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 (file)
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -1942,9 +1942,17 @@ static void chv_setup_private_ppat(struct drm_i915_private *dev_priv)
         * 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) |