[PATCH] drm/i915: Fallback to single PAGE_SIZE segments for DMA remapping

[Chris Wilson]

If we at first do not succeed with attempting to remap our physical
pages using a coalesced scattergather list, try again with one
scattergather entry per page. This should help with swiotlb as it uses a
limited buffer size and only searches for contiguous chunks within its
buffer aligned up to the next boundary - i.e. we may prematurely cause a
failure as we are unable to utilize the unused space between large
chunks and trigger an error such as:

	 i915 0000:00:02.0: swiotlb buffer is full (sz: 1630208 bytes)

Reported-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/i915/i915_gem.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 412f3513f269..509d98887e04 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2326,7 +2326,8 @@ 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);
-	int page_count, i;
+	const unsigned long page_count = obj->base.size / PAGE_SIZE;
+	unsigned long i;
 	struct address_space *mapping;
 	struct sg_table *st;
 	struct scatterlist *sg;
@@ -2352,7 +2353,7 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 	if (st == NULL)
 		return ERR_PTR(-ENOMEM);
 
-	page_count = obj->base.size / PAGE_SIZE;
+rebuild_st:
 	if (sg_alloc_table(st, page_count, GFP_KERNEL)) {
 		kfree(st);
 		return ERR_PTR(-ENOMEM);
@@ -2411,8 +2412,21 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 	i915_sg_trim(st);
 
 	ret = i915_gem_gtt_prepare_pages(obj, st);
-	if (ret)
-		goto err_pages;
+	if (ret) {
+		/* DMA remapping failed? One possible cause is that
+		 * it could not reserve enough large entries, asking
+		 * for PAGE_SIZE chunks may be helpful.
+		 */
+		if (max_segment > PAGE_SIZE) {
+			for_each_sgt_page(page, sgt_iter, st)
+				put_page(page);
+			sg_free_table(st);
+
+			max_segment = PAGE_SIZE;
+			goto rebuild_st;
+		} else
+			goto err_pages;
+	}
 
 	if (i915_gem_object_needs_bit17_swizzle(obj))
 		i915_gem_object_do_bit_17_swizzle(obj, st);
-- 
2.11.0


--HlL+5n6rz5pIUxbD--