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- <title>Common API Elements</title>
-
- <para>Programming a V4L2 device consists of these
-steps:</para>
-
- <itemizedlist>
- <listitem>
- <para>Opening the device</para>
- </listitem>
- <listitem>
- <para>Changing device properties, selecting a video and audio
-input, video standard, picture brightness a.&nbsp;o.</para>
- </listitem>
- <listitem>
- <para>Negotiating a data format</para>
- </listitem>
- <listitem>
- <para>Negotiating an input/output method</para>
- </listitem>
- <listitem>
- <para>The actual input/output loop</para>
- </listitem>
- <listitem>
- <para>Closing the device</para>
- </listitem>
- </itemizedlist>
-
- <para>In practice most steps are optional and can be executed out of
-order. It depends on the V4L2 device type, you can read about the
-details in <xref linkend="devices" />. In this chapter we will discuss
-the basic concepts applicable to all devices.</para>
-
- <section id="open">
- <title>Opening and Closing Devices</title>
-
- <section>
- <title>Device Naming</title>
-
- <para>V4L2 drivers are implemented as kernel modules, loaded
-manually by the system administrator or automatically when a device is
-first opened. The driver modules plug into the "videodev" kernel
-module. It provides helper functions and a common application
-interface specified in this document.</para>
-
- <para>Each driver thus loaded registers one or more device nodes
-with major number 81 and a minor number between 0 and 255. Assigning
-minor numbers to V4L2 devices is entirely up to the system administrator,
-this is primarily intended to solve conflicts between devices.<footnote>
- <para>Access permissions are associated with character
-device special files, hence we must ensure device numbers cannot
-change with the module load order. To this end minor numbers are no
-longer automatically assigned by the "videodev" module as in V4L but
-requested by the driver. The defaults will suffice for most people
-unless two drivers compete for the same minor numbers.</para>
- </footnote> The module options to select minor numbers are named
-after the device special file with a "_nr" suffix. For example "video_nr"
-for <filename>/dev/video</filename> video capture devices. The number is
-an offset to the base minor number associated with the device type.
-<footnote>
- <para>In earlier versions of the V4L2 API the module options
-where named after the device special file with a "unit_" prefix, expressing
-the minor number itself, not an offset. Rationale for this change is unknown.
-Lastly the naming and semantics are just a convention among driver writers,
-the point to note is that minor numbers are not supposed to be hardcoded
-into drivers.</para>
- </footnote> When the driver supports multiple devices of the same
-type more than one minor number can be assigned, separated by commas:
-<informalexample>
- <screen>
-&gt; insmod mydriver.o video_nr=0,1 radio_nr=0,1</screen>
- </informalexample></para>
-
- <para>In <filename>/etc/modules.conf</filename> this may be
-written as: <informalexample>
- <screen>
-alias char-major-81-0 mydriver
-alias char-major-81-1 mydriver
-alias char-major-81-64 mydriver <co id="alias" />
-options mydriver video_nr=0,1 radio_nr=0,1 <co id="options" />
- </screen>
- <calloutlist>
- <callout arearefs="alias">
- <para>When an application attempts to open a device
-special file with major number 81 and minor number 0, 1, or 64, load
-"mydriver" (and the "videodev" module it depends upon).</para>
- </callout>
- <callout arearefs="options">
- <para>Register the first two video capture devices with
-minor number 0 and 1 (base number is 0), the first two radio device
-with minor number 64 and 65 (base 64).</para>
- </callout>
- </calloutlist>
- </informalexample> When no minor number is given as module
-option the driver supplies a default. <xref linkend="devices" />
-recommends the base minor numbers to be used for the various device
-types. Obviously minor numbers must be unique. When the number is
-already in use the <emphasis>offending device</emphasis> will not be
-registered. <!-- Blessed by Linus Torvalds on
-linux-kernel@vger.kernel.org, 2002-11-20. --></para>
-
- <para>By convention system administrators create various
-character device special files with these major and minor numbers in
-the <filename>/dev</filename> directory. The names recommended for the
-different V4L2 device types are listed in <xref linkend="devices" />.
-</para>
-
- <para>The creation of character special files (with
-<application>mknod</application>) is a privileged operation and
-devices cannot be opened by major and minor number. That means
-applications cannot <emphasis>reliable</emphasis> scan for loaded or
-installed drivers. The user must enter a device name, or the
-application can try the conventional device names.</para>
-
- <para>Under the device filesystem (devfs) the minor number
-options are ignored. V4L2 drivers (or by proxy the "videodev" module)
-automatically create the required device files in the
-<filename>/dev/v4l</filename> directory using the conventional device
-names above.</para>
- </section>
-
- <section id="related">
- <title>Related Devices</title>
-
- <para>Devices can support several related functions. For example
-video capturing, video overlay and VBI capturing are related because
-these functions share, amongst other, the same video input and tuner
-frequency. V4L and earlier versions of V4L2 used the same device name
-and minor number for video capturing and overlay, but different ones
-for VBI. Experience showed this approach has several problems<footnote>
- <para>Given a device file name one cannot reliable find
-related devices. For once names are arbitrary and in a system with
-multiple devices, where only some support VBI capturing, a
-<filename>/dev/video2</filename> is not necessarily related to
-<filename>/dev/vbi2</filename>. The V4L
-<constant>VIDIOCGUNIT</constant> ioctl would require a search for a
-device file with a particular major and minor number.</para>
- </footnote>, and to make things worse the V4L videodev module
-used to prohibit multiple opens of a device.</para>
-
- <para>As a remedy the present version of the V4L2 API relaxed the
-concept of device types with specific names and minor numbers. For
-compatibility with old applications drivers must still register different
-minor numbers to assign a default function to the device. But if related
-functions are supported by the driver they must be available under all
-registered minor numbers. The desired function can be selected after
-opening the device as described in <xref linkend="devices" />.</para>
-
- <para>Imagine a driver supporting video capturing, video
-overlay, raw VBI capturing, and FM radio reception. It registers three
-devices with minor number 0, 64 and 224 (this numbering scheme is
-inherited from the V4L API). Regardless if
-<filename>/dev/video</filename> (81, 0) or
-<filename>/dev/vbi</filename> (81, 224) is opened the application can
-select any one of the video capturing, overlay or VBI capturing
-functions. Without programming (e.&nbsp;g. reading from the device
-with <application>dd</application> or <application>cat</application>)
-<filename>/dev/video</filename> captures video images, while
-<filename>/dev/vbi</filename> captures raw VBI data.
