gpu reset is a very important piece of our infrastructure.
Unfortunately we only really it test by actually hanging the gpu,
which often has bad side-effects for the entire system. And the gpu
hang handling code is one of the rather complicated pieces of code we
have, consisting of
- hang detection
- error capture
- actual gpu reset
- reset of all the gem bookkeeping
- reinitialition of the entire gpu
This patch adds a debugfs to selectively stopping rings by ceasing to
update the hw tail pointer, which will result in the gpu no longer
updating it's head pointer and eventually to the hangcheck firing.
This way we can exercise the gpu hang code under controlled conditions
without a dying gpu taking down the entire systems.
Patch motivated by me forgetting to properly reinitialize ppgtt after
a gpu reset.
Usage:
echo $((1 << $ringnum)) > i915_ring_stop # stops one ring
echo 0xffffffff > i915_ring_stop # stops all, future-proof version
then run whatever testload is desired. i915_ring_stop automatically
resets after a gpu hang is detected to avoid hanging the gpu to fast
and declaring it wedged.
v2: Incorporate feedback from Chris Wilson.
v3: Add the missing cleanup.