1 .. SPDX-License-Identifier: GPL-2.0
2
3 Introduction
4 ------------
5
6 The V4L2 drivers tend to be very complex due t
7 hardware: most devices have multiple ICs, expo
8 /dev, and create also non-V4L2 devices such as
9 (IR) devices.
10
11 Especially the fact that V4L2 drivers have to
12 do audio/video muxing/encoding/decoding makes
13 Usually these ICs are connected to the main br
14 more I2C buses, but other buses can also be us
15 called 'sub-devices'.
16
17 For a long time the framework was limited to t
18 creating V4L device nodes and video_buf for ha
19 (note that this document does not discuss the
20
21 This meant that all drivers had to do the setu
22 connecting to sub-devices themselves. Some of
23 to do right and many drivers never did do it c
24
25 There is also a lot of common code that could
26 the lack of a framework.
27
28 So this framework sets up the basic building b
29 need and this same framework should make it mu
30 common code into utility functions shared by a
31
32 A good example to look at as a reference is th
33 source that is available in samples/v4l/. It i
34 a PCI capture card, and demonstrates how to us
35 framework. It can be used as a template for re
36
37 Structure of a V4L driver
38 -------------------------
39
40 All drivers have the following structure:
41
42 1) A struct for each device instance containin
43
44 2) A way of initializing and commanding sub-de
45
46 3) Creating V4L2 device nodes (/dev/videoX, /d
47 and keeping track of device-node specific d
48
49 4) Filehandle-specific structs containing per-
50
51 5) video buffer handling.
52
53 This is a rough schematic of how it all relate
54
55 .. code-block:: none
56
57 device instances
58 |
59 +-sub-device instances
60 |
61 \-V4L2 device nodes
62 |
63 \-filehandle instances
64
65
66 Structure of the V4L2 framework
67 -------------------------------
68
69 The framework closely resembles the driver str
70 struct for the device instance data, a v4l2_su
71 sub-device instances, the video_device struct
72 and the v4l2_fh struct keeps track of filehand
73
74 The V4L2 framework also optionally integrates
75 driver sets the struct v4l2_device mdev field,
76 will automatically appear in the media framewo
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