-<filename>/dev/radio</filename> (81, 64) is invariable a radio device,
-unrelated to the video functions. Being unrelated does not imply the
-devices can be used at the same time, however. The &func-open;
-function may very well return an &EBUSY;.</para>
-
- <para>Besides video input or output the hardware may also
-support audio sampling or playback. If so, these functions are
-implemented as OSS or ALSA PCM devices and eventually OSS or ALSA
-audio mixer. The V4L2 API makes no provisions yet to find these
-related devices. If you have an idea please write to the linux-media
-mailing list: &v4l-ml;.</para>
- </section>
-
- <section>
- <title>Multiple Opens</title>
-
- <para>In general, V4L2 devices can be opened more than once.
-When this is supported by the driver, users can for example start a
-"panel" application to change controls like brightness or audio
-volume, while another application captures video and audio. In other words, panel
-applications are comparable to an OSS or ALSA audio mixer application.
-When a device supports multiple functions like capturing and overlay
-<emphasis>simultaneously</emphasis>, multiple opens allow concurrent
-use of the device by forked processes or specialized applications.</para>
-
- <para>Multiple opens are optional, although drivers should
-permit at least concurrent accesses without data exchange, &ie; panel
-applications. This implies &func-open; can return an &EBUSY; when the
-device is already in use, as well as &func-ioctl; functions initiating
-data exchange (namely the &VIDIOC-S-FMT; ioctl), and the &func-read;
-and &func-write; functions.</para>
-
- <para>Mere opening a V4L2 device does not grant exclusive
-access.<footnote>
- <para>Drivers could recognize the
-<constant>O_EXCL</constant> open flag. Presently this is not required,
-so applications cannot know if it really works.</para>
- </footnote> Initiating data exchange however assigns the right
-to read or write the requested type of data, and to change related
-properties, to this file descriptor. Applications can request
-additional access privileges using the priority mechanism described in
-<xref linkend="app-pri" />.</para>
- </section>
-
- <section>
- <title>Shared Data Streams</title>
-
- <para>V4L2 drivers should not support multiple applications
-reading or writing the same data stream on a device by copying
-buffers, time multiplexing or similar means. This is better handled by
-a proxy application in user space. When the driver supports stream
-sharing anyway it must be implemented transparently. The V4L2 API does
-not specify how conflicts are solved. <!-- For example O_EXCL when the
-application does not want to be preempted, PROT_READ mmapped buffers
-which can be mapped twice, what happens when image formats do not
-match etc.--></para>
- </section>
-
- <section>
- <title>Functions</title>
-
- <para>To open and close V4L2 devices applications use the
-&func-open; and &func-close; function, respectively. Devices are
-programmed using the &func-ioctl; function as explained in the
-following sections.</para>
- </section>
- </section>
-
- <section id="querycap">
- <title>Querying Capabilities</title>
-
- <para>Because V4L2 covers a wide variety of devices not all
-aspects of the API are equally applicable to all types of devices.
-Furthermore devices of the same type have different capabilities and
-this specification permits the omission of a few complicated and less
-important parts of the API.</para>
-
- <para>The &VIDIOC-QUERYCAP; ioctl is available to check if the kernel
-device is compatible with this specification, and to query the <link
-linkend="devices">functions</link> and <link linkend="io">I/O
-methods</link> supported by the device.</para>
-
- <para>Starting with kernel version 3.1, VIDIOC-QUERYCAP will return the
-V4L2 API version used by the driver, with generally matches the Kernel version.
-There's no need of using &VIDIOC-QUERYCAP; to check if an specific ioctl is
-supported, the V4L2 core now returns ENOIOCTLCMD if a driver doesn't provide
-support for an ioctl.</para>
-
- <para>Other features can be queried
-by calling the respective ioctl, for example &VIDIOC-ENUMINPUT;
-to learn about the number, types and names of video connectors on the
-device. Although abstraction is a major objective of this API, the
-ioctl also allows driver specific applications to reliable identify
-the driver.</para>
-
- <para>All V4L2 drivers must support
-<constant>VIDIOC_QUERYCAP</constant>. Applications should always call
-this ioctl after opening the device.</para>
- </section>
-
- <section id="app-pri">
- <title>Application Priority</title>
-
- <para>When multiple applications share a device it may be
-desirable to assign them different priorities. Contrary to the
-traditional "rm -rf /" school of thought a video recording application
-could for example block other applications from changing video
-controls or switching the current TV channel. Another objective is to
-permit low priority applications working in background, which can be
-preempted by user controlled applications and automatically regain
-control of the device at a later time.</para>
-
- <para>Since these features cannot be implemented entirely in user
-space V4L2 defines the &VIDIOC-G-PRIORITY; and &VIDIOC-S-PRIORITY;
-ioctls to request and query the access priority associate with a file
-descriptor. Opening a device assigns a medium priority, compatible
-with earlier versions of V4L2 and drivers not supporting these ioctls.
-Applications requiring a different priority will usually call
-<constant>VIDIOC_S_PRIORITY</constant> after verifying the device with
-the &VIDIOC-QUERYCAP; ioctl.</para>
-
- <para>Ioctls changing driver properties, such as &VIDIOC-S-INPUT;,
-return an &EBUSY; after another application obtained higher priority.
