Hi there.
Apologies if this isn't the right place to ask, and please refer me to the right place to ask as I'm new here.
I'm running Gentoo with Musl and a mainline Linux-libre kernel on a Raspberry Pi 4 with 8GB RAM.
Everything is working almost flawlessly, almost.
Video playback works rather well on the system, so long as the window in which the video is being played is of the same size as the resolution of the video or smaller. Should the window be larger, weird flickering occurs, as seen in the following video:
https://imgur.com/a/XawXANR
Note also that the following error message is also printed:
I've tried the exact same mainline libre kernel on a fresh install of RpiOS, and had no problems whatsoever.
Note that I've tried all variations of GPU memory allocations in config.txt and they haven't fixed the issue. I game on my Pi as well and the video memory presents no issues.
I've read somewhere that a possible solution is to build mpv and ffmpeg with some raspberry-pi-related USE flags such as "mmal" or "raspberrypi", but none of them are available to me in my profile as for some reason I can't install the "raspberrypi-userland" package, maybe because I have a 64bit userland contrary to the 32bit one required by the package. I can elaborate further on that if anyone deems it necessary.

Here is my kernel .config:
https://pastebin.com/ucPkrLLb

Here are my USE flags:
aac acl acpi alsa apparmor -bluetooth -dbus dri ffmpeg flac fluidsynth -gstreamer -gtk hardened jpeg -kde -kerberos -llvm mmal mp3 mp4 -nls offensive openal opengl pie png -policykit -pulseaudio -qt5 -qt6 quicktime raspberry-pi raw readline savedconfig sdl smp socks5 sound static static-libs svg -systemd threads unicode vorbis vulkan -webkit webp -wifi xinerama -zeroconf zlib zstd
And additionaly:
media-video/ffmpeg X bluray codec2 cpudetection frei0r fribidi gmp libass libv4l opencl opus theora truetype v4l vaapi x264 x265 xvid
media-video/mpv bluray cdda dvd tools xv zimg -libcaca libplacebo

Does anyone have any idea as to what could be causing this? I haven't seen any mention online of something addressing specifically the problem I'm having.

Highly appreciated

mmal is dead, move to v4l2-m2m but ffmpeg need patch for that.
btw, don't know about gentoo :ugeek: