[Intel-gfx] [PATCH v4 3/3] drm/i915: optimise i915_gem_object_vmap_range() for small objects

[Dave Gordon]

Now that we use this function for ringbuffers and other "small" objects,
it's worth avoiding an extra kmalloc()/kfree() 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).

Signed-off-by: Dave Gordon <david.s.gordon at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Alex Dai <yu.dai at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 14942cf..effb69b 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2422,6 +2422,7 @@ void *i915_gem_object_vmap_range(struct drm_i915_gem_object *obj,
 				 unsigned int npages)
 {
 	struct sg_page_iter sg_iter;
+	struct page *mypages[32];
 	struct page **pages;
 	void *addr;
 	int i;
@@ -2431,10 +2432,16 @@ void *i915_gem_object_vmap_range(struct drm_i915_gem_object *obj,
 		return NULL;
 	}
 
-	pages = drm_malloc_ab(npages, sizeof(*pages));
-	if (pages == NULL) {
-		DRM_DEBUG_DRIVER("Failed to get space for pages\n");
-		return NULL;
+	if (npages <= ARRAY_SIZE(mypages))
+		pages = mypages;
+	else {
+		pages = kmalloc(npages*sizeof(*pages), GFP_TEMPORARY | __GFP_NOWARN);
+		if (pages == NULL)
+			pages = drm_malloc_ab(npages, sizeof(*pages));
+		if (pages == NULL) {
+			DRM_DEBUG_DRIVER("Failed to get space for pages\n");
+			return NULL;
+		}
 	}
 
 	i = 0;
@@ -2447,7 +2454,8 @@ 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 != mypages)
+		drm_free_large(pages);
 
 	return addr;
 }
-- 
1.9.1