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