[PATCH 3/3] drm/i915: optimise i915_gem_object_map() for small objects

[Dave Gordon <david.s.gordon@xxxxxxxxx>]

We're using this function for ringbuffers and other "small" objects, so
it's worth avoiding an extra malloc()/free() cycle if the page array is
small enough to put on the stack. Here we've chosen an arbitrary cutoff
of 32 (4k) pages, which is big enough for a ringbuffer (4 pages) or a
context image (currently up to 22 pages).

v5:
    change name of local array [Chris Wilson]

Signed-off-by: Dave Gordon <david.s.gordon@xxxxxxxxx>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/i915/i915_gem.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index f459702..f97cd83 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2399,7 +2399,8 @@ static void *i915_gem_object_map(const struct drm_i915_gem_object *obj)
 	unsigned long n_pages = obj->base.size >> PAGE_SHIFT;
 	struct sg_table *sgt = obj->pages;
 	struct sg_page_iter sg_iter;
-	struct page **pages;
+	struct page *stack_pages[32];
+	struct page **pages = stack_pages;
 	unsigned long i = 0;
 	void *addr = NULL;
 
@@ -2407,11 +2408,14 @@ static void *i915_gem_object_map(const struct drm_i915_gem_object *obj)
 	if (n_pages == 1)
 		return kmap(sg_page(sgt->sgl));
 
-	pages = drm_malloc_gfp(n_pages, sizeof(*pages), GFP_TEMPORARY);
-	if (pages == NULL) {
-		DRM_DEBUG_DRIVER("Failed to get space for %lu pointers\n",
-				 n_pages);
-		return NULL;
+	if (n_pages > ARRAY_SIZE(stack_pages)) {
+		/* Too big for stack -- allocate temporary array instead */
+		pages = drm_malloc_gfp(n_pages, sizeof(*pages), GFP_TEMPORARY);
+		if (pages == NULL) {
+			DRM_DEBUG_DRIVER("Failed to get space for %lu pointers\n",
+					 n_pages);
+			return NULL;
+		}
 	}
 
 	for_each_sgt_page_range(&sg_iter, sgt, 0, n_pages)
@@ -2424,7 +2428,8 @@ static void *i915_gem_object_map(const struct drm_i915_gem_object *obj)
 	if (addr == NULL)
 		DRM_DEBUG_DRIVER("Failed to vmap %lu pages\n", n_pages);
 
-	drm_free_large(pages);
+	if (pages != stack_pages)
+		drm_free_large(pages);
 
 	return addr;
 }
-- 
1.9.1

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