building FFMPEG with HEVC and H264 HW acceleration on RPi

Hello,

I was not sure where to put my question, feel free to move this thread...

My question is related to FFMPEG that LibreElec is using. I have a Raspberry Pi 4 with official Raspberry OS.

In short - I am trying to achieve realtime transcoding of a HEVC video stream to H264. And for this I need hardware acceleration of both HEVC decoding and H264 encoding.

Unfortunatelly, the current stock FFMPEG provided with Raspberry OS only supports HW accelerated H264 encoding using h264_omx encoder. It also has h264_v4l2m2m encoder, but it produces video with a green tint (bug?). And regarding HEVC - I was not able to enable HW acceleration for it.

I have read that LibreElec uses a custom patched version of FFMPEG and supports HEVC HW accelerated decoding.

So I tried to make my own build of ffmpeg based on LibreElec. And now here is the part where I am failing

I investigated LibreElec build system (mainly LibreELEC.tv/gen-patches.sh at master · LibreELEC/LibreELEC.tv · GitHub).

I used FFMPEG sources from Kodi(XBMC) branch 4.3.1-Matrix-Alpha1-1 from GitHub - xbmc/FFmpeg: mirror of git://source.ffmpeg.org/ffmpeg.git.

Then I merged GitHub - jc-kynesim/rpi-ffmpeg: FFmpeg work for RPI (branch test/4.3/kodi_main), GitHub - lrusak/FFmpeg: mirror of git://source.ffmpeg.org/ffmpeg.git (v4l2-drmprime-v5) and GitHub - LibreELEC/FFmpeg (branch 4.3-libreelec-misc).

Then I configured build according to package.mk:

./configure --prefix="/usr" \
              --cpu=cortex-a7 \
              --arch=armv6t2 \
              --target-os="linux" \
              --disable-shared \
              --disable-rpi \
              --enable-sand \
              --enable-hwaccels \
              --disable-hwaccel=h264_v4l2request \
              --disable-hwaccel=mpeg2_v4l2request \
              --disable-hwaccel=vp8_v4l2request \
              --disable-hwaccel=vp9_v4l2request \
              --enable-libudev \
              --enable-v4l2-request \
              --enable-libdrm \
              --enable-v4l2_m2m \
              --enable-gpl \
              --disable-version3 \
              --enable-avdevice \
              --enable-avcodec \
              --enable-avformat \
              --enable-swscale \
              --enable-postproc \
              --enable-avfilter \
              --enable-libx264 \
              --enable-libx265 

and succesfully builded FFMPEG.

The resulting FFMPEG however does not work as expected For some reason I get these errors:

[h264_v4l2m2m @ 0x23bdd70] All capture buffers returned to userspace. Increase num_capture_buffers to prevent device deadlock or dropped packets/frames.

And it also seems that HW acceleration of HEVC decoding is not working either.

My FFMPEG command line is:

./ffmpeg -hwaccel drm -c:v hevc -i Udalosti.ts -c:v h264_v4l2m2m -c:a copy -b:v 1500k -pix_fmt yuv420p Udalosti.avi

I am pretty sure that I am doing something wrong, as I am not an expert...

Can someone please point me to the right direction, what might be wrong?

Or if there is another way... I just need HEVC -> H264 HW transcoding of a video stream through pipes.

Thank you very much for any useful info.