-An event mechanism to notify applications about asynchronous property
-changes has been proposed but not added yet.</para>
- </section>
-
- <section id="video">
- <title>Video Inputs and Outputs</title>
-
- <para>Video inputs and outputs are physical connectors of a
-device. These can be for example RF connectors (antenna/cable), CVBS
-a.k.a. Composite Video, S-Video or RGB connectors. Only video and VBI
-capture devices have inputs, output devices have outputs, at least one
-each. Radio devices have no video inputs or outputs.</para>
-
- <para>To learn about the number and attributes of the
-available inputs and outputs applications can enumerate them with the
-&VIDIOC-ENUMINPUT; and &VIDIOC-ENUMOUTPUT; ioctl, respectively. The
-&v4l2-input; returned by the <constant>VIDIOC_ENUMINPUT</constant>
-ioctl also contains signal status information applicable when the
-current video input is queried.</para>
-
- <para>The &VIDIOC-G-INPUT; and &VIDIOC-G-OUTPUT; ioctl return the
-index of the current video input or output. To select a different
-input or output applications call the &VIDIOC-S-INPUT; and
-&VIDIOC-S-OUTPUT; ioctl. Drivers must implement all the input ioctls
-when the device has one or more inputs, all the output ioctls when the
-device has one or more outputs.</para>
-
- <!--
- <figure id=io-tree>
- <title>Input and output enumeration is the root of most device properties.</title>
- <mediaobject>
- <imageobject>
- <imagedata fileref="links.pdf" format="ps" />
- </imageobject>
- <imageobject>
- <imagedata fileref="links.gif" format="gif" />
- </imageobject>
- <textobject>
- <phrase>Links between various device property structures.</phrase>
- </textobject>
- </mediaobject>
- </figure>
- -->
-
- <example>
- <title>Information about the current video input</title>
-
- <programlisting>
-&v4l2-input; input;
-int index;
-
-if (-1 == ioctl (fd, &VIDIOC-G-INPUT;, &amp;index)) {
- perror ("VIDIOC_G_INPUT");
- exit (EXIT_FAILURE);
-}
-
-memset (&amp;input, 0, sizeof (input));
-input.index = index;
-
-if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &amp;input)) {
- perror ("VIDIOC_ENUMINPUT");
- exit (EXIT_FAILURE);
-}
-
-printf ("Current input: %s\n", input.name);
- </programlisting>
- </example>
-
- <example>
- <title>Switching to the first video input</title>
-
- <programlisting>
-int index;
-
-index = 0;
-
-if (-1 == ioctl (fd, &VIDIOC-S-INPUT;, &amp;index)) {
- perror ("VIDIOC_S_INPUT");
- exit (EXIT_FAILURE);
-}
- </programlisting>
- </example>
- </section>
-
- <section id="audio">
- <title>Audio Inputs and Outputs</title>
-
- <para>Audio inputs and outputs are physical connectors of a
-device. Video capture devices have inputs, output devices have
-outputs, zero or more each. Radio devices have no audio inputs or
-outputs. They have exactly one tuner which in fact
-<emphasis>is</emphasis> an audio source, but this API associates
-tuners with video inputs or outputs only, and radio devices have
-none of these.<footnote>
- <para>Actually &v4l2-audio; ought to have a
-<structfield>tuner</structfield> field like &v4l2-input;, not only
-making the API more consistent but also permitting radio devices with
-multiple tuners.</para>
- </footnote> A connector on a TV card to loop back the received
-audio signal to a sound card is not considered an audio output.</para>
-
- <para>Audio and video inputs and outputs are associated. Selecting
-a video source also selects an audio source. This is most evident when
-the video and audio source is a tuner. Further audio connectors can
-combine with more than one video input or output. Assumed two
-composite video inputs and two audio inputs exist, there may be up to
-four valid combinations. The relation of video and audio connectors
-is defined in the <structfield>audioset</structfield> field of the
-respective &v4l2-input; or &v4l2-output;, where each bit represents
-the index number, starting at zero, of one audio input or output.</para>
-
- <para>To learn about the number and attributes of the
-available inputs and outputs applications can enumerate them with the
-&VIDIOC-ENUMAUDIO; and &VIDIOC-ENUMAUDOUT; ioctl, respectively. The
-&v4l2-audio; returned by the <constant>VIDIOC_ENUMAUDIO</constant> ioctl
-also contains signal status information applicable when the current
-audio input is queried.</para>
-
- <para>The &VIDIOC-G-AUDIO; and &VIDIOC-G-AUDOUT; ioctl report
-the current audio input and output, respectively. Note that, unlike
-&VIDIOC-G-INPUT; and &VIDIOC-G-OUTPUT; these ioctls return a structure
-as <constant>VIDIOC_ENUMAUDIO</constant> and
-<constant>VIDIOC_ENUMAUDOUT</constant> do, not just an index.</para>
-
- <para>To select an audio input and change its properties
-applications call the &VIDIOC-S-AUDIO; ioctl. To select an audio
-output (which presently has no changeable properties) applications
-call the &VIDIOC-S-AUDOUT; ioctl.</para>
-
- <para>Drivers must implement all input ioctls when the device
-has one or more inputs, all output ioctls when the device has one
-or more outputs. When the device has any audio inputs or outputs the
-driver must set the <constant>V4L2_CAP_AUDIO</constant> flag in the
-&v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl.</para>
-
- <example>
- <title>Information about the current audio input</title>
-
- <programlisting>
-&v4l2-audio; audio;
-
-memset (&amp;audio, 0, sizeof (audio));
-
-if (-1 == ioctl (fd, &VIDIOC-G-AUDIO;, &amp;audio)) {
- perror ("VIDIOC_G_AUDIO");
- exit (EXIT_FAILURE);
-}
-
-printf ("Current input: %s\n", audio.name);
- </programlisting>
- </example>
-
- <example>
- <title>Switching to the first audio input</title>
-
- <programlisting>
-&v4l2-audio; audio;
-
-memset (&amp;audio, 0, sizeof (audio)); /* clear audio.mode, audio.reserved */
-
-audio.index = 0;
-
-if (-1 == ioctl (fd, &VIDIOC-S-AUDIO;, &amp;audio)) {
- perror ("VIDIOC_S_AUDIO");
- exit (EXIT_FAILURE);
-}
- </programlisting>
- </example>
- </section>
-
- <section id="tuner">
- <title>Tuners and Modulators</title>
-
- <section>
- <title>Tuners</title>
-
- <para>Video input devices can have one or more tuners
-demodulating a RF signal. Each tuner is associated with one or more
-video inputs, depending on the number of RF connectors on the tuner.
