]>
Commit | Line | Data |
---|---|---|
2a1fcdf0 HV |
1 | Introduction |
2 | ------------ | |
3 | ||
4 | The V4L2 drivers tend to be very complex due to the complexity of the | |
5 | hardware: most devices have multiple ICs, export multiple device nodes in | |
6 | /dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input | |
7 | (IR) devices. | |
8 | ||
9 | Especially the fact that V4L2 drivers have to setup supporting ICs to | |
10 | do audio/video muxing/encoding/decoding makes it more complex than most. | |
11 | Usually these ICs are connected to the main bridge driver through one or | |
12 | more I2C busses, but other busses can also be used. Such devices are | |
13 | called 'sub-devices'. | |
14 | ||
15 | For a long time the framework was limited to the video_device struct for | |
16 | creating V4L device nodes and video_buf for handling the video buffers | |
17 | (note that this document does not discuss the video_buf framework). | |
18 | ||
19 | This meant that all drivers had to do the setup of device instances and | |
20 | connecting to sub-devices themselves. Some of this is quite complicated | |
21 | to do right and many drivers never did do it correctly. | |
22 | ||
23 | There is also a lot of common code that could never be refactored due to | |
24 | the lack of a framework. | |
25 | ||
26 | So this framework sets up the basic building blocks that all drivers | |
27 | need and this same framework should make it much easier to refactor | |
28 | common code into utility functions shared by all drivers. | |
29 | ||
926977e0 | 30 | A good example to look at as a reference is the v4l2-pci-skeleton.c |
0185f850 | 31 | source that is available in samples/v4l/. It is a skeleton driver for |
926977e0 HV |
32 | a PCI capture card, and demonstrates how to use the V4L2 driver |
33 | framework. It can be used as a template for real PCI video capture driver. | |
2a1fcdf0 | 34 | |
f6fa883b MCC |
35 | Structure of a V4L driver |
36 | ------------------------- | |
2a1fcdf0 HV |
37 | |
38 | All drivers have the following structure: | |
39 | ||
40 | 1) A struct for each device instance containing the device state. | |
41 | ||
42 | 2) A way of initializing and commanding sub-devices (if any). | |
43 | ||
f44026db HV |
44 | 3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX and /dev/radioX) |
45 | and keeping track of device-node specific data. | |
2a1fcdf0 | 46 | |
44061c05 MCC |
47 | 4) Filehandle-specific structs containing per-filehandle data; |
48 | ||
49 | 5) video buffer handling. | |
2a1fcdf0 HV |
50 | |
51 | This is a rough schematic of how it all relates: | |
52 | ||
4c21d4fb MCC |
53 | .. code-block:: none |
54 | ||
2a1fcdf0 HV |
55 | device instances |
56 | | | |
57 | +-sub-device instances | |
58 | | | |
59 | \-V4L2 device nodes | |
60 | | | |
61 | \-filehandle instances | |
62 | ||
63 | ||
f6fa883b MCC |
64 | Structure of the V4L2 framework |
65 | ------------------------------- | |
2a1fcdf0 HV |
66 | |
67 | The framework closely resembles the driver structure: it has a v4l2_device | |
68 | struct for the device instance data, a v4l2_subdev struct to refer to | |
69 | sub-device instances, the video_device struct stores V4L2 device node data | |
f818b358 | 70 | and the v4l2_fh struct keeps track of filehandle instances. |
2a1fcdf0 | 71 | |
2c0ab67b LP |
72 | The V4L2 framework also optionally integrates with the media framework. If a |
73 | driver sets the struct v4l2_device mdev field, sub-devices and video nodes | |
74 | will automatically appear in the media framework as entities. |