75 lines
2.7 KiB
ReStructuredText
75 lines
2.7 KiB
ReStructuredText
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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 busses, but other busses 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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