - return -EINVAL; - if(v.norm != VIDEO_MODE_AUTO) - return -EINVAL; - return 0; - } - - - </programlisting> - <para> - The implementation of this call in our driver is remarkably easy. Because we - are assuming fixed format hardware we need only check that the user has not - tried to change anything. - </para> - <para> - The user also needs to be able to configure and adjust the picture they are - seeing. This is much like adjusting a television set. A user application - also needs to know the palette being used so that it knows how to display - the image that has been captured. The VIDIOCGPICT and VIDIOCSPICT ioctl - calls provide this information. - </para> - <programlisting> - - - case VIDIOCGPICT - { - struct video_picture v; - v.brightness = hardware_brightness(); - v.hue = hardware_hue(); - v.colour = hardware_saturation(); - v.contrast = hardware_brightness(); - /* Not settable */ - v.whiteness = 32768; - v.depth = 24; /* 24bit */ - v.palette = VIDEO_PALETTE_RGB24; - if(copy_to_user(&v, arg, - sizeof(v))) - return -EFAULT; - return 0; - } - - - </programlisting> - <para> - The brightness, hue, color, and contrast provide the picture controls that - are akin to a conventional television. Whiteness provides additional - control for greyscale images. All of these values are scaled between 0-65535 - and have 32768 as the mid point setting. The scaling means that applications - do not have to worry about the capability range of the hardware but can let - it make a best effort attempt. - </para> - <para> - Our depth is 24, as this is in bits. We will be returning RGB24 format. This - has one byte of red, then one of green, then one of blue. This then repeats - for every other pixel in the image. The other common formats the interface - defines are - </para> - <table frame="all" id="Framebuffer_Encodings"><title>Framebuffer Encodings</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>GREY</entry><entry>Linear greyscale. This is for simple cameras and the - like</entry> - </row><row> - <entry>RGB565</entry><entry>The top 5 bits hold 32 red levels, the next six bits - hold green and the low 5 bits hold blue. </entry> - </row><row> - <entry>RGB555</entry><entry>The top bit is clear. The red green and blue levels - each occupy five bits.</entry> - </row> - </tbody> - </tgroup> - </table> - <para> - Additional modes are support for YUV capture formats. These are common for - TV and video conferencing applications. - </para> - <para> - The VIDIOCSPICT ioctl allows a user to set some of the picture parameters. - Exactly which ones are supported depends heavily on the card itself. It is - possible to support many modes and effects in software. In general doing - this in the kernel is a bad idea. Video capture is a performance-sensitive - application and the programs can often do better if they aren't being - 'helped' by an overkeen driver writer. Thus for our device we will report - RGB24 only and refuse to allow a change. - </para> - <programlisting> - - - case VIDIOCSPICT: - { - struct video_picture v; - if(copy_from_user(&v, arg, sizeof(v))) - return -EFAULT; - if(v.depth!=24 || - v.palette != VIDEO_PALETTE_RGB24) - return -EINVAL; - set_hardware_brightness(v.brightness); - set_hardware_hue(v.hue); - set_hardware_saturation(v.colour); - set_hardware_brightness(v.contrast); - return 0; - } - - - </programlisting> - <para> - We check the user has not tried to change the palette or the depth. We do - not want to carry out some of the changes and then return an error. This may - confuse the application which will be assuming no change occurred. - </para> - <para> - In much the same way as you need to be able to set the picture controls to - get the right capture images, many cards need to know what they are - displaying onto when generating overlay output. In some cases getting this - wrong even makes a nasty mess or may crash the computer. For that reason - the VIDIOCSBUF ioctl used to set up the frame buffer information may well - only be usable by root. - </para> - <para> - We will assume our card is one of the old ISA devices with feature connector - and only supports a couple of standard video modes. Very common for older - cards although the PCI devices are way smarter than this. - </para> - <programlisting> - - -static struct video_buffer capture_fb; - - case VIDIOCGFBUF: - { - if(copy_to_user(arg, &capture_fb, - sizeof(capture_fb))) - return -EFAULT; - return 0; - - } - - - </programlisting> - <para> - We keep the frame buffer information in the format the ioctl uses. This - makes it nice and easy to work with in the ioctl calls. - </para> - <programlisting> - - case VIDIOCSFBUF: - { - struct video_buffer v; - - if(!capable(CAP_SYS_ADMIN)) - return -EPERM; - - if(copy_from_user(&v, arg, sizeof(v))) - return -EFAULT; - if(v.width!=320 && v.width!=640) - return -EINVAL; - if(v.height!=200 && v.height!=240 - && v.height!=400 - && v.height !=480) - return -EINVAL; - memcpy(&capture_fb, &v, sizeof(v)); - hardware_set_fb(&v); - return 0; - } - - - - </programlisting> - <para> - The capable() function checks a user has the required capability. The Linux - operating system has a set of about 30 capabilities indicating privileged - access to services. The default set up gives the superuser (uid 0) all of - them and nobody else has any. - </para> - <para> - We check that the user has the SYS_ADMIN capability, that is they are - allowed to operate as the machine administrator. We don't want anyone but - the administrator making a mess of the display. - </para> - <para> - Next we check for standard PC video modes (320 or 640 wide with either - EGA or VGA depths). If the mode is not a standard video mode we reject it as - not supported by our card. If the mode is acceptable we save it so that - VIDIOCFBUF will give the right answer next time it is called. The - hardware_set_fb() function is some undescribed card specific function to - program the card for the desired mode. - </para> - <para> - Before the driver can display an overlay window it needs to know where the - window should be placed, and also how large it should be. If the card - supports clipping it needs to know which rectangles to omit from the - display. The video_window structure is used to describe the way the image - should be displayed. - </para> - <table frame="all" id="video_window_fields"><title>struct video_window fields</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>width</entry><entry>The width in pixels of the desired image. The card - may use a smaller size if this size is not available</entry> - </row><row> - <entry>height</entry><entry>The height of the image. The card may use a smaller - size if this size is not available.