[Intel-gfx] [PATCH] drm/i915: enable semaphores on gen6 if dmar is not active

[Daniel Vetter]

Inspired by the recent ppgtt regression report, where switching of
dmar only for the gpu seems to fix things completely, I've looked
again at the semaphores+vt-d situation.

Contrary to my earlier testing a few months back my system is now
stable with dmar disabled for the igd, and not only when disabling
dmar completely.

So I'm rather hopeful that all our recent fixes for snb have changed
things for code and it's time to try enabling semaphores again. We've
also had issues with enabling semaphores which are not vt-d related,
but I guess these are all fixed by the autoreport-disabling and lazy
request fix. And there's only one way to find out whether there are
still other issues ...

Signed-Off-by: Daniel Vetter <daniel.vetter at ffwll.ch>

---

If no further vt-d regressions show up in the 3.4 cycle I plan to
merge this into -next for 3.5 (in a month or so). Comments about how
unfeasibly you deem this highly welcome.
-Daniel
---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 8e0b686..ac52433 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -849,9 +849,11 @@ intel_enable_semaphores(struct drm_device *dev)
 	if (i915_semaphores >= 0)
 		return i915_semaphores;
 
-	/* Disable semaphores on SNB */
-	if (INTEL_INFO(dev)->gen == 6)
-		return 0;
+#ifdef CONFIG_INTEL_IOMMU
+	/* Disable semaphores on SNB if VT-d is on. */
+	if (INTEL_INFO(dev)->gen == 6 && intel_iommu_gfx_mapped)
+		return false;
+#endif
 
 	return 1;
 }
-- 
1.7.7.6