After a walk and a think I sensibly stepped back and realised Xorg.0.log
reports problems and with that clue so does the kernel (but I hadn't
noticed!). I *suspect* this may be related to having kexec-ed into the
current kernel. I'm going to try a poweroff restart after this report:
$ grep -E 'DRI|rendering' /var/log/Xorg.0.log
[ 60.448] (==) RADEON(0): DRI3 disabled
[ 60.448] (WW) RADEON(0): Direct rendering disabled
[ 60.510] (II) Initializing extension DRI3
[ 60.510] (II) AIGLX: Screen 0 is not DRI2 capable
[ 60.852] (II) GLX: Initialized DRISWRAST GL provider for screen 0
[ 60.852] (II) Initializing extension XFree86-DRI
[ 60.852] (II) Initializing extension DRI2
Jul 14 14:52:59 sunny kernel: [drm:r600_ring_test [radeon]] *ERROR*
radeon: ring 0 test failed (scratch(0x850C)=0xCAFEDEAD)
Jul 14 14:52:59 sunny kernel: radeon 0000:0a:00.0: disabling GPU
acceleration
$ uname -r
6.4.3+debian+tj+
