drm/i915: Disable semaphores on Sandybridge

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]

In comparison, v3.4:
with semaphores:
Sequential (3 engines, 1 processes): average 16.066us per cycle [expected 11.842us]
w/o semaphores:
Sequential (3 engines, 1 processes): average 23.460us per cycle [expected 11.839us]

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54226 #and 100+ dupes
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171120205504.21892-3-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index d470e10b2edd0c43a5259505d8bb78bdc442e9d1..d53bb8e872ba6025d8d160f9abe56e23289d55c9 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4999,20 +4999,12 @@ out:
 
 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;
 }