drm/xe/bo: optimise CCS case for WB pages

Dealing with CCS state is significant on LNL+, where we end up clearing
the compression state on every page alloc using the blitter for user
buffers, including also saving and restoring it when moving between
domains, plus we need to alloc extra pages to hold the raw CCS state for
the save step.

However all compression PAT modes, on platforms like LNL, also require
coh_none, meaning that only WC memory can use compression in the first
place. With this we can be sneaky and completely ignore CCS for WB
buffers, which is likely the common case anyway. This would then skip
all blitter moves/clears between sys <-> tt and then also means we can
drop the extra CCS pages.

This should be safe since there is no way to interact with the
compression state (potentially uncleared) without using a PAT enabled
index (which is rejected at bind), including if trying to be malicious
and copy the raw CCS state from userpace, which should give back all
zeroes if the src surface (indirect) is lacking compressed PAT index.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Satyanarayana K V P <satyanarayana.k.v.p@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Satyanarayana K V P <satyanarayana.k.v.p@intel.com>
Link: https://lore.kernel.org/r/20250516153810.223530-2-matthew.auld@intel.com

drivers/gpu/drm/xe/xe_bo.c

@@ -2982,6 +2982,14 @@ bool xe_bo_needs_ccs_pages(struct xe_bo *bo)
 	if (IS_DGFX(xe) && (bo->flags & XE_BO_FLAG_SYSTEM))
 		return false;
 
+	/*
+	 * Compression implies coh_none, therefore we know for sure that WB
+	 * memory can't currently use compression, which is likely one of the
+	 * common cases.
+	 */
+	if (bo->cpu_caching == DRM_XE_GEM_CPU_CACHING_WB)
+		return false;
+
 	return true;
 }
 

drivers/gpu/drm/xe/xe_pat.c

@@ -103,7 +103,8 @@ static const struct xe_pat_table_entry xelpg_pat_table[] = {
  *
  * Note: There is an implicit assumption in the driver that compression and
  * coh_1way+ are mutually exclusive. If this is ever not true then userptr
- * and imported dma-buf from external device will have uncleared ccs state.
+ * and imported dma-buf from external device will have uncleared ccs state. See
+ * also xe_bo_needs_ccs_pages().
  */
 #define XE2_PAT(no_promote, comp_en, l3clos, l3_policy, l4_policy, __coh_mode) \
 	{ \