[PATCH 1/3] drm/i915: Disable sempahores on Sandybridge

[Chris Wilson <chris@chris-wilson.co.uk>]

I should have admitted defeat long ago as there has been a rare but
persistent error on Sandybridge where semaphore signaling did not
propagate to the waiter, leading to a GPU hang.

With the work on fence signaling for v4.9, the impact of using CPU driven
signaling was greatly reduced wrt to the latency of GPU semaphores,
though without logical rings support, the benefit of reordering work to
avoid bubbles is not realised (i.e. as it stands fence signaling is just
a slower, more costly version of HW semaphores; but works more
consistently). As a rough indicator of the difference,

with semaphores:
Sequential (3 engines, 1 processes): average 5.470us per cycle [expected 4.988us]

w/o semaphores:
Sequential (3 engines, 1 processes): average 15.771us per cycle [expected 4.923us]

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54226
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
---
 drivers/gpu/drm/i915/i915_gem.c | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 24d795802eaf..eee699dd376e 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4950,20 +4950,12 @@ int i915_gem_init_hw(struct drm_i915_private *dev_priv)
 
 bool intel_sanitize_semaphores(struct drm_i915_private *dev_priv, int value)
 {
-	if (INTEL_GEN(dev_priv) < 6)
-		return false;
-
-	/* TODO: make semaphores and Execlists play nicely together */
-	if (HAS_EXECLISTS(dev_priv))
+	if (!IS_GEN7(dev_priv))
 		return false;
 
 	if (value >= 0)
 		return value;
 
-	/* Enable semaphores on SNB when IO remapping is off */
-	if (IS_GEN6(dev_priv) && intel_vtd_active())
-		return false;
-
 	return true;
 }
 
-- 
2.15.0.rc2

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