drm/i915: Use Write-Through cacheing for the display plane on Iris

Haswell GT3e has the unique feature of supporting Write-Through cacheing
of objects within the eLLC/LLC. The purpose of this is to enable the display
plane to remain coherent whilst objects lie resident in the eLLC/LLC - so
that we, in theory, get the best of both worlds, perfect display and fast
access.

However, we still need to be careful as the CPU does not see the WT when
accessing the cache. In particular, this means that we need to flush the
cache lines after writing to an object through the CPU, and on
transitioning from a cached state to WT.

v2: Actually do the clflush on transition to WT, nagging by Ville.
v3: Flush the CPU cache after writes into WT objects.
v4: Rease onto LLC updates and report WT as "uncached" for
get_cache_level_ioctl to remain symmetric with set_cache_level_ioctl.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 10 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index c9420c280cf0..212f6d8c35ec 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -55,6 +55,7 @@
 #define HSW_WB_LLC_AGE3                 HSW_CACHEABILITY_CONTROL(0x2)
 #define HSW_WB_LLC_AGE0                 HSW_CACHEABILITY_CONTROL(0x3)
 #define HSW_WB_ELLC_LLC_AGE0            HSW_CACHEABILITY_CONTROL(0xb)
+#define HSW_WT_ELLC_LLC_AGE0            HSW_CACHEABILITY_CONTROL(0x6)
 
 static gen6_gtt_pte_t snb_pte_encode(dma_addr_t addr,
                                      enum i915_cache_level level)
@@ -138,8 +139,16 @@ static gen6_gtt_pte_t iris_pte_encode(dma_addr_t addr,
         gen6_gtt_pte_t pte = GEN6_PTE_VALID;
         pte |= HSW_PTE_ADDR_ENCODE(addr);
 
-        if (level != I915_CACHE_NONE)
+        switch (level) {
+        case I915_CACHE_NONE:
+                break;
+        case I915_CACHE_WT:
+                pte |= HSW_WT_ELLC_LLC_AGE0;
+                break;
+        default:
                 pte |= HSW_WB_ELLC_LLC_AGE0;
+                break;
+        }
 
         return pte;
 }