drm/i915: Disable caches for Global GTT.

Global GTT doesn't have pat_sel[2:0] so it always point to pat_sel = 000;
So the only way to avoid screen corruptions is setting PAT 0 to Uncached.

MOCS can still be used though. But if userspace is trusting PTE for
cache selection the safest thing to do is to let caches disabled.

BSpec: "For GGTT, there is NO pat_sel[2:0] from the entry,
so RTL will always use the value corresponding to pat_sel = 000"

- System agent ggtt writes (i.e. cpu gtt mmaps) already work before
this patch, i.e. the same uncached + snooping access like on gen6/7
seems to be in effect.
- So this just fixes blitter/render access. Again it looks like it's
not just uncached access, but uncached + snooping. So we can still
hold onto all our assumptions wrt cpu clflushing on LLC machines.

v2: Cleaner patch as suggested by Chris.
v3: Add Daniel's comment

Reference: https://bugs.freedesktop.org/show_bug.cgi?id=85576
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: James Ausmus <james.ausmus@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Stable@vger.kernel.org
Tested-by: James Ausmus <james.ausmus@intel.com>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index b672b84..728938f 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -1902,6 +1902,22 @@ static void bdw_setup_private_ppat(struct drm_i915_private *dev_priv)
              GEN8_PPAT(6, GEN8_PPAT_WB | GEN8_PPAT_LLCELLC | GEN8_PPAT_AGE(2)) |
              GEN8_PPAT(7, GEN8_PPAT_WB | GEN8_PPAT_LLCELLC | GEN8_PPAT_AGE(3));
 
+       if (!USES_PPGTT(dev_priv->dev))
+               /* Spec: "For GGTT, there is NO pat_sel[2:0] from the entry,
+                * so RTL will always use the value corresponding to
+                * pat_sel = 000".
+                * So let's disable cache for GGTT to avoid screen corruptions.
+                * MOCS still can be used though.
+                * - System agent ggtt writes (i.e. cpu gtt mmaps) already work
+                * before this patch, i.e. the same uncached + snooping access
+                * like on gen6/7 seems to be in effect.
+                * - So this just fixes blitter/render access. Again it looks
+                * like it's not just uncached access, but uncached + snooping.
+                * So we can still hold onto all our assumptions wrt cpu
+                * clflushing on LLC machines.
+                */
+               pat = GEN8_PPAT(0, GEN8_PPAT_UC);
+
        /* XXX: spec defines this as 2 distinct registers. It's unclear if a 64b
         * write would work. */
        I915_WRITE(GEN8_PRIVATE_PAT, pat);