From: Chris Wilson <chris.p.wilson@xxxxxxxxx>
The scratch page should never be accessed, and is only assigned as a
filler page to redirection invalid userspace access. It is not of a
performance concern and so we prefer to have a single consistent
configuration across all platforms, reducing the pressure on device
memory and avoiding the direct device access that would be required to
initialise the scratch page.
Signed-off-by: Chris Wilson <chris.p.wilson@xxxxxxxxx>
Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
---
drivers/gpu/drm/i915/gt/gen8_ppgtt.c | 43 ++++++++++++++--------------
1 file changed, 22 insertions(+), 21 deletions(-)
diff --git a/drivers/gpu/drm/i915/gt/gen8_ppgtt.c b/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
index c7bd5d71b03e..9604edf7d7c2 100644
--- a/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
+++ b/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
@@ -922,29 +922,30 @@ struct i915_ppgtt *gen8_ppgtt_create(struct intel_gt *gt,
*/
ppgtt->vm.has_read_only = !IS_GRAPHICS_VER(gt->i915, 11, 12);
- if (HAS_LMEM(gt->i915)) {
+ if (HAS_LMEM(gt->i915))
ppgtt->vm.alloc_pt_dma = alloc_pt_lmem;
-
- /*
- * On some platforms the hw has dropped support for 4K GTT pages
- * when dealing with LMEM, and due to the design of 64K GTT
- * pages in the hw, we can only mark the *entire* page-table as
- * operating in 64K GTT mode, since the enable bit is still on
- * the pde, and not the pte. And since we still need to allow
- * 4K GTT pages for SMEM objects, we can't have a "normal" 4K
- * page-table with scratch pointing to LMEM, since that's
- * undefined from the hw pov. The simplest solution is to just
- * move the 64K scratch page to SMEM on such platforms and call
- * it a day, since that should work for all configurations.
- */
- if (HAS_64K_PAGES(gt->i915))
- ppgtt->vm.alloc_scratch_dma = alloc_pt_dma;
- else
- ppgtt->vm.alloc_scratch_dma = alloc_pt_lmem;
- } else {
+ else
ppgtt->vm.alloc_pt_dma = alloc_pt_dma;
- ppgtt->vm.alloc_scratch_dma = alloc_pt_dma;
- }
+
+ /*
+ * On some platforms the hw has dropped support for 4K GTT pages
+ * when dealing with LMEM, and due to the design of 64K GTT
+ * pages in the hw, we can only mark the *entire* page-table as
+ * operating in 64K GTT mode, since the enable bit is still on
+ * the pde, and not the pte. And since we still need to allow
+ * 4K GTT pages for SMEM objects, we can't have a "normal" 4K
+ * page-table with scratch pointing to LMEM, since that's
+ * undefined from the hw pov. The simplest solution is to just
+ * move the 64K scratch page to SMEM on all platforms and call
+ * it a day, since that should work for all configurations.
+ *
+ * Using SMEM instead of LMEM has the additional advantage of
+ * not reserving high performance memory for a "never" used
+ * filler page. It also removes the device access that would
+ * be required to initialise the scratch page, reducing pressure
+ * on an even scarcer resource.
+ */
+ ppgtt->vm.alloc_scratch_dma = alloc_pt_dma;
err = gen8_init_scratch(&ppgtt->vm);
if (err)
--
2.37.3
