[040/118] drm/i915: Workaround incoherence between fences and LLCacross multiple CPUs

[Ben Hutchings]

3.2.45-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>

commit 25ff1195f8a0b3724541ae7bbe331b4296de9c06 upstream.

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
manually flush writes to memory prior to writing the fence register in
conjunction with the memory barriers placed around the register write.

Fixes i-g-t/gem_fence_thrash

v2: Bring a bigger gun
v3: Switch the bigger gun for heavier bullets (Arjan van de Ven)
v4: Remove changes for working generations.
v5: Reduce to a per-cpu wbinvd() call prior to updating the fences.
v6: Rewrite comments to ellide forgotten history.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=62191
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Jon Bloomfield <jon.bloomfield@xxxxxxxxx>
Tested-by: Jon Bloomfield <jon.bloomfield@xxxxxxxxx> (v2)
Reviewed-by: Jesse Barnes <jbarnes@xxxxxxxxxxxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
[bwh: Backported to 3.2: insert the cache flush in i915_gem_object_get_fence()]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2520,6 +2520,11 @@ i915_find_fence_reg(struct drm_device *d
 	return avail;
 }
 
+static void i915_gem_write_fence__ipi(void *data)
+{
+	wbinvd();
+}
+
 /**
  * i915_gem_object_get_fence - set up a fence reg for an object
  * @obj: object to map through a fence reg
@@ -2640,6 +2645,17 @@ update:
 	switch (INTEL_INFO(dev)->gen) {
 	case 7:
 	case 6:
+		/* 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+. In theory, the write to the fence register
+		 * flushes all memory transactions before, and coupled with the
+		 * mb() placed around the register write we serialise all memory
+		 * operations with respect to the changes in the tiler. Yet, on
+		 * SNB+ we need to take a step further and emit an explicit wbinvd()
+		 * on each processor in order to manually flush all memory
+		 * transactions before updating the fence register.
+		 */
+		on_each_cpu(i915_gem_write_fence__ipi, NULL, 1);
 		ret = sandybridge_write_fence_reg(obj, pipelined);
 		break;
 	case 5:

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