drm/i915/selftests: Replace opencoded clflush with drm_clflush_virt_range

We occasionally see that the clflush prior to a read of GPU data is
returning stale data, reminiscent of much earlier bugs fixed by adding a
second clflush for serialisation. As drm_clflush_virt_range() already
supplies the workaround, use it rather than open code the clflush
instruction.

References: 396f5d62 ("drm: Restore double clflush on the last partial cacheline")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180730075351.15569-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_gem_coherency.c

@@ -33,7 +33,8 @@ static int cpu_set(struct drm_i915_gem_object *obj,
 {
 	unsigned int needs_clflush;
 	struct page *page;
-	u32 *map;
+	void *map;
+	u32 *cpu;
 	int err;
 
 	err = i915_gem_obj_prepare_shmem_write(obj, &needs_clflush);
@@ -42,24 +43,19 @@ static int cpu_set(struct drm_i915_gem_object *obj,
 
 	page = i915_gem_object_get_page(obj, offset >> PAGE_SHIFT);
 	map = kmap_atomic(page);
+	cpu = map + offset_in_page(offset);
 
-	if (needs_clflush & CLFLUSH_BEFORE) {
-		mb();
-		clflush(map+offset_in_page(offset) / sizeof(*map));
-		mb();
-	}
+	if (needs_clflush & CLFLUSH_BEFORE)
+		drm_clflush_virt_range(cpu, sizeof(*cpu));
 
-	map[offset_in_page(offset) / sizeof(*map)] = v;
+	*cpu = v;
 
-	if (needs_clflush & CLFLUSH_AFTER) {
-		mb();
-		clflush(map+offset_in_page(offset) / sizeof(*map));
-		mb();
-	}
+	if (needs_clflush & CLFLUSH_AFTER)
+		drm_clflush_virt_range(cpu, sizeof(*cpu));
 
 	kunmap_atomic(map);
-
 	i915_gem_obj_finish_shmem_access(obj);
+
 	return 0;
 }
 
@@ -69,7 +65,8 @@ static int cpu_get(struct drm_i915_gem_object *obj,
 {
 	unsigned int needs_clflush;
 	struct page *page;
-	u32 *map;
+	void *map;
+	u32 *cpu;
 	int err;
 
 	err = i915_gem_obj_prepare_shmem_read(obj, &needs_clflush);
@@ -78,17 +75,16 @@ static int cpu_get(struct drm_i915_gem_object *obj,
 
 	page = i915_gem_object_get_page(obj, offset >> PAGE_SHIFT);
 	map = kmap_atomic(page);
+	cpu = map + offset_in_page(offset);
 
-	if (needs_clflush & CLFLUSH_BEFORE) {
-		mb();
-		clflush(map+offset_in_page(offset) / sizeof(*map));
-		mb();
-	}
+	if (needs_clflush & CLFLUSH_BEFORE)
+		drm_clflush_virt_range(cpu, sizeof(*cpu));
 
-	*v = map[offset_in_page(offset) / sizeof(*map)];
-	kunmap_atomic(map);
+	*v = *cpu;
 
+	kunmap_atomic(map);
 	i915_gem_obj_finish_shmem_access(obj);
+
 	return 0;
 }