</entry> - </row><row> - <entry>x</entry><entry> The X position of the top left of the window. This - is in pixels relative to the left hand edge of the - picture. Not all cards can display images aligned on - any pixel boundary. If the position is unsuitable - the card adjusts the image right and reduces the - width.</entry> - </row><row> - <entry>y</entry><entry> The Y position of the top left of the window. This - is counted in pixels relative to the top edge of the - picture. As with the width if the card cannot - display starting on this line it will adjust the - values.</entry> - </row><row> - <entry>chromakey</entry><entry>The colour (expressed in RGB32 format) for the - chromakey colour if chroma keying is being used. </entry> - </row><row> - <entry>clips</entry><entry>An array of rectangles that must not be drawn - over.</entry> - </row><row> - <entry>clipcount</entry><entry>The number of clips in this array.</entry> - </row> - </tbody> - </tgroup> - </table> - <para> - Each clip is a struct video_clip which has the following fields - </para> - <table frame="all" id="video_clip_fields"><title>video_clip fields</title> - <tgroup cols="2" align="left"> - <tbody> - <row> - <entry>x, y</entry><entry>Co-ordinates relative to the display</entry> - </row><row> - <entry>width, height</entry><entry>Width and height in pixels</entry> - </row><row> - <entry>next</entry><entry>A spare field for the application to use</entry> - </row> - </tbody> - </tgroup> - </table> - <para> - The driver is required to ensure it always draws in the area requested or a smaller area, and that it never draws in any of the areas that are clipped. - This may well mean it has to leave alone. small areas the application wished to be - drawn. - </para> - <para> - Our example card uses chromakey so does not have to address most of the - clipping. We will add a video_window structure to our global variables to - remember our parameters, as we did with the frame buffer. - </para> - <programlisting> - - - case VIDIOCGWIN: - { - if(copy_to_user(arg, &capture_win, - sizeof(capture_win))) - return -EFAULT; - return 0; - } - - - case VIDIOCSWIN: - { - struct video_window v; - if(copy_from_user(&v, arg, sizeof(v))) - return -EFAULT; - if(v.width > 640 || v.height > 480) - return -EINVAL; - if(v.width < 16 || v.height < 16) - return -EINVAL; - hardware_set_key(v.chromakey); - hardware_set_window(v); - memcpy(&capture_win, &v, sizeof(v)); - capture_w = v.width; - capture_h = v.height; - return 0; - } - - - </programlisting> - <para> - Because we are using Chromakey our setup is fairly simple. Mostly we have to - check the values are sane and load them into the capture card. - </para> - <para> - With all the setup done we can now turn on the actual capture/overlay. This - is done with the VIDIOCCAPTURE ioctl. This takes a single integer argument - where 0 is on and 1 is off. - </para> - <programlisting> - - - case VIDIOCCAPTURE: - { - int v; - if(get_user(v, (int *)arg)) - return -EFAULT; - if(v==0) - hardware_capture_off(); - else - { - if(capture_fb.width == 0 - || capture_w == 0) - return -EINVAL; - hardware_capture_on(); - } - return 0; - } - - - </programlisting> - <para> - We grab the flag from user space and either enable or disable according to - its value. There is one small corner case we have to consider here. Suppose - that the capture was requested before the video window or the frame buffer - had been set up. In those cases there will be unconfigured fields in our - card data, as well as unconfigured hardware settings. We check for this case and - return an error if the frame buffer or the capture window width is zero. - </para> - <programlisting> - - - default: - return -ENOIOCTLCMD; - } -} - </programlisting> - <para> - - We don't need to support any other ioctls, so if we get this far, it is time - to tell the video layer that we don't now what the user is talking about. - </para> - </sect1> - <sect1 id="endvid"> - <title>Other Functionality</title> - <para> - The Video4Linux layer supports additional features, including a high - performance mmap() based capture mode and capturing part of the image. - These features are out of the scope of the book. You should however have enough - example code to implement most simple video4linux devices for radio and TV - cards. - </para> - </sect1> - </chapter> - <chapter id="bugs"> - <title>Known Bugs And Assumptions</title> - <para> - <variablelist> - <varlistentry><term>Multiple Opens</term> - <listitem> - <para> - The driver assumes multiple opens should not be allowed. A driver - can work around this but not cleanly. - </para> - </listitem></varlistentry> - - <varlistentry><term>API Deficiencies</term> - <listitem> - <para> - The existing API poorly reflects compression capable devices. There - are plans afoot to merge V4L, V4L2 and some other ideas into a - better interface. - </para> - </listitem></varlistentry> - </variablelist> - - </para> - </chapter> - - <chapter id="pubfunctions"> - <title>Public Functions Provided</title> -!Edrivers/media/video/v4l2-dev.c - </chapter> - -</book> |