drm/xe: Use write-back caching mode for system memory on DGFX

The caching mode for buffer objects with VRAM as a possible
placement was forced to write-combined, regardless of placement.

However, write-combined system memory is expensive to allocate and
even though it is pooled, the pool is expensive to shrink, since
it involves global CPU TLB flushes.

Moreover write-combined system memory from TTM is only reliably
available on x86 and DGFX doesn't have an x86 restriction.

So regardless of the cpu caching mode selected for a bo,
internally use write-back caching mode for system memory on DGFX.

Coherency is maintained, but user-space clients may perceive a
difference in cpu access speeds.

v2:
- Update RB- and Ack tags.
- Rephrase wording in xe_drm.h (Matt Roper)
v3:
- Really rephrase wording.
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: 622f709c ("drm/xe/uapi: Add support for CPU caching mode")
Cc: Pallavi Mishra <pallavi.mishra@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Effie Yu <effie.yu@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Jose Souza <jose.souza@intel.com>
Cc: Michal Mrozek <michal.mrozek@intel.com>
Cc: <stable@vger.kernel.org> # v6.8+
Acked-by: Matthew Auld <matthew.auld@intel.com>
Acked-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Fixes: 622f709c ("drm/xe/uapi: Add support for CPU caching mode")
Acked-by: Michal Mrozek <michal.mrozek@intel.com>
Acked-by: Effie Yu <effie.yu@intel.com> #On chat
Link: https://patchwork.freedesktop.org/patch/msgid/20240705132828.27714-1-thomas.hellstrom@linux.intel.com

drivers/gpu/drm/xe/xe_bo.c

@@ -343,7 +343,7 @@ static struct ttm_tt *xe_ttm_tt_create(struct ttm_buffer_object *ttm_bo,
 	struct xe_device *xe = xe_bo_device(bo);
 	struct xe_ttm_tt *tt;
 	unsigned long extra_pages;
-	enum ttm_caching caching;
+	enum ttm_caching caching = ttm_cached;
 	int err;
 
 	tt = kzalloc(sizeof(*tt), GFP_KERNEL);
@@ -357,26 +357,35 @@ static struct ttm_tt *xe_ttm_tt_create(struct ttm_buffer_object *ttm_bo,
 		extra_pages = DIV_ROUND_UP(xe_device_ccs_bytes(xe, bo->size),
 					   PAGE_SIZE);
 
-	switch (bo->cpu_caching) {
-	case DRM_XE_GEM_CPU_CACHING_WC:
-		caching = ttm_write_combined;
-		break;
-	default:
-		caching = ttm_cached;
-		break;
-	}
-
-	WARN_ON((bo->flags & XE_BO_FLAG_USER) && !bo->cpu_caching);
-
 	/*
-	 * Display scanout is always non-coherent with the CPU cache.
-	 *
-	 * For Xe_LPG and beyond, PPGTT PTE lookups are also non-coherent and
-	 * require a CPU:WC mapping.
+	 * DGFX system memory is always WB / ttm_cached, since
+	 * other caching modes are only supported on x86. DGFX
+	 * GPU system memory accesses are always coherent with the
+	 * CPU.
 	 */
-	if ((!bo->cpu_caching && bo->flags & XE_BO_FLAG_SCANOUT) ||
-	    (xe->info.graphics_verx100 >= 1270 && bo->flags & XE_BO_FLAG_PAGETABLE))
-		caching = ttm_write_combined;
+	if (!IS_DGFX(xe)) {
+		switch (bo->cpu_caching) {
+		case DRM_XE_GEM_CPU_CACHING_WC:
+			caching = ttm_write_combined;
+			break;
+		default:
+			caching = ttm_cached;
+			break;
+		}
+
+		WARN_ON((bo->flags & XE_BO_FLAG_USER) && !bo->cpu_caching);
+
+		/*
+		 * Display scanout is always non-coherent with the CPU cache.
+		 *
+		 * For Xe_LPG and beyond, PPGTT PTE lookups are also
+		 * non-coherent and require a CPU:WC mapping.
+		 */
+		if ((!bo->cpu_caching && bo->flags & XE_BO_FLAG_SCANOUT) ||
+		    (xe->info.graphics_verx100 >= 1270 &&
+		     bo->flags & XE_BO_FLAG_PAGETABLE))
+			caching = ttm_write_combined;
+	}
 
 	if (bo->flags & XE_BO_FLAG_NEEDS_UC) {
 		/*

drivers/gpu/drm/xe/xe_bo_types.h

@@ -68,7 +68,8 @@ struct xe_bo {
 
 	/**
 	 * @cpu_caching: CPU caching mode. Currently only used for userspace
-	 * objects.
+	 * objects. Exceptions are system memory on DGFX, which is always
+	 * WB.
 	 */
 	u16 cpu_caching;
 

include/uapi/drm/xe_drm.h

@@ -783,7 +783,13 @@ struct drm_xe_gem_create {
 #define DRM_XE_GEM_CPU_CACHING_WC                      2
 	/**
 	 * @cpu_caching: The CPU caching mode to select for this object. If
-	 * mmaping the object the mode selected here will also be used.
+	 * mmaping the object the mode selected here will also be used. The
+	 * exception is when mapping system memory (including data evicted
+	 * to system) on discrete GPUs. The caching mode selected will
+	 * then be overridden to DRM_XE_GEM_CPU_CACHING_WB, and coherency
+	 * between GPU- and CPU is guaranteed. The caching mode of
+	 * existing CPU-mappings will be updated transparently to
+	 * user-space clients.
 	 */
 	__u16 cpu_caching;
 	/** @pad: MBZ */