drm/xe: Use ttm_uncached for BO with NEEDS_UC flag

We should honor requested uncached mode also at the TTM layer.
Otherwise, we risk losing updates to the memory based interrupts
source or status vectors, as those require uncached memory.
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240618104947.729-1-michal.wajdeczko@intel.com

drivers/gpu/drm/xe/xe_bo.c

@@ -378,6 +378,15 @@ static struct ttm_tt *xe_ttm_tt_create(struct ttm_buffer_object *ttm_bo,
 	    (xe->info.graphics_verx100 >= 1270 && bo->flags & XE_BO_FLAG_PAGETABLE))
 		caching = ttm_write_combined;
 
+	if (bo->flags & XE_BO_FLAG_NEEDS_UC) {
+		/*
+		 * Valid only for internally-created buffers only, for
+		 * which cpu_caching is never initialized.
+		 */
+		xe_assert(xe, bo->cpu_caching == 0);
+		caching = ttm_uncached;
+	}
+
 	err = ttm_tt_init(&tt->ttm, &bo->ttm, page_flags, caching, extra_pages);
 	if (err) {
 		kfree(tt);