Re: [PATCH] drm/i915: stop using swiotlb

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On 21/07/2022 18:43, Robert Beckett wrote:
Calling swiotlb functions directly is nowadays considered harmful. See
https://lore.kernel.org/intel-gfx/20220711082614.GA29487@xxxxxx/

Replace swiotlb_max_segment() calls with dma_max_mapping_size().
In i915_gem_object_get_pages_internal() no longer consider max_segment
only if CONFIG_SWIOTLB is enabled. There can be other (iommu related)
causes of specific max segment sizes.

Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
Cc: Thomas Hellstrom <thomas.hellstrom@xxxxxxxxx>
Cc: Matthew Auld <matthew.auld@xxxxxxxxx>

Signed-off-by: Robert Beckett <bob.beckett@xxxxxxxxxxxxx>
---
  drivers/gpu/drm/i915/gem/i915_gem_internal.c | 20 +++++---------------
  drivers/gpu/drm/i915/gem/i915_gem_shmem.c    |  2 +-
  drivers/gpu/drm/i915/gem/i915_gem_ttm.c      |  4 ++--
  drivers/gpu/drm/i915/gem/i915_gem_userptr.c  |  2 +-
  drivers/gpu/drm/i915/i915_scatterlist.h      | 16 ----------------
  5 files changed, 9 insertions(+), 35 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_internal.c b/drivers/gpu/drm/i915/gem/i915_gem_internal.c
index c698f95af15f..e1aca378d90f 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_internal.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_internal.c
@@ -6,7 +6,6 @@
#include <linux/scatterlist.h>
  #include <linux/slab.h>
-#include <linux/swiotlb.h>
#include "i915_drv.h"
  #include "i915_gem.h"
@@ -38,22 +37,13 @@ static int i915_gem_object_get_pages_internal(struct drm_i915_gem_object *obj)
  	struct scatterlist *sg;
  	unsigned int sg_page_sizes;
  	unsigned int npages;
-	int max_order;
+	int max_order = MAX_ORDER;
+	size_t max_segment;
  	gfp_t gfp;
- max_order = MAX_ORDER;
-#ifdef CONFIG_SWIOTLB
-	if (is_swiotlb_active(obj->base.dev->dev)) {
-		unsigned int max_segment;
-
-		max_segment = swiotlb_max_segment();
-		if (max_segment) {
-			max_segment = max_t(unsigned int, max_segment,
-					    PAGE_SIZE) >> PAGE_SHIFT;
-			max_order = min(max_order, ilog2(max_segment));
-		}
-	}
-#endif
+	max_segment = dma_max_mapping_size(i915->drm.dev);
+	max_segment = max_t(size_t, max_segment, PAGE_SIZE) >> PAGE_SHIFT;
+	max_order = min(max_order, ilog2(max_segment));
gfp = GFP_KERNEL | __GFP_HIGHMEM | __GFP_RECLAIMABLE;
  	if (IS_I965GM(i915) || IS_I965G(i915)) {
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
index 4eed3dd90ba8..b0ec65b7c1da 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
@@ -194,7 +194,7 @@ static int shmem_get_pages(struct drm_i915_gem_object *obj)
  	struct intel_memory_region *mem = obj->mm.region;
  	struct address_space *mapping = obj->base.filp->f_mapping;
  	const unsigned long page_count = obj->base.size / PAGE_SIZE;
-	unsigned int max_segment = i915_sg_segment_size();
+	unsigned int max_segment = dma_max_mapping_size(i915->drm.dev);
  	struct sg_table *st;
  	struct sgt_iter sgt_iter;
  	struct page *page;
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
index 5a5cf332d8a5..882f046f4d18 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
@@ -189,7 +189,7 @@ static int i915_ttm_tt_shmem_populate(struct ttm_device *bdev,
  	struct drm_i915_private *i915 = container_of(bdev, typeof(*i915), bdev);
  	struct intel_memory_region *mr = i915->mm.regions[INTEL_MEMORY_SYSTEM];
  	struct i915_ttm_tt *i915_tt = container_of(ttm, typeof(*i915_tt), ttm);
-	const unsigned int max_segment = i915_sg_segment_size();
+	const unsigned int max_segment = dma_max_mapping_size(i915->drm.dev);
  	const size_t size = (size_t)ttm->num_pages << PAGE_SHIFT;
  	struct file *filp = i915_tt->filp;
  	struct sgt_iter sgt_iter;
@@ -568,7 +568,7 @@ static struct i915_refct_sgt *i915_ttm_tt_get_st(struct ttm_tt *ttm)
  	ret = sg_alloc_table_from_pages_segment(st,
  			ttm->pages, ttm->num_pages,
  			0, (unsigned long)ttm->num_pages << PAGE_SHIFT,
-			i915_sg_segment_size(), GFP_KERNEL);
+			dma_max_mapping_size(i915_tt->dev), GFP_KERNEL);
  	if (ret) {
  		st->sgl = NULL;
  		return ERR_PTR(ret);
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
index 094f06b4ce33..8a62a71859e6 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
@@ -129,7 +129,7 @@ static void i915_gem_object_userptr_drop_ref(struct drm_i915_gem_object *obj)
  static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
  {
  	const unsigned long num_pages = obj->base.size >> PAGE_SHIFT;
-	unsigned int max_segment = i915_sg_segment_size();
+	unsigned int max_segment = dma_max_mapping_size(obj->base.dev->dev);
  	struct sg_table *st;
  	unsigned int sg_page_sizes;
  	struct page **pvec;
diff --git a/drivers/gpu/drm/i915/i915_scatterlist.h b/drivers/gpu/drm/i915/i915_scatterlist.h
index 9ddb3e743a3e..c9a61b51e99d 100644
--- a/drivers/gpu/drm/i915/i915_scatterlist.h
+++ b/drivers/gpu/drm/i915/i915_scatterlist.h
@@ -9,7 +9,6 @@
#include <linux/pfn.h>
  #include <linux/scatterlist.h>
-#include <linux/swiotlb.h>
#include "i915_gem.h" @@ -127,21 +126,6 @@ static inline unsigned int i915_sg_dma_sizes(struct scatterlist *sg)
  	return page_sizes;
  }
-static inline unsigned int i915_sg_segment_size(void)
-{
-	unsigned int size = swiotlb_max_segment();
-
-	if (size == 0)
-		size = UINT_MAX;

On a more detailed look, there was a CI failure which makes me think this cap might need to stay. Because max sg segment is unsigned int. So I wonder if sg contstruction overflows without it.

If this quick analysis is right, you could leave i915_sg_segment_size helper and cap the return from dma_max_mapping_size to UINT_MAX in it.

Regards,

Tvrtko

*) https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_106589v1/shard-tglb2/igt@gem_mmap_offset@xxxxxxxxxx

-
-	size = rounddown(size, PAGE_SIZE);
-	/* swiotlb_max_segment_size can return 1 byte when it means one page. */
-	if (size < PAGE_SIZE)
-		size = PAGE_SIZE;
-
-	return size;
-}
-
  bool i915_sg_trim(struct sg_table *orig_st);
/**



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