So with some testing this evening, vlc under 5.4.42-v8+ will play HEVC happily, and anything software-rendered, using 'dtoverlay=vc4-fkms-v3d'. You get audio out of H.264 streams, but no video. omxplayer just barfs. With the 32b 5.4.42-v7l+, both vlc and omxplayer do H.264, and vlc does HEVC and software-rendered MPEG 2 as well. Neither does 10b yet, but that I expected. All on the raw framebuffer, over ssh, no X / Wayland / whatever involved.
So aside from the CMA problem, the -v8+ kernel still isn't feature-compatible with the -v7l+, despite being notionally the same code.
This is all with the latest firmware and latest EEPROM image, mostly because I wanted to see what the new HDMI diagnostics display looked like, and I must admit, I really like what I see, when I see it.
Good work, though. I'm always slighly amazed any of this works at all.
As it is apparently board policy to disallow any criticism of anything, as it appears to criticise something is to criticise all the users of that something, I will no longer be commenting in threads which are not directly relevant to my uses of the Pi.