[PATCH v6 4/7] drm/i915: optimise i915_gem_object_vmap_range() 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>
Cc: Alex Dai <yu.dai@xxxxxxxxx>
---
 drivers/gpu/drm/i915/i915_gem.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 8f12e73..9410c72 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2420,7 +2420,8 @@ void *i915_gem_object_vmap_range(struct drm_i915_gem_object *obj,
 {
 	struct scatterlist *sg = obj->pages->sgl;
 	struct sg_page_iter sg_iter;
-	struct page **pages;
+	struct page *stack_pages[32];
+	struct page **pages = stack_pages;
 	unsigned long first, npages, i;
 	int nents;
 	void *addr;
@@ -2436,10 +2437,13 @@ void *i915_gem_object_vmap_range(struct drm_i915_gem_object *obj,
 	first = start >> PAGE_SHIFT;
 	npages = DIV_ROUND_UP(start + nbytes, PAGE_SIZE) - first;
 
-	pages = drm_malloc_gfp(npages, sizeof(*pages), GFP_TEMPORARY);
-	if (pages == NULL) {
-		DRM_DEBUG_DRIVER("Failed to get space for pages\n");
-		return NULL;
+	if (npages > ARRAY_SIZE(stack_pages)) {
+		/* Too big for stack -- allocate temporary array instead */
+		pages = drm_malloc_gfp(npages, sizeof(*pages), GFP_TEMPORARY);
+		if (pages == NULL) {
+			DRM_DEBUG_DRIVER("Failed to get space for pages\n");
+			return NULL;
+		}
 	}
 
 	i = 0;
@@ -2453,7 +2457,8 @@ void *i915_gem_object_vmap_range(struct drm_i915_gem_object *obj,
 	addr = vmap(pages, npages, 0, PAGE_KERNEL);
 	if (addr == NULL)
 		DRM_DEBUG_DRIVER("Failed to vmap pages\n");
-	drm_free_large(pages);
+	if (pages != stack_pages)
+		drm_free_large(pages);
 
 	return addr;
 }
-- 
1.9.1

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