Chris Wilson chris@chris-wilson.co.uk writes:
When running gem_exec_nop, it floods the system with many requests (with the goal of userspace submitting faster than the HW can process a single empty batch). This causes the driver to continually resubmit new requests onto the end of an active context, a flood of lite-restore preemptions. If we time this just right, Tigerlake hangs.
Inserting a small delay between the processing of CS events and submitting the next context, prevents the hang. Naturally it does not occur with debugging enabled. The suspicion then is that this is related to the issues with the CS event buffer, and inserting an mmio read of the CS pointer status appears to be very successful in preventing the hang. Other registers, or uncached reads, or plain mb, do not prevent the hang, suggesting that register is key -- but that the hang can be prevented by a simple udelay, suggests it is just a timing issue like that encountered by commit 233c1ae3c83f ("drm/i915/gt: Wait for CSB entries on Tigerlake"). Also note that the hang is not prevented by applying CTX_DESC_FORCE_RESTORE, or by inserting a delay on the GPU between requests.
Signed-off-by: Chris Wilson chris@chris-wilson.co.uk Cc: Mika Kuoppala mika.kuoppala@linux.intel.com Cc: Bruce Chang yu.bruce.chang@intel.com Cc: Joonas Lahtinen joonas.lahtinen@linux.intel.com Cc: stable@vger.kernel.org
Acked-by: Mika Kuoppala mika.kuoppala@linux.intel.com
drivers/gpu/drm/i915/gt/intel_lrc.c | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c index 6170f6874f52..d15d561152ba 100644 --- a/drivers/gpu/drm/i915/gt/intel_lrc.c +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c @@ -2711,6 +2711,9 @@ static void process_csb(struct intel_engine_cs *engine) smp_wmb(); /* complete the seqlock */ WRITE_ONCE(execlists->active, execlists->inflight);
/* Magic delay for tgl */ENGINE_POSTING_READ(engine, RING_CONTEXT_STATUS_PTR);WRITE_ONCE(execlists->pending[0], NULL);-- 2.20.1