Thank you, Rich. Thanks to your suggestion, I compared the output of
lsmod with and without ffmpeg4.tcz in onboot.lst. The only difference is that when ffmpeg4.tcz
is in onboot.lst, there are 5 additional kernel modules loaded:
uvcvideo and its dependencies (videobuf2_common, videobuf2_memops, videobuf2_v4l2 and videobuf2_malloc).
If I blacklist uvcvideo (via
blacklist=uvcvideo boot code), ffmpeg4.tcz can be loaded via onboot.lst and my keyboard works just fine after booting.
Just for comparison, I booted into my Devuan ASCII (Debian Stretch without systemd) partition on this same laptop. Allowing uvcvideo to load at boot does not interfere with the keyboard in Devuan. Strange.
I don't mind blacklisting uvcvideo because it disables the webcam, which is a salubrious side effect. So even though numerous questions remain unanswered, I think we can consider this solved. As always, thank you for your incredibly helpful input
[BTW, this whole adventure was just to try to get video playback support in chromium, which remains broken. So we won the ffmpeg4.tcz battle but I'm still losing the war. Please send reinforcements if you can--see the other thread.]