I spent most of yesterday trying to get some UHD h265 videos I have that play fine under Raspbian's VLC (when using the MMAL-based vout plugin video output setting) to play under the armv7l build of ALARM for the RPi4 with no luck. The videos are stored on the same SATA3 SSD that I'm running Arch from.
I have installed ffmpeg-mmal from the AUR and I've been trying commands such as:
$this->bbcode_second_pass_code('', '$ mpv --vo=gpu --gpu-context=drm --drm-connector=1.HDMI-A-1 --hwdec=v4l2m2m-copy vid.mp4')
If you have been able to get hw accelerated h265 playback working with mpv under ALARM on a Pi4, please tell me what kernel you're running, the mpv command you are using and what options you are using in /boot/config.txt plus anything else you had to do to get it to work.
I have tried using vc4-kms-v3d and vc4-kms-v3d-pi4 for the dtoverlay option in config.txt but vc4-fkms-v3d is the only one that works at all for me.