drm/msm: Temporarily disable stall-on-fault after a page fault

drm/msm: Temporarily disable stall-on-fault after a page fault

When things go wrong, the GPU is capable of quickly generating millions
of faulting translation requests per second. When that happens, in the
stall-on-fault model each access will stall until it wins the race to
signal the fault and then the RESUME register is written. This slows
processing page faults to a crawl as the GPU can generate faults much
faster than the CPU can acknowledge them. It also means that all
available resources in the SMMU are saturated waiting for the stalled
transactions, so that other transactions such as transactions generated
by the GMU, which shares translation resources with the GPU, cannot
proceed. This causes a GMU watchdog timeout, which leads to a failed
reset because GX cannot collapse when there is a transaction pending and
a permanently hung GPU.

On older platforms with qcom,smmu-v2, it seems that when one transaction
is stalled subsequent faulting transactions are terminated, which avoids
this problem, but the MMU-500 follows the spec here.

To work around these problems, disable stall-on-fault as soon as we get a
page fault until a cooldown period after pagefaults stop. This allows
the GMU some guaranteed time to continue working. We only use
stall-on-fault to halt the GPU while we collect a devcoredump and we
always terminate the transaction afterward, so it's fine to miss some
subsequent page faults. We also keep it disabled so long as the current
devcoredump hasn't been deleted, because in that case we likely won't
capture another one if there's a fault.

After this commit HFI messages still occasionally time out, because the
crashdump handler doesn't run fast enough to let the GMU resume, but the
driver seems to recover from it. This will probably go away after the
HFI timeout is increased.

Signed-off-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/654891/
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>