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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _open:
+
+***************************
+Opening and Closing Devices
+***************************
+
+
+Device Naming
+=============
+
+V4L2 drivers are implemented as kernel modules, loaded manually by the
+system administrator or automatically when a device is first discovered.
+The driver modules plug into the "videodev" kernel module. It provides
+helper functions and a common application interface specified in this
+document.
+
+Each driver thus loaded registers one or more device nodes 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.).
+
+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:
+
+.. code-block:: none
+
+   # modprobe mydriver video_nr=0,1 radio_nr=0,1
+
+In ``/etc/modules.conf`` this may be written as:
+
+::
+
+    options mydriver video_nr=0,1 radio_nr=0,1
+
+When no device node number is given as module option the driver supplies
+a default.
+
+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 :ref:`devices`.
+
+The creation of character special files (with mknod) is a privileged
+operation and devices cannot be opened by major and minor number. That
+means applications cannot *reliable* scan for loaded or installed
+drivers. The user must enter a device name, or the application can try
+the conventional device names.
+
+
+.. _related:
+
+Related Devices
+===============
+
+Devices can support several functions. For example video capturing, VBI
+capturing and radio support.
+
+The V4L2 API creates different nodes for each of these functions.
+
+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
+:ref:`read() <func-read>`/\ :ref:`write() <func-write>` API.
+
+Today each device node supports just one function.
+
+Besides video input or output the hardware may also support audio
+sampling or playback. If so, these functions are implemented as ALSA PCM
+devices with optional ALSA audio mixer devices.
+
+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 :ref:`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
+`v4l-utils <http://git.linuxtv.org/cgit.cgi/v4l-utils.git/>`__ 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:
+`https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__.
+
+
+Multiple Opens
+==============
+
+V4L2 devices can be opened more than once. [#f1]_ 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 ALSA audio mixer application. Just opening a V4L2
+device should not change the state of the device. [#f2]_
+
+Once an application has allocated the memory buffers needed for
+streaming data (by calling the :ref:`VIDIOC_REQBUFS`
+or :ref:`VIDIOC_CREATE_BUFS` ioctls, or
+implicitly by calling the :ref:`read() <func-read>` or
+:ref:`write() <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
+:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl) and other applications are
+no longer allowed to allocate buffers or start or stop streaming. The
+EBUSY error code will be returned instead.
+
+Merely opening a V4L2 device does not grant exclusive access. [#f3]_
+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 :ref:`app-pri`.
+
+
+Shared Data Streams
+===================
+
+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.
+
+
+Functions
+=========
+
+To open and close V4L2 devices applications use the
+:ref:`open() <func-open>` and :ref:`close() <func-close>` function,
+respectively. Devices are programmed using the
+:ref:`ioctl() <func-ioctl>` function as explained in the following
+sections.
+
+.. [#f1]
+   There are still some old and obscure drivers that have not been
+   updated to allow for multiple opens. This implies that for such
+   drivers :ref:`open() <func-open>` can return an ``EBUSY`` error code
+   when the device is already in use.
+
+.. [#f2]
+   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.
+
+.. [#f3]
+   Drivers could recognize the ``O_EXCL`` open flag. Presently this is
+   not required, so applications cannot know if it really works.