Re: [PATCH v4] drm/i915: stop using swiotlb

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On 26/07/2022 14:12, Tvrtko Ursulin wrote:

On 25/07/2022 15:18, 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>

v2: - restore UINT_MAX clamp in i915_sg_segment_size()
     - drop PAGE_SIZE check as it will always be >= PAGE_SIZE
v3: - actually clamp to UINT_MAX in i915_sg_segment_size()
v4: - round down max segment size to PAGE_SIZE

Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Signed-off-by: Robert Beckett <bob.beckett@xxxxxxxxxxxxx>
---
  drivers/gpu/drm/i915/gem/i915_gem_internal.c | 19 ++++---------------
  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      | 17 ++++-------------
  5 files changed, 12 insertions(+), 32 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..24f37658f1bb 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 +37,12 @@ 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;
+    unsigned int 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 = i915_sg_segment_size(i915->drm.dev) >> 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..34b9c76cd8e6 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 = i915_sg_segment_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..7a828c9c0f6d 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 = i915_sg_segment_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);
+            i915_sg_segment_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..dfc35905dba2 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 = i915_sg_segment_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..1377a97a20cf 100644
--- a/drivers/gpu/drm/i915/i915_scatterlist.h
+++ b/drivers/gpu/drm/i915/i915_scatterlist.h
@@ -9,7 +9,7 @@
  #include <linux/pfn.h>
  #include <linux/scatterlist.h>
-#include <linux/swiotlb.h>
+#include <linux/dma-mapping.h>
  #include "i915_gem.h"
@@ -127,19 +127,10 @@ static inline unsigned int i915_sg_dma_sizes(struct scatterlist *sg)
      return page_sizes;
  }
-static inline unsigned int i915_sg_segment_size(void)
+static inline unsigned int i915_sg_segment_size(struct device *dev)
  {
-    unsigned int size = swiotlb_max_segment();
-
-    if (size == 0)
-        size = UINT_MAX;
-
-    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;
+    size_t max = min_t(size_t, UINT_MAX, dma_max_mapping_size(dev));

Hope your patience is not growing thin but blank line here as per checkpatch. I could fudge when applying though.

no worries, I'll issue a new version shortly.
I only recently guided a mentee to "always do your checkpatch, even if only doing a quick 1 character change, you never know if it you make a small mistake"
I guess do as I say, not as I do applies here :)


Otherwise it finally passed CI, yay! Now do we dig deeper to a) understand how/where it failed and b) check what are the possible return values from dma_max_mapping_size(), or call it done?

I will have to call it done myself as I am unable to repo it locally.
I could spend a lot of time debugging it via trybot, but I expect it will be easier, quicker and cheaper to get someone with local hw with a repro case to debug it if you want to root cause it.


Regards,

Tvrtko

+    return round_down(max, PAGE_SIZE);
  }
  bool i915_sg_trim(struct sg_table *orig_st);



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