xref: /openbmc/linux/Documentation/userspace-api/media/v4l/open.rst (revision 407e84cd)

.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L

.. _open:

***************************
Opening and Closing Devices
***************************

.. _v4l2_hardware_control:

Controlling a hardware peripheral via V4L2
==========================================

Hardware that is supported using the V4L2 uAPI often consists of multiple
devices or peripherals, each of which have their own driver.

The bridge driver exposes one or more V4L2 device nodes
(see :ref:`v4l2_device_naming`).

There are other drivers providing support for other components of
the hardware, which may also expose device nodes, called V4L2 sub-devices.

When such V4L2 sub-devices are exposed, they allow controlling those
other hardware components - usually connected via a serial bus (like
I²C, SMBus or SPI). Depending on the bridge driver, those sub-devices
can be controlled indirectly via the bridge driver or explicitly via
the :ref:`Media Controller <media_controller>` and via the
:ref:`V4L2 sub-devices <subdev>`.

The devices that require the use of the
:ref:`Media Controller <media_controller>` are called **MC-centric**
devices. The devices that are fully controlled via V4L2 device nodes
are called **video-node-centric**.

Userspace can check if a V4L2 hardware peripheral is MC-centric by
calling :ref:`VIDIOC_QUERYCAP` and checking the
:ref:`device_caps field <device-capabilities>`.

If the device returns ``V4L2_CAP_IO_MC`` flag at ``device_caps``,
then it is MC-centric, otherwise, it is video-node-centric.

It is required for MC-centric drivers to identify the V4L2
sub-devices and to configure the pipelines via the
:ref:`media controller API <media_controller>` before using the peripheral.
Also, the sub-devices' configuration shall be controlled via the
:ref:`sub-device API <subdev>`.

.. note::

   A video-node-centric may still provide media-controller and
   sub-device interfaces as well.

  However, in that case the media-controller and the sub-device
  interfaces are read-only and just provide information about the
  device. The actual configuration is done via the video nodes.

.. _v4l2_device_naming:

V4L2 Device Node 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. 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.

The device nodes supported by the Video4Linux subsystem are:

======================== ====================================================
Default device node name Usage
======================== ====================================================
``/dev/videoX``		 Video and metadata for capture/output devices
``/dev/vbiX``		 Vertical blank data (i.e. closed captions, teletext)
``/dev/radioX``		 Radio tuners and modulators
``/dev/swradioX``	 Software Defined Radio tuners and modulators
``/dev/v4l-touchX``	 Touch sensors
``/dev/v4l-subdevX``	 Video sub-devices (used by sensors and other
			 components of the hardware peripheral)\ [#]_
======================== ====================================================

Where ``X`` is a non-negative integer.

.. note::

   1. The actual device node name is system-dependent, as udev rules may apply.
   2. There is no guarantee that ``X`` will remain the same for the same
      device, as the number depends on the device driver's probe order.
      If you need an unique name, udev default rules produce
      ``/dev/v4l/by-id/`` and ``/dev/v4l/by-path/`` directories containing
      links that can be used uniquely to identify a V4L2 device node::

	$ tree /dev/v4l
	/dev/v4l
	├── by-id
	│   └── usb-OmniVision._USB_Camera-B4.04.27.1-video-index0 -> ../../video0
	└── by-path
	    └── pci-0000:00:14.0-usb-0:2:1.0-video-index0 -> ../../video0

.. [#] **V4L2 sub-device nodes** (e. g. ``/dev/v4l-subdevX``) use a different
       set of system calls, as covered at :ref:`subdev`.

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 *reliably* 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 V4L2 device 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
:c:func:`read()`/\ :c:func:`write()` API.

Today each V4L2 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 V4L2 device nodes. Some really complex
hardware use the Media Controller (see :ref:`media_controller`) which can
be used for this purpose. But several drivers do not use it, and while some
code exists that uses sysfs to discover related V4L2 device nodes (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 :c:func:`read()` or
:c: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
:c:func:`open()` and :c: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 :c: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.