Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+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 busses, but other busses 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.