[PATCH 16/24] drm/i915: postpone the fill-dma operation for large objects

[Matthew Auld]

If we are going to fill the entire page anyway when inserting our
entries then there's not much point in clearing it before hand. For
particularly large objects the fill_page_dma() tends to rank rather
highly in perf when running benchmarks like gem_exec_fault, for both the
non-huge and especially the huge page cases.

Signed-off-by: Matthew Auld <matthew.auld at intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem_gtt.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index 35ba39883094..bfb6430c6992 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -1344,7 +1344,14 @@ static int gen8_ppgtt_alloc_pd(struct i915_address_space *vm,
 			if (IS_ERR(pt))
 				goto unwind;
 
-			gen8_initialize_pt(vm, pt);
+			/* As a small optimisation we can postpone clearing the
+			 * pt if we are going fill every PTE when we later
+			 * insert our entries. This is especially true for 1G
+			 * and 2M pages where the pt remains unused.
+			 */
+			if (length - start < (1 << GEN8_PDE_SHIFT) ||
+			    !IS_ALIGNED(start, 1 << GEN8_PDE_SHIFT))
+				gen8_initialize_pt(vm, pt);
 
 			gen8_ppgtt_set_pde(vm, pd, pt, pde);
 			pd->used_pdes++;
@@ -1375,7 +1382,15 @@ static int gen8_ppgtt_alloc_pdp(struct i915_address_space *vm,
 			if (IS_ERR(pd))
 				goto unwind;
 
-			gen8_initialize_pd(vm, pd);
+			/* As a small optimisation we can postpone clearing the
+			 * pd if we are going fill every PDE when we later
+			 * insert the entries. This is especially true for 1G
+			 * pages we the pd remains unused.
+			 */
+			if (length - start < (1 << GEN8_PDPE_SHIFT) ||
+			    !IS_ALIGNED(start, 1 << GEN8_PDPE_SHIFT))
+				gen8_initialize_pd(vm, pd);
+
 			gen8_ppgtt_set_pdpe(vm, pdp, pd, pdpe);
 			pdp->used_pdpes++;
 			GEM_BUG_ON(pdp->used_pdpes > i915_pdpes_per_pdp(vm));
-- 
2.13.5