[Intel-gfx] [PATCH] drm/i915: don't bail out of intel_wait_ring_buffer too early

[Daniel Vetter]

In the pre-gem days with non-existing hangcheck and gpu reset code,
this timeout of 3 seconds was pretty important to avoid stuck
processes.

But now we have the hangcheck code in gem that goes to great length
to ensure that the gpu is really dead before declaring it wedged.

So there's no need for this timeout anymore. Actually it's even harmful
because we can bail out too early (e.g. with xscreensaver slip)
when running giant batchbuffers. And our code isn't robust enough
to properly unroll any state-changes, we pretty much rely on the gpu
reset code cleaning up the mess (like cache tracking, fencing state,
active list/request tracking, ...).

With this change intel_begin_ring can only fail when the gpu is
wedged, and it will return -EAGAIN (like wait_request in case the
gpu reset is still outstanding).

v2: Chris Wilson noted that on resume timers aren't running and hence
we won't ever get kicked out of this loop by the hangcheck code. Use
an insanely large timeout instead for the HAS_GEM case to prevent
resume bugs from totally hanging the machine.

Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/intel_ringbuffer.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 0e99589..d33cbf7 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1006,7 +1006,16 @@ int intel_wait_ring_buffer(struct intel_ring_buffer *ring, int n)
 	}
 
 	trace_i915_ring_wait_begin(ring);
-	end = jiffies + 3 * HZ;
+	if (drm_core_check_feature(dev, DRIVER_GEM))
+		/* With GEM the hangcheck timer should kick us out of the loop,
+		 * leaving it early runs the risk of corrupting GEM state (due
+		 * to running on almost untested codepaths). But on resume
+		 * timers don't work yet, so prevent a complete hang in that
+		 * case by choosing an insanely large timeout. */
+		end = jiffies + 60 * HZ;
+	else
+		end = jiffies + 3 * HZ;
+
 	do {
 		ring->head = I915_READ_HEAD(ring);
 		ring->space = ring_space(ring);
-- 
1.7.6.4