Aka, are you working on this? If not, we should make it Available, for anyone interested to be able to pick it.

Assigning to Francois, who kindly offered to help with this.

Some hints on how to proceed with this:

The drain sequence is documented in the memory-to-memory statful encoder specification:
https://hverkuil.home.xs4all.nl/request-api/uapi/v4l/dev-encoder.html#drain

There is also a bit more condensed form in the general description of the VIDIOC_ENCODER_CMD ioctl:
https://hverkuil.home.xs4all.nl/request-api/uapi/v4l/vidioc-encoder-cmd.html#c.VIDIOC_TRY_ENCODER_CMD

The Rockchip VPU driver is currently implementing the sequence relatively close to the specification:
https://chromium.googlesource.com/chromiumos/third_party/kernel/+/ed8d49d095e6a87b7fd2b5d346031c2b5b86efbe/drivers/media/platform/rockchip-vpu/rockchip_vpu_enc.c#1077

It still has some differences in few corner cases, since it was implemented for the earlier version of the specification:
 - doesn't restart decoding when CAPTURE queue streaming is restarted,
 - returns -EINVAL on V4L2_ENC_CMD_START, if the OUTPUT queue is not streaming - it's currently an undocumented case, so we can ignore it for now.
These shouldn't affect the video_encode_accelerator_unittest results.

For reference, the CL that added the implementation for this driver:
https://chromium.googlesource.com/chromiumos/third_party/kernel/+/cc6b4e8c5741666a247af3bf3faae0c7d0017981%5E%21/

There is also a brand new document on how to run the video "unit" tests:
https://chromium.googlesource.com/chromium/src/+/HEAD/docs/media/gpu/vdatest_usage.md

Note that the test currently skips the flush testing on drivers which don't implement the VIDIOC_TRY_ENCODER_CMD ioctl: https://cs.chromium.org/chromium/src/media/gpu/v4l2/v4l2_video_encode_accelerator.cc?rcl=a007ecdfe31c796dca77305da540e5a1b71add23&l=185.
You should be able to see a log message in such case.