drm/i915: Use maximum write flush for pwrite_gtt

As recently disovered by forcing big-core (!llc) machines to use the GTT
paths, we need our full GTT write flush before manipulating the GTT PTE
or else the writes may be directed to the wrong page.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718145407.21352-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -610,7 +610,8 @@ i915_gem_gtt_pwrite_fast(struct drm_i915_gem_object *obj,
 		unsigned int page_length = PAGE_SIZE - page_offset;
 		page_length = remain < page_length ? remain : page_length;
 		if (node.allocated) {
-			wmb(); /* flush the write before we modify the GGTT */
+			/* flush the write before we modify the GGTT */
+			intel_gt_flush_ggtt_writes(ggtt->vm.gt);
 			ggtt->vm.insert_page(&ggtt->vm,
 					     i915_gem_object_get_dma_address(obj, offset >> PAGE_SHIFT),
 					     node.start, I915_CACHE_NONE, 0);
@@ -639,8 +640,8 @@ i915_gem_gtt_pwrite_fast(struct drm_i915_gem_object *obj,
 	i915_gem_object_unlock_fence(obj, fence);
 out_unpin:
 	mutex_lock(&i915->drm.struct_mutex);
+	intel_gt_flush_ggtt_writes(ggtt->vm.gt);
 	if (node.allocated) {
-		wmb();
 		ggtt->vm.clear_range(&ggtt->vm, node.start, node.size);
 		remove_mappable_node(&node);
 	} else {