drm/i915: Fix comments about CHV snoop behaviour
Replace the misinformed notes about CHV snoop behaviour with something
that's hopefully closer to reality.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index 29a4486..2b00852 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -1942,9 +1942,17 @@
* Only the snoop bit has meaning for CHV, the rest is
* ignored.
*
- * Note that the harware enforces snooping for all page
- * table accesses. The snoop bit is actually ignored for
- * PDEs.
+ * The hardware will never snoop for certain types of accesses:
+ * - CPU GTT (GMADR->GGTT->no snoop->memory)
+ * - PPGTT page tables
+ * - some other special cycles
+ *
+ * As with BDW, we also need to consider the following for GT accesses:
+ * "For GGTT, there is NO pat_sel[2:0] from the entry,
+ * so RTL will always use the value corresponding to
+ * pat_sel = 000".
+ * Which means we must set the snoop bit in PAT entry 0
+ * in order to keep the global status page working.
*/
pat = GEN8_PPAT(0, CHV_PPAT_SNOOP) |
GEN8_PPAT(1, 0) |