On 19/12/2016 12:43, Chris Wilson wrote:
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>
Fixes: 871dfbd67d4e ("drm/i915: Allow compaction upto SWIOTLB max segment size")
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: Imre Deak <imre.deak@xxxxxxxxx>
Cc: <drm-intel-fixes@xxxxxxxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/i915_gem.c | 26 ++++++++++++++++++++++----
1 file changed, 22 insertions(+), 4 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 412f3513f269..4e263df2afc3 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 +2412,25 @@ 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 instead 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 {
+ dev_warn(&dev_priv->drm.pdev->dev,
+ "Failed to DMA remap %lu pages\n",
+ page_count);
+ goto err_pages;
+ }
+ }
if (i915_gem_object_needs_bit17_swizzle(obj))
i915_gem_object_do_bit_17_swizzle(obj, st);
How much is the cost of freeing and re-acquiring pages in the fall back
case? It could be avoidable by using the table and adding something like
sgt = i915_sg_copy(sgt, table_max_segment). But it depends on how likely
is this path to be hit on swiotlb platforms. I have no idea. Our
datasets are much bigger than the swiotlb space - if that is true on
such platforms?
Regards,
Tvrtko
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