77 lines
2.7 KiB
ReStructuredText
77 lines
2.7 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
Introduction
|
|
------------
|
|
|
|
The V4L2 drivers tend to be very complex due to the complexity of the
|
|
hardware: most devices have multiple ICs, export multiple device nodes in
|
|
/dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input
|
|
(IR) devices.
|
|
|
|
Especially the fact that V4L2 drivers have to setup supporting ICs to
|
|
do audio/video muxing/encoding/decoding makes it more complex than most.
|
|
Usually these ICs are connected to the main bridge driver through one or
|
|
more I2C buses, but other buses can also be used. Such devices are
|
|
called 'sub-devices'.
|
|
|
|
For a long time the framework was limited to the video_device struct for
|
|
creating V4L device nodes and video_buf for handling the video buffers
|
|
(note that this document does not discuss the video_buf framework).
|
|
|
|
This meant that all drivers had to do the setup of device instances and
|
|
connecting to sub-devices themselves. Some of this is quite complicated
|
|
to do right and many drivers never did do it correctly.
|
|
|
|
There is also a lot of common code that could never be refactored due to
|
|
the lack of a framework.
|
|
|
|
So this framework sets up the basic building blocks that all drivers
|
|
need and this same framework should make it much easier to refactor
|
|
common code into utility functions shared by all drivers.
|
|
|
|
A good example to look at as a reference is the v4l2-pci-skeleton.c
|
|
source that is available in samples/v4l/. It is a skeleton driver for
|
|
a PCI capture card, and demonstrates how to use the V4L2 driver
|
|
framework. It can be used as a template for real PCI video capture driver.
|
|
|
|
Structure of a V4L driver
|
|
-------------------------
|
|
|
|
All drivers have the following structure:
|
|
|
|
1) A struct for each device instance containing the device state.
|
|
|
|
2) A way of initializing and commanding sub-devices (if any).
|
|
|
|
3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX and /dev/radioX)
|
|
and keeping track of device-node specific data.
|
|
|
|
4) Filehandle-specific structs containing per-filehandle data;
|
|
|
|
5) video buffer handling.
|
|
|
|
This is a rough schematic of how it all relates:
|
|
|
|
.. code-block:: none
|
|
|
|
device instances
|
|
|
|
|
+-sub-device instances
|
|
|
|
|
\-V4L2 device nodes
|
|
|
|
|
\-filehandle instances
|
|
|
|
|
|
Structure of the V4L2 framework
|
|
-------------------------------
|
|
|
|
The framework closely resembles the driver structure: it has a v4l2_device
|
|
struct for the device instance data, a v4l2_subdev struct to refer to
|
|
sub-device instances, the video_device struct stores V4L2 device node data
|
|
and the v4l2_fh struct keeps track of filehandle instances.
|
|
|
|
The V4L2 framework also optionally integrates with the media framework. If a
|
|
driver sets the struct v4l2_device mdev field, sub-devices and video nodes
|
|
will automatically appear in the media framework as entities.
|