[PATCH] drm/i915: stop using swiotlb

[PATCH] drm/i915: stop using swiotlb

[Robert Beckett]

Calling swiotlb functions directly is nowadays considered harmful. See
https://lore.kernel.org/intel-gfx/20220711082614.GA29487@lst.de/

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@lst.de>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Thomas Hellstrom <thomas.hellstrom@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
---
 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;
-
-	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);
 
 /**
-- 
2.25.1

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

[Tvrtko Ursulin]

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@lst.de/
> 
> 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.

This matches my understanding as well. And thanks for writing the patch 
up, I actually copied you to comment on the timeline of code removal 
only and did not expect you'd take it on fully. Thanks!

Christoph - ack from you? Also, if we merge it via the normal process it 
will hit 5.21 only. Does that work for you?

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko

> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> Cc: Thomas Hellstrom <thomas.hellstrom@intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> 
> Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
> ---
>   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;
> -
> -	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);
>   
>   /**

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

[Tvrtko Ursulin]

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@lst.de/
> 
> 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@lst.de>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> Cc: Thomas Hellstrom <thomas.hellstrom@intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> 
> Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
> ---
>   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@clear.html

> -
> -	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);
>   
>   /**

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

[Christoph Hellwig]

On Thu, Jul 21, 2022 at 06:43:07PM +0100, Robert Beckett wrote:
> +	max_segment = dma_max_mapping_size(i915->drm.dev);
> +	max_segment = max_t(size_t, max_segment, PAGE_SIZE) >> PAGE_SHIFT;

dma_max_mapping_size is always larger than PAGE_SIZE, so I don't think
the max is needed.

Otherwise this loks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

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

[Christoph Hellwig]

On Fri, Jul 22, 2022 at 10:45:54AM +0100, Tvrtko Ursulin wrote:
>> -	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.

As dma_max_mapping_size retuns a size_t it would be good to make
all variables using it a size_t as well.  In places where that gets
lower to an unsigned int your probably want this cap.

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

[Christoph Hellwig]

On Fri, Jul 22, 2022 at 10:13:48AM +0100, Tvrtko Ursulin wrote:
> Christoph - ack from you? Also, if we merge it via the normal process it 
> will hit 5.21 only. Does that work for you?

While I'd like to kill off swiotlb_max_segment rather sooner than
later I'm fine with following the normal process.  But I don't think
there will be a 5.21 with the recent Linux versioning practice :)

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

[Tvrtko Ursulin]

On 22/07/2022 10:58, Christoph Hellwig wrote:
> On Fri, Jul 22, 2022 at 10:45:54AM +0100, Tvrtko Ursulin wrote:
>>> -	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.
> 
> As dma_max_mapping_size retuns a size_t it would be good to make
> all variables using it a size_t as well.  In places where that gets
> lower to an unsigned int your probably want this cap.

Yep. Problem we have is struct scatterlist length field, which is 
unsigned int, since all our backing storage handling is built on top of 
it. Therefore I think capping in this helper should be good.

Regards,

Tvrtko