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54f38fcae5
Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee38 ("docs: Create a user-space API guide").
As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.
Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
166 lines
6.6 KiB
ReStructuredText
166 lines
6.6 KiB
ReStructuredText
.. Permission is granted to copy, distribute and/or modify this
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.. document under the terms of the GNU Free Documentation License,
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.. Version 1.1 or any later version published by the Free Software
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.. Foundation, with no Invariant Sections, no Front-Cover Texts
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.. and no Back-Cover Texts. A copy of the license is included at
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.. Documentation/userspace-api/media/fdl-appendix.rst.
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..
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.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
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.. _open:
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***************************
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Opening and Closing Devices
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***************************
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Device Naming
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=============
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V4L2 drivers are implemented as kernel modules, loaded manually by the
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system administrator or automatically when a device is first discovered.
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The driver modules plug into the "videodev" kernel module. It provides
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helper functions and a common application interface specified in this
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document.
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Each driver thus loaded registers one or more device nodes with major
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number 81 and a minor number between 0 and 255. Minor numbers are
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allocated dynamically unless the kernel is compiled with the kernel
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option CONFIG_VIDEO_FIXED_MINOR_RANGES. In that case minor numbers
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are allocated in ranges depending on the device node type (video, radio,
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etc.).
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Many drivers support "video_nr", "radio_nr" or "vbi_nr" module
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options to select specific video/radio/vbi node numbers. This allows the
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user to request that the device node is named e.g. /dev/video5 instead
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of leaving it to chance. When the driver supports multiple devices of
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the same type more than one device node number can be assigned,
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separated by commas:
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.. code-block:: none
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# modprobe mydriver video_nr=0,1 radio_nr=0,1
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In ``/etc/modules.conf`` this may be written as:
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::
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options mydriver video_nr=0,1 radio_nr=0,1
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When no device node number is given as module option the driver supplies
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a default.
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Normally udev will create the device nodes in /dev automatically for
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you. If udev is not installed, then you need to enable the
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CONFIG_VIDEO_FIXED_MINOR_RANGES kernel option in order to be able to
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correctly relate a minor number to a device node number. I.e., you need
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to be certain that minor number 5 maps to device node name video5. With
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this kernel option different device types have different minor number
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ranges. These ranges are listed in :ref:`devices`.
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The creation of character special files (with mknod) is a privileged
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operation and devices cannot be opened by major and minor number. That
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means applications cannot *reliably* scan for loaded or installed
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drivers. The user must enter a device name, or the application can try
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the conventional device names.
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.. _related:
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Related Devices
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===============
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Devices can support several functions. For example video capturing, VBI
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capturing and radio support.
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The V4L2 API creates different nodes for each of these functions.
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The V4L2 API was designed with the idea that one device node could
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support all functions. However, in practice this never worked: this
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'feature' was never used by applications and many drivers did not
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support it and if they did it was certainly never tested. In addition,
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switching a device node between different functions only works when
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using the streaming I/O API, not with the
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:ref:`read() <func-read>`/\ :ref:`write() <func-write>` API.
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Today each device node supports just one function.
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Besides video input or output the hardware may also support audio
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sampling or playback. If so, these functions are implemented as ALSA PCM
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devices with optional ALSA audio mixer devices.
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One problem with all these devices is that the V4L2 API makes no
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provisions to find these related devices. Some really complex devices
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use the Media Controller (see :ref:`media_controller`) which can be
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used for this purpose. But most drivers do not use it, and while some
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code exists that uses sysfs to discover related devices (see
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libmedia_dev in the
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`v4l-utils <http://git.linuxtv.org/cgit.cgi/v4l-utils.git/>`__ git
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repository), there is no library yet that can provide a single API
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towards both Media Controller-based devices and devices that do not use
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the Media Controller. If you want to work on this please write to the
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linux-media mailing list:
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`https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__.
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Multiple Opens
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==============
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V4L2 devices can be opened more than once. [#f1]_ When this is supported
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by the driver, users can for example start a "panel" application to
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change controls like brightness or audio volume, while another
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application captures video and audio. In other words, panel applications
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are comparable to an ALSA audio mixer application. Just opening a V4L2
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device should not change the state of the device. [#f2]_
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Once an application has allocated the memory buffers needed for
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streaming data (by calling the :ref:`VIDIOC_REQBUFS`
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or :ref:`VIDIOC_CREATE_BUFS` ioctls, or
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implicitly by calling the :ref:`read() <func-read>` or
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:ref:`write() <func-write>` functions) that application (filehandle)
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becomes the owner of the device. It is no longer allowed to make changes
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that would affect the buffer sizes (e.g. by calling the
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl) and other applications are
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no longer allowed to allocate buffers or start or stop streaming. The
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EBUSY error code will be returned instead.
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Merely opening a V4L2 device does not grant exclusive access. [#f3]_
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Initiating data exchange however assigns the right to read or write the
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requested type of data, and to change related properties, to this file
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descriptor. Applications can request additional access privileges using
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the priority mechanism described in :ref:`app-pri`.
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Shared Data Streams
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===================
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V4L2 drivers should not support multiple applications reading or writing
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the same data stream on a device by copying buffers, time multiplexing
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or similar means. This is better handled by a proxy application in user
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space.
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Functions
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=========
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To open and close V4L2 devices applications use the
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:ref:`open() <func-open>` and :ref:`close() <func-close>` function,
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respectively. Devices are programmed using the
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:ref:`ioctl() <func-ioctl>` function as explained in the following
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sections.
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.. [#f1]
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There are still some old and obscure drivers that have not been
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updated to allow for multiple opens. This implies that for such
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drivers :ref:`open() <func-open>` can return an ``EBUSY`` error code
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when the device is already in use.
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.. [#f2]
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Unfortunately, opening a radio device often switches the state of the
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device to radio mode in many drivers. This behavior should be fixed
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eventually as it violates the V4L2 specification.
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.. [#f3]
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Drivers could recognize the ``O_EXCL`` open flag. Presently this is
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not required, so applications cannot know if it really works.
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