This libVA backend is designed to work with the Linux Video4Linux2 Request API that is used by a number of video codecs drivers, including the Video Engine found in most Allwinner SoCs.
The v4l2-request libVA backend currently is not in a usable state. Development has ceased in 2019, before the V4L2-m2m interface has been stable. Work on reviving the project is in progress.
Your system's V4L2 capabilities can be interrogated with v4l2-ctl:
v4l2-ctl -d $LIBVA_V4L2_REQUEST_VIDEO_PATH {-l,--list-formats{,-out}}
In particular, your device needs to offer the "Video Memory-to-Memory Multiplanar", "Streaming", and "Device" capabilities.
Specify which V4L2 device is to be used:
LIBVA_V4L2_REQUEST_VIDEO_PATH=/dev/videoXLIBVA_V4L2_REQUEST_MEDIA_PATH=/dev/mediaY
libVA can be instructed to load drivers from additional paths, and to fix the used driver if multiple are present:
LIBVA_DRIVERS_PATH=<project dir>/build/srcLIBVA_DRIVER_NAME=v4l2_request
When using gstreamer, opt-in to non-whitelisted drivers to be used:
GST_VAAPI_ALL_DRIVERS=1