Just for reference, the ring 0 test failed error comes from the line:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/radeon/r600.c#n2848

Basically, the r600 driver writes 0xCAFEDEAD into a scratch register, then uses
a ring write to try and write 0xDEADBEEF into the scratch register, and then
waits and then reads the scratch register again to see what is in it.  If it is
not 0xDEADBEEF, then the test fails and hardware acceleration is turned off.

if 60db3a4d8cc9073cf56264785197ba75ee1caca4 caused it to start,
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=60db3a4d8cc9073cf56264785197ba75ee1caca4
then maybe the hardware doesn't support 256 concurrent requests?