-The <structfield>type</structfield> field of the respective
-&v4l2-input; returned by the &VIDIOC-ENUMINPUT; ioctl is set to
-<constant>V4L2_INPUT_TYPE_TUNER</constant> and its
-<structfield>tuner</structfield> field contains the index number of
-the tuner.</para>
-
- <para>Radio devices have exactly one tuner with index zero, no
-video inputs.</para>
-
- <para>To query and change tuner properties applications use the
-&VIDIOC-G-TUNER; and &VIDIOC-S-TUNER; ioctl, respectively. The
-&v4l2-tuner; returned by <constant>VIDIOC_G_TUNER</constant> also
-contains signal status information applicable when the tuner of the
-current video input, or a radio tuner is queried. Note that
-<constant>VIDIOC_S_TUNER</constant> does not switch the current tuner,
-when there is more than one at all. The tuner is solely determined by
-the current video input. Drivers must support both ioctls and set the
-<constant>V4L2_CAP_TUNER</constant> flag in the &v4l2-capability;
-returned by the &VIDIOC-QUERYCAP; ioctl when the device has one or
-more tuners.</para>
- </section>
-
- <section>
- <title>Modulators</title>
-
- <para>Video output devices can have one or more modulators, uh,
-modulating a video signal for radiation or connection to the antenna
-input of a TV set or video recorder. Each modulator is associated with
-one or more video outputs, depending on the number of RF connectors on
-the modulator. The <structfield>type</structfield> field of the
-respective &v4l2-output; returned by the &VIDIOC-ENUMOUTPUT; ioctl is
-set to <constant>V4L2_OUTPUT_TYPE_MODULATOR</constant> and its
-<structfield>modulator</structfield> field contains the index number
-of the modulator. This specification does not define radio output
-devices.</para>
-
- <para>To query and change modulator properties applications use
-the &VIDIOC-G-MODULATOR; and &VIDIOC-S-MODULATOR; ioctl. Note that
-<constant>VIDIOC_S_MODULATOR</constant> does not switch the current
-modulator, when there is more than one at all. The modulator is solely
-determined by the current video output. Drivers must support both
-ioctls and set the <constant>V4L2_CAP_MODULATOR</constant> flag in
-the &v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl when the
-device has one or more modulators.</para>
- </section>
-
- <section>
- <title>Radio Frequency</title>
-
- <para>To get and set the tuner or modulator radio frequency
-applications use the &VIDIOC-G-FREQUENCY; and &VIDIOC-S-FREQUENCY;
-ioctl which both take a pointer to a &v4l2-frequency;. These ioctls
-are used for TV and radio devices alike. Drivers must support both
-ioctls when the tuner or modulator ioctls are supported, or
-when the device is a radio device.</para>
- </section>
- </section>
-
- <section id="standard">
- <title>Video Standards</title>
-
- <para>Video devices typically support one or more different video
-standards or variations of standards. Each video input and output may
-support another set of standards. This set is reported by the
-<structfield>std</structfield> field of &v4l2-input; and
-&v4l2-output; returned by the &VIDIOC-ENUMINPUT; and
-&VIDIOC-ENUMOUTPUT; ioctl, respectively.</para>
-
- <para>V4L2 defines one bit for each analog video standard
-currently in use worldwide, and sets aside bits for driver defined
-standards, &eg; hybrid standards to watch NTSC video tapes on PAL TVs
-and vice versa. Applications can use the predefined bits to select a
-particular standard, although presenting the user a menu of supported
-standards is preferred. To enumerate and query the attributes of the
-supported standards applications use the &VIDIOC-ENUMSTD; ioctl.</para>
-
- <para>Many of the defined standards are actually just variations
-of a few major standards. The hardware may in fact not distinguish
-between them, or do so internal and switch automatically. Therefore
-enumerated standards also contain sets of one or more standard
-bits.</para>
-
- <para>Assume a hypothetic tuner capable of demodulating B/PAL,
-G/PAL and I/PAL signals. The first enumerated standard is a set of B
-and G/PAL, switched automatically depending on the selected radio
-frequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I"
-choice. Similar a Composite input may collapse standards, enumerating
-"PAL-B/G/H/I", "NTSC-M" and "SECAM-D/K".<footnote>
- <para>Some users are already confused by technical terms PAL,
-NTSC and SECAM. There is no point asking them to distinguish between
-B, G, D, or K when the software or hardware can do that
-automatically.</para>
- </footnote></para>
-
- <para>To query and select the standard used by the current video
-input or output applications call the &VIDIOC-G-STD; and
-&VIDIOC-S-STD; ioctl, respectively. The <emphasis>received</emphasis>
-standard can be sensed with the &VIDIOC-QUERYSTD; ioctl. Note parameter of all these ioctls is a pointer to a &v4l2-std-id; type (a standard set), <emphasis>not</emphasis> an index into the standard enumeration.<footnote>
- <para>An alternative to the current scheme is to use pointers
-to indices as arguments of <constant>VIDIOC_G_STD</constant> and
-<constant>VIDIOC_S_STD</constant>, the &v4l2-input; and
-&v4l2-output; <structfield>std</structfield> field would be a set of
-indices like <structfield>audioset</structfield>.</para>
- <para>Indices are consistent with the rest of the API
-and identify the standard unambiguously. In the present scheme of
-things an enumerated standard is looked up by &v4l2-std-id;. Now the
-standards supported by the inputs of a device can overlap. Just
-assume the tuner and composite input in the example above both
-exist on a device. An enumeration of "PAL-B/G", "PAL-H/I" suggests
-a choice which does not exist. We cannot merge or omit sets, because
-applications would be unable to find the standards reported by
