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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>
This commit is contained in:
Rodrigo Vivi 2014-11-05 16:56:36 -08:00 committed by Jani Nikula
parent 0df1f2487d
commit d6a8b72edc
1 changed files with 16 additions and 0 deletions
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16
drivers/gpu/drm/i915/i915_gem_gtt.c
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@ -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);