[Intel-gfx] [PATCH] drm/i915: Use the existing pages when retrying to DMA map

[Tvrtko Ursulin]

From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

Rather than freeing and re-allocating the pages when DMA mapping
in large chunks fails, we can just rebuild the sg table with no
coalescing.

Also change back the page counter to unsigned int because that
is what the sg API supports.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
---
Only compile tested!
---
 drivers/gpu/drm/i915/i915_gem.c | 40 ++++++++++++++++++++++++++++++----------
 1 file changed, 30 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 5275f6248ce3..e73f9f5a5d23 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2340,12 +2340,36 @@ static void i915_sg_trim(struct sg_table *orig_st)
 	*orig_st = new_st;
 }
 
+static void i915_sg_uncoalesce(struct sg_table *orig_st, unsigned long nents)
+{
+	struct sg_table new_st;
+	struct scatterlist *new_sg;
+	struct sgt_iter sgt_iter;
+	struct page *page;
+
+	if (sg_alloc_table(&new_st, nents, GFP_KERNEL))
+		return;
+
+	new_sg = new_st.sgl;
+	for_each_sgt_page(page, sgt_iter, orig_st) {
+		sg_set_page(new_sg, page, PAGE_SIZE, 0);
+		/* called before being DMA mapped, no need to copy sg->dma_* */
+		new_sg = sg_next(new_sg);
+	}
+
+	GEM_BUG_ON(new_sg); /* Should walk exactly nents and hit the end */
+
+	sg_free_table(orig_st);
+
+	*orig_st = new_st;
+}
+
 static struct sg_table *
 i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 {
 	struct drm_i915_private *dev_priv = to_i915(obj->base.dev);
-	const unsigned long page_count = obj->base.size / PAGE_SIZE;
-	unsigned long i;
+	const unsigned int page_count = obj->base.size / PAGE_SIZE;
+	unsigned int i;
 	struct address_space *mapping;
 	struct sg_table *st;
 	struct scatterlist *sg;
@@ -2371,7 +2395,6 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 	if (st == NULL)
 		return ERR_PTR(-ENOMEM);
 
-rebuild_st:
 	if (sg_alloc_table(st, page_count, GFP_KERNEL)) {
 		kfree(st);
 		return ERR_PTR(-ENOMEM);
@@ -2429,6 +2452,7 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 	/* Trim unused sg entries to avoid wasting memory. */
 	i915_sg_trim(st);
 
+prepare_gtt:
 	ret = i915_gem_gtt_prepare_pages(obj, st);
 	if (ret) {
 		/* DMA remapping failed? One possible cause is that
@@ -2436,16 +2460,12 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 		 * for PAGE_SIZE chunks instead may be helpful.
 		 */
 		if (max_segment > PAGE_SIZE) {
-			for_each_sgt_page(page, sgt_iter, st)
-				put_page(page);
-			sg_free_table(st);
-
+			i915_sg_uncoalesce(st, page_count);
 			max_segment = PAGE_SIZE;
-			goto rebuild_st;
+			goto prepare_gtt;
 		} else {
 			dev_warn(&dev_priv->drm.pdev->dev,
-				 "Failed to DMA remap %lu pages\n",
-				 page_count);
+				 "Failed to DMA remap %u pages\n", page_count);
 			goto err_pages;
 		}
 	}
-- 
2.7.4