-<constant>VIDIOC_G_STD</constant>. That leaves separate enumerations
-for each input. Also selecting a standard by &v4l2-std-id; can be
-ambiguous. Advantage of this method is that applications need not
-identify the standard indirectly, after enumerating.</para><para>So in
-summary, the lookup itself is unavoidable. The difference is only
-whether the lookup is necessary to find an enumerated standard or to
-switch to a standard by &v4l2-std-id;.</para>
- </footnote> Drivers must implement all video standard ioctls
-when the device has one or more video inputs or outputs.</para>
-
- <para>Special rules apply to USB cameras where the notion of video
-standards makes little sense. More generally any capture device,
-output devices accordingly, which is <itemizedlist>
- <listitem>
- <para>incapable of capturing fields or frames at the nominal
-rate of the video standard, or</para>
- </listitem>
- <listitem>
- <para>where <link linkend="buffer">timestamps</link> refer
-to the instant the field or frame was received by the driver, not the
-capture time, or</para>
- </listitem>
- <listitem>
- <para>where <link linkend="buffer">sequence numbers</link>
-refer to the frames received by the driver, not the captured
-frames.</para>
- </listitem>
- </itemizedlist> Here the driver shall set the
-<structfield>std</structfield> field of &v4l2-input; and &v4l2-output;
-to zero, the <constant>VIDIOC_G_STD</constant>,
-<constant>VIDIOC_S_STD</constant>,
-<constant>VIDIOC_QUERYSTD</constant> and
-<constant>VIDIOC_ENUMSTD</constant> ioctls shall return the
-&EINVAL;.<footnote>
- <para>See <xref linkend="buffer" /> for a rationale. Probably
-even USB cameras follow some well known video standard. It might have
-been better to explicitly indicate elsewhere if a device cannot live
-up to normal expectations, instead of this exception.</para>
- </footnote></para>
-
- <example>
- <title>Information about the current video standard</title>
-
- <programlisting>
-&v4l2-std-id; std_id;
-&v4l2-standard; standard;
-
-if (-1 == ioctl (fd, &VIDIOC-G-STD;, &amp;std_id)) {
- /* Note when VIDIOC_ENUMSTD always returns EINVAL this
- is no video device or it falls under the USB exception,
- and VIDIOC_G_STD returning EINVAL is no error. */
-
- perror ("VIDIOC_G_STD");
- exit (EXIT_FAILURE);
-}
-
-memset (&amp;standard, 0, sizeof (standard));
-standard.index = 0;
-
-while (0 == ioctl (fd, &VIDIOC-ENUMSTD;, &amp;standard)) {
- if (standard.id &amp; std_id) {
- printf ("Current video standard: %s\n", standard.name);
- exit (EXIT_SUCCESS);
- }
-
- standard.index++;
-}
-
-/* EINVAL indicates the end of the enumeration, which cannot be
- empty unless this device falls under the USB exception. */
-
-if (errno == EINVAL || standard.index == 0) {
- perror ("VIDIOC_ENUMSTD");
- exit (EXIT_FAILURE);
-}
- </programlisting>
- </example>
-
- <example>
- <title>Listing the video standards supported by the current
-input</title>
-
- <programlisting>
-&v4l2-input; input;
-&v4l2-standard; standard;
-
-memset (&amp;input, 0, sizeof (input));
-
-if (-1 == ioctl (fd, &VIDIOC-G-INPUT;, &amp;input.index)) {
- perror ("VIDIOC_G_INPUT");
- exit (EXIT_FAILURE);
-}
-
-if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &amp;input)) {
- perror ("VIDIOC_ENUM_INPUT");
- exit (EXIT_FAILURE);
-}
-
-printf ("Current input %s supports:\n", input.name);
-
-memset (&amp;standard, 0, sizeof (standard));
-standard.index = 0;
-
-while (0 == ioctl (fd, &VIDIOC-ENUMSTD;, &amp;standard)) {
- if (standard.id &amp; input.std)
- printf ("%s\n", standard.name);
-
- standard.index++;
-}
-
-/* EINVAL indicates the end of the enumeration, which cannot be
- empty unless this device falls under the USB exception. */
-
-if (errno != EINVAL || standard.index == 0) {
- perror ("VIDIOC_ENUMSTD");
- exit (EXIT_FAILURE);
-}
- </programlisting>
- </example>
-
- <example>
- <title>Selecting a new video standard</title>
-
- <programlisting>
-&v4l2-input; input;
-&v4l2-std-id; std_id;
-
-memset (&amp;input, 0, sizeof (input));
-
-if (-1 == ioctl (fd, &VIDIOC-G-INPUT;, &amp;input.index)) {
- perror ("VIDIOC_G_INPUT");
- exit (EXIT_FAILURE);
-}
-
-if (-1 == ioctl (fd, &VIDIOC-ENUMINPUT;, &amp;input)) {
- perror ("VIDIOC_ENUM_INPUT");
- exit (EXIT_FAILURE);
-}
-
-if (0 == (input.std &amp; V4L2_STD_PAL_BG)) {
- fprintf (stderr, "Oops. B/G PAL is not supported.\n");
- exit (EXIT_FAILURE);
-}
-
-/* Note this is also supposed to work when only B
- <emphasis>or</emphasis> G/PAL is supported. */
-
-std_id = V4L2_STD_PAL_BG;
-
-if (-1 == ioctl (fd, &VIDIOC-S-STD;, &amp;std_id)) {
- perror ("VIDIOC_S_STD");
- exit (EXIT_FAILURE);
-}
- </programlisting>
- </example>
- </section>
- <section id="dv-timings">
- <title>Digital Video (DV) Timings</title>
- <para>
- The video standards discussed so far have been dealing with Analog TV and the
-corresponding video timings. Today there are many more different hardware interfaces
-such as High Definition TV interfaces (HDMI), VGA, DVI connectors etc., that carry
-video signals and there is a need to extend the API to select the video timings
-for these interfaces. Since it is not possible to extend the &v4l2-std-id; due to
-the limited bits available, a new set of IOCTLs was added to set/get video timings at
-the input and output: </para><itemizedlist>
- <listitem>
- <para>DV Timings: This will allow applications to define detailed
-video timings for the interface. This includes parameters such as width, height,
-polarities, frontporch, backporch etc. The <filename>linux/v4l2-dv-timings.h</filename>
-header can be used to get the timings of the formats in the <xref linkend="cea861" /> and
-<xref linkend="vesadmt" /> standards.
- </para>
- </listitem>
- <listitem>
- <para>DV Presets: Digital Video (DV) presets (<emphasis role="bold">deprecated</emphasis>).
