Support the MFX Dispatcher from Intel Media SDK

[TheTroll]

Summary of the bug:

The current implementation of QSV supposes that the MFX dispatcher will open the VA session.

This is true with the MFX dispatcher available on github, but this version does not work properly, specially when closing the session.

With the official MFX dispatcher from the Intel Media SDK package, FFMPEG needs to open itself HW accelerated VA session.

This can be done like this:
​https://github.com/TheTroll/FFmpeg/commit/69bff7abfcd321a29c23606cec973b75314f3b94
+
​https://github.com/TheTroll/FFmpeg/commit/725f96bf2c71406ec535b8fed5d5879ec32a9881

[Marek Fort]

The proposed patch is linux centric. There is no libva on Windows.

[TheTroll]

Indeed...
Well you have the idea here, in Linux, this is how it works with the latest Intel MediaSDK

[Gwenole Beauchesne]

I think you need to supply two ways of allocating the VA display: one from within FFmpeg (libavcodec), and another one handed over by the user (e.g. through some vaapi_context).

Next, if FFmpeg was to create the VA display handle, using an environment variable to derive the device name is a no-go. You have the option to use libudev to enumerate and pick an Intel device + VGA class device. You can also avoid libudev if you were to parse /sys/bus/pci/devices entries yourself and check for class = 0x030000, vendor_id = 0x8086 for Intel.

Last but not least, DRM render nodes are now also the preferred way, unless you really want to ultimately render frames through KMS for instance. A render node id is built as 0x80|i where i has a /dev/dri/card<i> counterpart. With udev, it's generally enough to match "renderD[0-9]*" devices.