Re: [Mesa-dev] [PATCH 12/12] i965: Prefer to use the GPU copy if we need to stall for reads

[Chris Wilson]

Quoting Chris Wilson (2017-08-04 21:01:16)
> If we need to stall to read the bo, ask the GPU to copy it into the CPU
> cache whilst we wait.

This is more food for thought, as I think we need to change the priority
ladder first. Aiui, miptree_map is the last resort so we don't want
needless complexity for cases we can identify and handle before hitting
this fallback. (I have an untidy series to blorpify read/drawpixels
get/subteximage.)
-Chris
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[Mesa-dev] [PATCH 12/12] i965: Prefer to use the GPU copy if we need to stall for reads

[Chris Wilson]

If we need to stall to read the bo, ask the GPU to copy it into the CPU
cache whilst we wait.
---
 src/mesa/drivers/dri/i965/intel_mipmap_tree.c | 14 +-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/intel_mipmap_tree.c 
b/src/mesa/drivers/dri/i965/intel_mipmap_tree.c
index 3b5e5595d7..5cd8d24f1e 100644
--- a/src/mesa/drivers/dri/i965/intel_mipmap_tree.c
+++ b/src/mesa/drivers/dri/i965/intel_mipmap_tree.c
@@ -3421,6 +3421,18 @@ can_blit_slice(struct intel_mipmap_tree *mt,
 }
 
 static bool
+map_will_stall(struct brw_bo *bo, GLbitfield mode)
+{
+   /* If we need to stall for reading the buffer, offload the cost
+* of clflushing it to the GPU.
+*/
+   if (!bo->cache_coherent && !(mode & GL_MAP_INVALIDATE_RANGE_BIT))
+  mode |= GL_MAP_READ_BIT;
+
+   return brw_bo_map_busy(bo, mode);
+}
+
+static bool
 use_intel_mipree_map_blit(struct brw_context *brw,
   struct intel_mipmap_tree *mt,
   GLbitfield mode,
@@ -3431,7 +3443,7 @@ use_intel_mipree_map_blit(struct brw_context *brw,
 * all the overhead involved.
 */
 
-   if (!(mode & GL_MAP_WRITE_BIT) &&
+   if (map_will_stall(mt->bo, mode) &&
!mt->compressed &&
(mt->surf.tiling == ISL_TILING_X ||
 /* Prior to Sandybridge, the blitter can't handle Y tiling */
-- 
2.13.3

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