[Intel-gfx] [PATCH] drm/i915: Workaround incoherence between fences and LLC across multiple CPUs

[Chris Wilson]

In order to fully serialize access to the fenced region and the update
to the fence register we need to take extreme measures on SNB+, and
write the fence from each cpu taking care to serialise memory accesses
on each.  The usual mb(), or even a mb() on each CPU is not enough to
ensure that access to the fenced region is coherent across the change in
fence register.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=62191
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: stable at vger.kernel.org
---
 drivers/gpu/drm/i915/i915_gem.c |   34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 19fc21b..fe34240 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2684,17 +2684,43 @@ static inline int fence_number(struct drm_i915_private *dev_priv,
 	return fence - dev_priv->fence_regs;
 }
 
+struct write_fence {
+	struct drm_device *dev;
+	struct drm_i915_gem_object *obj;
+	int fence;
+};
+
+static void i915_gem_write_fence__ipi(void *data)
+{
+	struct write_fence *args = data;
+	i915_gem_write_fence(args->dev, args->fence, args->obj);
+}
+
 static void i915_gem_object_update_fence(struct drm_i915_gem_object *obj,
 					 struct drm_i915_fence_reg *fence,
 					 bool enable)
 {
 	struct drm_i915_private *dev_priv = obj->base.dev->dev_private;
-	int reg = fence_number(dev_priv, fence);
-
-	i915_gem_write_fence(obj->base.dev, reg, enable ? obj : NULL);
+	struct write_fence args = {
+		.dev = obj->base.dev,
+		.fence = fence_number(dev_priv, fence),
+		.obj = enable ? obj : NULL,
+	};
+
+	/* In order to fully serialize access to the fenced region and
+	 * the update to the fence register we need to take extreme
+	 * measures on SNB+, and write the fence from each cpu taking
+	 * care to serialise memory accesses on each. The usual mb(),
+	 * or even a mb() on each CPU is not enough to ensure that access
+	 * to the fenced region is coherent across the change in fence
+	 * register.
+	 */
+	if (!HAS_LLC(obj->base.dev) ||
+	    on_each_cpu(i915_gem_write_fence__ipi, &args, 1) != 0)
+		i915_gem_write_fence__ipi(&args);
 
 	if (enable) {
-		obj->fence_reg = reg;
+		obj->fence_reg = args.fence;
 		fence->obj = obj;
 		list_move_tail(&fence->lru_list, &dev_priv->mm.fence_list);
 	} else {
-- 
1.7.10.4