<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
+first discovered. 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>
+with major number 81 and a minor number between 0 and 255. Minor numbers
+are allocated dynamically unless the kernel is compiled with the kernel
+option CONFIG_VIDEO_FIXED_MINOR_RANGES. In that case minor numbers are
+allocated in ranges depending on the device node type (video, radio, etc.).</para>
+
+ <para>Many drivers support "video_nr", "radio_nr" or "vbi_nr"
+module options to select specific video/radio/vbi node numbers. This allows
+the user to request that the device node is named e.g. /dev/video5 instead
+of leaving it to chance. When the driver supports multiple devices of the same
+type more than one device node number can be assigned, separated by commas:
+ <informalexample>
<screen>
-> insmod mydriver.o video_nr=0,1 radio_nr=0,1</screen>
+> modprobe mydriver 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" />
+options mydriver video_nr=0,1 radio_nr=0,1
</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" />.
+ </informalexample> When no device node number is given as module
+option the driver supplies a default.</para>
+
+ <para>Normally udev will create the device nodes in /dev automatically
+for you. If udev is not installed, then you need to enable the
+CONFIG_VIDEO_FIXED_MINOR_RANGES kernel option in order to be able to correctly
+relate a minor number to a device node number. I.e., you need to be certain
+that minor number 5 maps to device node name video5. With this kernel option
+different device types have different minor number ranges. These ranges are
+listed in <xref linkend="devices" />.
</para>
<para>The creation of character special files (with
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. 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>Devices can support several functions. For example
+video capturing, VBI capturing and radio support.</para>
+
+ <para>The V4L2 API creates different nodes for each of these functions.</para>
+
+ <para>The V4L2 API was designed with the idea that one device node could support
+all functions. However, in practice this never worked: this 'feature'
+was never used by applications and many drivers did not support it and if
+they did it was certainly never tested. In addition, switching a device
+node between different functions only works when using the streaming I/O
+API, not with the read()/write() API.</para>
+
+ <para>Today each device node supports just one function.</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>
+implemented as ALSA PCM devices with optional ALSA audio mixer
+devices.</para>
+
+ <para>One problem with all these devices is that the V4L2 API
+makes no provisions to find these related devices. Some really
+complex devices use the Media Controller (see <xref linkend="media_controller" />)
+which can be used for this purpose. But most drivers do not use it,
+and while some code exists that uses sysfs to discover related devices
+(see libmedia_dev in the <ulink url="http://git.linuxtv.org/v4l-utils/">v4l-utils</ulink>
+git repository), there is no library yet that can provide a single API towards
+both Media Controller-based devices and devices that do not use the Media Controller.
+If you want to work on this 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.
+ <para>V4L2 devices can be opened more than once.<footnote><para>
+There are still some old and obscure drivers that have not been updated to
+allow for multiple opens. This implies that for such drivers &func-open; can
+return an &EBUSY; when the device is already in use.</para></footnote>
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
+applications are comparable to an ALSA audio mixer application.
+Just opening a V4L2 device should not change the state of the device.<footnote>
+<para>Unfortunately, opening a radio device often switches the state of the
+device to radio mode in many drivers. This behavior should be fixed eventually
+as it violates the V4L2 specification.</para></footnote></para>
+
+ <para>Once an application has allocated the memory buffers needed for
+streaming data (by calling the &VIDIOC-REQBUFS; or &VIDIOC-CREATE-BUFS; ioctls,
+or implicitly by calling the &func-read; or &func-write; functions) that
+application (filehandle) becomes the owner of the device. It is no longer
+allowed to make changes that would affect the buffer sizes (e.g. by calling
+the &VIDIOC-S-FMT; ioctl) and other applications are no longer allowed to allocate
+buffers or start or stop streaming. The &EBUSY; will be returned instead.</para>
+
+ <para>Merely 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,
<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>
+a proxy application in user space.</para>
</section>
<section>
projects / cascardo / linux.git / commitdiff