I tried using hardware video encoding with amdgpu/vaapi, but every attempt
immediately crashes the system.

The latest command line (amongst many with the same crash result) I tried:

ffmpeg -vaapi_device /dev/dri/renderD128 -t 10 -i somevideo.mp4 -vf
'scale=1920:1072,hwupload' -aspect 1920:1072 -map 0:0 -y -c:a copy -threads 1
-slices 1 -vcodec h264_vaapi -b:v 12500k -profile:v 77 -bf 0 test.mkv

ffmpeg/libva/mesa/dri etc. are the most recent ones currently shipped with Arch
Linux (ffmpeg version 3.3.3 built with gcc 7.1.1).

I am currently using
https://cgit.freedesktop.org/~agd5f/linux/log/?h=amd-staging-drm-next as my
kernel, but experienced the same error also with previous kernel versions (like
4.12)

Video card is a Radeon RX 460.