[PATCH v6] drm/xe/migrate: Revamp PAT index selection for migrate PTEs

[Sanjay Yadav <sanjay.kumar.yadav@intel.com>]

Improve PAT index selection logic in xe_migrate.c to avoid unnecessary
coherency overhead when host-side memory is uncached. Previously,
we defaulted to XE_CACHE_WB, which enforces 2-way coherency and may
trigger cacheline pulls from CPU even when host-side memory is never
dirty.

This change introduces xe_migrate_pat_index() to choose the appropriate
PAT index based on the actual TTM caching mode of the buffer object
being mapped. For iGPUs with WC host mappings, we now prefer
XE_CACHE_NONE to skip coherency snoops. For compressed PTEs on newer
platforms, we select XE_CACHE_NONE_COMPRESSION.

This avoids unnecessary cache traffic for uncached host mappings.

v6: (sashiko)
- Only apply the BO's host-side caching for system-memory PTEs.

v5: (Matt A)
- Simplify emit_pte() to derive caching from res->bo directly, removing
  the separate bo parameter
- Leave changes in __xe_migrate_update_pgtables() and
  build_pt_update_batch_sram()
- Fix comment about page-walker coherency in xe_migrate_pat_index()

v4:
- Keep xe_migrate_prepare_vm() on XE_CACHE_WB since page tables require
  page-walker coherency.
- Pass BO into emit_pte() and select PAT attributes from the BO's TTM
  caching mode.

Assisted-by: GitHub Copilot:claude-opus-4.8
Signed-off-by: Sanjay Yadav <sanjay.kumar.yadav@intel.com>
Suggested-by: Matthew Auld <matthew.auld@intel.com>
---
 drivers/gpu/drm/xe/xe_migrate.c | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_migrate.c b/drivers/gpu/drm/xe/xe_migrate.c
index 9428dd5e7760..cad8f0e443af 100644
--- a/drivers/gpu/drm/xe/xe_migrate.c
+++ b/drivers/gpu/drm/xe/xe_migrate.c
@@ -117,6 +117,27 @@ static void xe_migrate_fini(void *arg)
 	xe_exec_queue_put(m->q);
 }
 
+static inline u16 xe_migrate_pat_index(struct xe_device *xe,
+				       enum ttm_caching caching,
+				       bool is_comp_pte)
+{
+	enum xe_cache_level cache_level;
+
+	/*
+	 * Select the appropriate PAT index for buffer object PTEs programmed
+	 * by emit_pte(). We choose not to mess with xe_migrate_prepare_vm()
+	 * yet, for simplicity.
+	 */
+	if (is_comp_pte && GRAPHICS_VERx100(xe) >= 2000)
+		cache_level = XE_CACHE_NONE_COMPRESSION;
+	else if (caching == ttm_cached)
+		cache_level = XE_CACHE_WB;
+	else
+		cache_level = XE_CACHE_NONE;
+
+	return xe_cache_pat_idx(xe, cache_level);
+}
+
 static u64 xe_migrate_vm_addr(u64 slot, u32 level)
 {
 	XE_WARN_ON(slot >= NUM_PT_SLOTS);
@@ -631,17 +652,17 @@ static void emit_pte(struct xe_migrate *m,
 {
 	struct xe_device *xe = tile_to_xe(m->tile);
 	struct xe_vm *vm = m->q->vm;
+	struct xe_bo *bo = ttm_to_xe_bo(res->bo);
+	enum ttm_caching caching = ttm_cached;
 	u16 pat_index;
 	u32 ptes;
 	u64 ofs = (u64)at_pt * XE_PAGE_SIZE;
 	u64 cur_ofs;
 
-	/* Indirect access needs compression enabled uncached PAT index */
-	if (GRAPHICS_VERx100(xe) >= 2000)
-		pat_index = is_comp_pte ? xe_cache_pat_idx(xe, XE_CACHE_NONE_COMPRESSION) :
-					  xe_cache_pat_idx(xe, XE_CACHE_WB);
-	else
-		pat_index = xe_cache_pat_idx(xe, XE_CACHE_WB);
+	if (!is_vram && bo->ttm.ttm)
+		caching = bo->ttm.ttm->caching;
+
+	pat_index = xe_migrate_pat_index(xe, caching, is_comp_pte);
 
 	ptes = DIV_ROUND_UP(size, XE_PAGE_SIZE);
 
-- 
2.52.0