- These are IDs representing a
-video timing at the input/output. Presets are pre-defined timings implemented
-by the hardware according to video standards. A __u32 data type is used to represent
-a preset unlike the bit mask that is used in &v4l2-std-id; allowing future extensions
-to support as many different presets as needed. This API is deprecated in favor of the DV Timings
-API.</para>
- </listitem>
- </itemizedlist>
- <para>To enumerate and query the attributes of the DV timings supported by a device,
- applications use the &VIDIOC-ENUM-DV-TIMINGS; and &VIDIOC-DV-TIMINGS-CAP; ioctls.
- To set DV timings for the device, applications use the
-&VIDIOC-S-DV-TIMINGS; ioctl and to get current DV timings they use the
-&VIDIOC-G-DV-TIMINGS; ioctl. To detect the DV timings as seen by the video receiver applications
-use the &VIDIOC-QUERY-DV-TIMINGS; ioctl.</para>
- <para>To enumerate and query the attributes of DV presets supported by a device,
-applications use the &VIDIOC-ENUM-DV-PRESETS; ioctl. To get the current DV preset,
-applications use the &VIDIOC-G-DV-PRESET; ioctl and to set a preset they use the
-&VIDIOC-S-DV-PRESET; ioctl. To detect the preset as seen by the video receiver applications
-use the &VIDIOC-QUERY-DV-PRESET; ioctl.</para>
- <para>Applications can make use of the <xref linkend="input-capabilities" /> and
-<xref linkend="output-capabilities"/> flags to decide what ioctls are available to set the
-video timings for the device.</para>
- </section>
-
- &sub-controls;
-
- <section id="format">
- <title>Data Formats</title>
-
- <section>
- <title>Data Format Negotiation</title>
-
- <para>Different devices exchange different kinds of data with
-applications, for example video images, raw or sliced VBI data, RDS
-datagrams. Even within one kind many different formats are possible,
-in particular an abundance of image formats. Although drivers must
-provide a default and the selection persists across closing and
-reopening a device, applications should always negotiate a data format
-before engaging in data exchange. Negotiation means the application
-asks for a particular format and the driver selects and reports the
-best the hardware can do to satisfy the request. Of course
-applications can also just query the current selection.</para>
-
- <para>A single mechanism exists to negotiate all data formats
-using the aggregate &v4l2-format; and the &VIDIOC-G-FMT; and
-&VIDIOC-S-FMT; ioctls. Additionally the &VIDIOC-TRY-FMT; ioctl can be
-used to examine what the hardware <emphasis>could</emphasis> do,
-without actually selecting a new data format. The data formats
-supported by the V4L2 API are covered in the respective device section
-in <xref linkend="devices" />. For a closer look at image formats see
-<xref linkend="pixfmt" />.</para>
-
- <para>The <constant>VIDIOC_S_FMT</constant> ioctl is a major
-turning-point in the initialization sequence. Prior to this point
-multiple panel applications can access the same device concurrently to
-select the current input, change controls or modify other properties.
-The first <constant>VIDIOC_S_FMT</constant> assigns a logical stream
-(video data, VBI data etc.) exclusively to one file descriptor.</para>
-
- <para>Exclusive means no other application, more precisely no
-other file descriptor, can grab this stream or change device
-properties inconsistent with the negotiated parameters. A video
-standard change for example, when the new standard uses a different
-number of scan lines, can invalidate the selected image format.
-Therefore only the file descriptor owning the stream can make
-invalidating changes. Accordingly multiple file descriptors which
-grabbed different logical streams prevent each other from interfering
-with their settings. When for example video overlay is about to start
-or already in progress, simultaneous video capturing may be restricted
-to the same cropping and image size.</para>
-
- <para>When applications omit the
-<constant>VIDIOC_S_FMT</constant> ioctl its locking side effects are
-implied by the next step, the selection of an I/O method with the
-&VIDIOC-REQBUFS; ioctl or implicit with the first &func-read; or
-&func-write; call.</para>
-
- <para>Generally only one logical stream can be assigned to a
-file descriptor, the exception being drivers permitting simultaneous
-video capturing and overlay using the same file descriptor for
-compatibility with V4L and earlier versions of V4L2. Switching the
-logical stream or returning into "panel mode" is possible by closing
-and reopening the device. Drivers <emphasis>may</emphasis> support a
-switch using <constant>VIDIOC_S_FMT</constant>.</para>
-
- <para>All drivers exchanging data with
-applications must support the <constant>VIDIOC_G_FMT</constant> and
-<constant>VIDIOC_S_FMT</constant> ioctl. Implementation of the
-<constant>VIDIOC_TRY_FMT</constant> is highly recommended but
-optional.</para>
- </section>
-
- <section>
- <title>Image Format Enumeration</title>
-
- <para>Apart of the generic format negotiation functions
-a special ioctl to enumerate all image formats supported by video
-capture, overlay or output devices is available.<footnote>
- <para>Enumerating formats an application has no a-priori
-knowledge of (otherwise it could explicitly ask for them and need not
-enumerate) seems useless, but there are applications serving as proxy
-between drivers and the actual video applications for which this is
-useful.</para>
- </footnote></para>
-
- <para>The &VIDIOC-ENUM-FMT; ioctl must be supported
-by all drivers exchanging image data with applications.</para>
-
- <important>
- <para>Drivers are not supposed to convert image formats in
-kernel space. They must enumerate only formats directly supported by
-the hardware. If necessary driver writers should publish an example
-conversion routine or library for integration into applications.</para>
- </important>
- </section>
- </section>
-
- &sub-planar-apis;
-
- <section id="crop">
- <title>Image Cropping, Insertion and Scaling</title>
-
- <para>Some video capture devices can sample a subsection of the
-picture and shrink or enlarge it to an image of arbitrary size. We
-call these abilities cropping and scaling. Some video output devices
-can scale an image up or down and insert it at an arbitrary scan line
-and horizontal offset into a video signal.</para>
-
- <para>Applications can use the following API to select an area in
-the video signal, query the default area and the hardware limits.
-<emphasis>Despite their name, the &VIDIOC-CROPCAP;, &VIDIOC-G-CROP;
-and &VIDIOC-S-CROP; ioctls apply to input as well as output
-devices.</emphasis></para>
-
- <para>Scaling requires a source and a target. On a video capture
-or overlay device the source is the video signal, and the cropping
-ioctls determine the area actually sampled. The target are images
-read by the application or overlaid onto the graphics screen. Their
-size (and position for an overlay) is negotiated with the
-&VIDIOC-G-FMT; and &VIDIOC-S-FMT; ioctls.</para>
-
- <para>On a video output device the source are the images passed in
-by the application, and their size is again negotiated with the
-<constant>VIDIOC_G/S_FMT</constant> ioctls, or may be encoded in a
-compressed video stream. The target is the video signal, and the
-cropping ioctls determine the area where the images are
-inserted.</para>
-
- <para>Source and target rectangles are defined even if the device
-does not support scaling or the <constant>VIDIOC_G/S_CROP</constant>
-ioctls. Their size (and position where applicable) will be fixed in
-this case. <emphasis>All capture and output device must support the
-<constant>VIDIOC_CROPCAP</constant> ioctl such that applications can
-determine if scaling takes place.</emphasis></para>
-
- <section>
- <title>Cropping Structures</title>
-
- <figure id="crop-scale">
- <title>Image Cropping, Insertion and Scaling</title>
- <mediaobject>
- <imageobject>
- <imagedata fileref="crop.pdf" format="PS" />
- </imageobject>
- <imageobject>
- <imagedata fileref="crop.gif" format="GIF" />
- </imageobject>
- <textobject>
- <phrase>The cropping, insertion and scaling process</phrase>
- </textobject>
- </mediaobject>
- </figure>
-
- <para>For capture devices the coordinates of the top left
-corner, width and height of the area which can be sampled is given by
-the <structfield>bounds</structfield> substructure of the
-&v4l2-cropcap; returned by the <constant>VIDIOC_CROPCAP</constant>
-ioctl. To support a wide range of hardware this specification does not
-define an origin or units. However by convention drivers should
-horizontally count unscaled samples relative to 0H (the leading edge
-of the horizontal sync pulse, see <xref linkend="vbi-hsync" />).
-Vertically ITU-R line
-numbers of the first field (<xref linkend="vbi-525" />, <xref
-linkend="vbi-625" />), multiplied by two if the driver can capture both
-fields.</para>
-
- <para>The top left corner, width and height of the source
-rectangle, that is the area actually sampled, is given by &v4l2-crop;
-using the same coordinate system as &v4l2-cropcap;. Applications can
-use the <constant>VIDIOC_G_CROP</constant> and
-<constant>VIDIOC_S_CROP</constant> ioctls to get and set this
-rectangle. It must lie completely within the capture boundaries and
-the driver may further adjust the requested size and/or position
-according to hardware limitations.</para>
-
- <para>Each capture device has a default source rectangle, given
-by the <structfield>defrect</structfield> substructure of
-&v4l2-cropcap;. The center of this rectangle shall align with the
-center of the active picture area of the video signal, and cover what
-the driver writer considers the complete picture. Drivers shall reset
-the source rectangle to the default when the driver is first loaded,
-but not later.</para>
-
- <para>For output devices these structures and ioctls are used
-accordingly, defining the <emphasis>target</emphasis> rectangle where
-the images will be inserted into the video signal.</para>
-
- </section>
-
- <section>
- <title>Scaling Adjustments</title>
-
- <para>Video hardware can have various cropping, insertion and
-scaling limitations. It may only scale up or down, support only
-discrete scaling factors, or have different scaling abilities in
-horizontal and vertical direction. Also it may not support scaling at
-all. At the same time the &v4l2-crop; rectangle may have to be
-aligned, and both the source and target rectangles may have arbitrary
-upper and lower size limits. In particular the maximum
-<structfield>width</structfield> and <structfield>height</structfield>
-in &v4l2-crop; may be smaller than the
-&v4l2-cropcap;.<structfield>bounds</structfield> area. Therefore, as
-usual, drivers are expected to adjust the requested parameters and
-return the actual values selected.</para>
-
- <para>Applications can change the source or the target rectangle
-first, as they may prefer a particular image size or a certain area in
-the video signal. If the driver has to adjust both to satisfy hardware
-limitations, the last requested rectangle shall take priority, and the
-driver should preferably adjust the opposite one. The &VIDIOC-TRY-FMT;
-ioctl however shall not change the driver state and therefore only
-adjust the requested rectangle.</para>
-
- <para>Suppose scaling on a video capture device is restricted to
-a factor 1:1 or 2:1 in either direction and the target image size must
-be a multiple of 16&nbsp;&times;&nbsp;16 pixels. The source cropping
-rectangle is set to defaults, which are also the upper limit in this
-example, of 640&nbsp;&times;&nbsp;400 pixels at offset 0,&nbsp;0. An
-application requests an image size of 300&nbsp;&times;&nbsp;225
-pixels, assuming video will be scaled down from the "full picture"
-accordingly. The driver sets the image size to the closest possible
-values 304&nbsp;&times;&nbsp;224, then chooses the cropping rectangle
-closest to the requested size, that is 608&nbsp;&times;&nbsp;224
-(224&nbsp;&times;&nbsp;2:1 would exceed the limit 400). The offset
-0,&nbsp;0 is still valid, thus unmodified. Given the default cropping
-rectangle reported by <constant>VIDIOC_CROPCAP</constant> the
-application can easily propose another offset to center the cropping
-rectangle.</para>
-
- <para>Now the application may insist on covering an area using a
-picture aspect ratio closer to the original request, so it asks for a
-cropping rectangle of 608&nbsp;&times;&nbsp;456 pixels. The present
-scaling factors limit cropping to 640&nbsp;&times;&nbsp;384, so the
-driver returns the cropping size 608&nbsp;&times;&nbsp;384 and adjusts
-the image size to closest possible 304&nbsp;&times;&nbsp;192.</para>
-
- </section>
-
- <section>
- <title>Examples</title>
-
- <para>Source and target rectangles shall remain unchanged across
-closing and reopening a device, such that piping data into or out of a
-device will work without special preparations. More advanced
-applications should ensure the parameters are suitable before starting
-I/O.</para>
-
- <example>
- <title>Resetting the cropping parameters</title>
-
- <para>(A video capture device is assumed; change
-<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> for other
-devices.)</para>
-
- <programlisting>
-&v4l2-cropcap; cropcap;
-&v4l2-crop; crop;
-
-memset (&amp;cropcap, 0, sizeof (cropcap));
-cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
-
-if (-1 == ioctl (fd, &VIDIOC-CROPCAP;, &amp;cropcap)) {
- perror ("VIDIOC_CROPCAP");
- exit (EXIT_FAILURE);
-}
-
-memset (&amp;crop, 0, sizeof (crop));
-crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
-crop.c = cropcap.defrect;
-
-/* Ignore if cropping is not supported (EINVAL). */
-
-if (-1 == ioctl (fd, &VIDIOC-S-CROP;, &amp;crop)
- &amp;&amp; errno != EINVAL) {
- perror ("VIDIOC_S_CROP");
- exit (EXIT_FAILURE);
-}
- </programlisting>
- </example>
-
- <example>
- <title>Simple downscaling</title>
-
- <para>(A video capture device is assumed.)</para>
-
- <programlisting>
-&v4l2-cropcap; cropcap;
-&v4l2-format; format;
-
-reset_cropping_parameters ();
-
-/* Scale down to 1/4 size of full picture. */
-
-memset (&amp;format, 0, sizeof (format)); /* defaults */
-
-format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
-
-format.fmt.pix.width = cropcap.defrect.width &gt;&gt; 1;
-format.fmt.pix.height = cropcap.defrect.height &gt;&gt; 1;
-format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
-
-if (-1 == ioctl (fd, &VIDIOC-S-FMT;, &amp;format)) {
- perror ("VIDIOC_S_FORMAT");
- exit (EXIT_FAILURE);
-}
-
-/* We could check the actual image size now, the actual scaling factor
- or if the driver can scale at all. */
- </programlisting>
- </example>
-
- <example>
- <title>Selecting an output area</title>
-
- <programlisting>
-&v4l2-cropcap; cropcap;
-&v4l2-crop; crop;
-
-memset (&amp;cropcap, 0, sizeof (cropcap));
-cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
-
-if (-1 == ioctl (fd, VIDIOC_CROPCAP;, &amp;cropcap)) {
- perror ("VIDIOC_CROPCAP");
- exit (EXIT_FAILURE);
-}
-
-memset (&amp;crop, 0, sizeof (crop));
-
-crop.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
-crop.c = cropcap.defrect;
-
-/* Scale the width and height to 50 % of their original size
- and center the output. */
-
-crop.c.width /= 2;
-crop.c.height /= 2;
-crop.c.left += crop.c.width / 2;
-crop.c.top += crop.c.height / 2;
-
-/* Ignore if cropping is not supported (EINVAL). */
-
-if (-1 == ioctl (fd, VIDIOC_S_CROP, &amp;crop)
- &amp;&amp; errno != EINVAL) {
- perror ("VIDIOC_S_CROP");
- exit (EXIT_FAILURE);
-}
-</programlisting>
- </example>
-
- <example>
- <title>Current scaling factor and pixel aspect</title>
-
- <para>(A video capture device is assumed.)</para>
-
- <programlisting>
-&v4l2-cropcap; cropcap;
-&v4l2-crop; crop;
-&v4l2-format; format;
-double hscale, vscale;
-double aspect;
-int dwidth, dheight;
-
-memset (&amp;cropcap, 0, sizeof (cropcap));
-cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
-
-if (-1 == ioctl (fd, &VIDIOC-CROPCAP;, &amp;cropcap)) {
- perror ("VIDIOC_CROPCAP");
- exit (EXIT_FAILURE);
-}
-
-memset (&amp;crop, 0, sizeof (crop));
-crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
-
-if (-1 == ioctl (fd, &VIDIOC-G-CROP;, &amp;crop)) {
- if (errno != EINVAL) {
- perror ("VIDIOC_G_CROP");
- exit (EXIT_FAILURE);
- }
-
- /* Cropping not supported. */
- crop.c = cropcap.defrect;
-}
-
-memset (&amp;format, 0, sizeof (format));
-format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
-
-if (-1 == ioctl (fd, &VIDIOC-G-FMT;, &amp;format)) {
- perror ("VIDIOC_G_FMT");
- exit (EXIT_FAILURE);
-}
-
-/* The scaling applied by the driver. */
-
-hscale = format.fmt.pix.width / (double) crop.c.width;
-vscale = format.fmt.pix.height / (double) crop.c.height;
-
-aspect = cropcap.pixelaspect.numerator /
- (double) cropcap.pixelaspect.denominator;
-aspect = aspect * hscale / vscale;
-
-/* Devices following ITU-R BT.601 do not capture
- square pixels. For playback on a computer monitor
- we should scale the images to this size. */
-
-dwidth = format.fmt.pix.width / aspect;
-dheight = format.fmt.pix.height;
- </programlisting>
- </example>
- </section>
- </section>
-
- &sub-selection-api;
-
- <section id="streaming-par">
- <title>Streaming Parameters</title>
-
- <para>Streaming parameters are intended to optimize the video
-capture process as well as I/O. Presently applications can request a
-high quality capture mode with the &VIDIOC-S-PARM; ioctl.</para>
-
- <para>The current video standard determines a nominal number of
-frames per second. If less than this number of frames is to be
-captured or output, applications can request frame skipping or
-duplicating on the driver side. This is especially useful when using
-the &func-read; or &func-write;, which are not augmented by timestamps
-or sequence counters, and to avoid unnecessary data copying.</para>
-
- <para>Finally these ioctls can be used to determine the number of
-buffers used internally by a driver in read/write mode. For
-implications see the section discussing the &func-read;
-function.</para>
-
- <para>To get and set the streaming parameters applications call
-the &VIDIOC-G-PARM; and &VIDIOC-S-PARM; ioctl, respectively. They take
-a pointer to a &v4l2-streamparm;, which contains a union holding
-separate parameters for input and output devices.</para>
-
- <para>These ioctls are optional, drivers need not implement
-them. If so, they return the &EINVAL;.</para>
- </section>