[PATCH] drm/i915: stop abusing swiotlb_max_segment

[Tvrtko Ursulin]

On 20/10/2022 12:03, Christoph Hellwig wrote:
> From: Robert Beckett <bob.beckett at collabora.com>
> 
> swiotlb_max_segment used to return either the maximum size that swiotlb
> could bounce, or for Xen PV PAGE_SIZE even if swiotlb could bounce buffer
> larger mappings.  This made i915 on Xen PV work as it bypasses the
> coherency aspect of the DMA API and can't cope with bounce buffering
> and this avoided bounce buffering for the Xen/PV case.
> 
> So instead of adding this hack back, check for Xen/PV directly in i915
> for the Xen case and otherwise use the proper DMA API helper to query
> the maximum mapping size.
> 
> 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.
> 
> Fixes: a2daa27c0c61 ("swiotlb: simplify swiotlb_max_segment")
> Reported-by: Marek Marczykowski-Górecki <marmarek at invisiblethingslab.com>
> Signed-off-by: Robert Beckett <bob.beckett at collabora.com>
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> [hch: added the Xen hack, rewrote the changelog]
> ---
>   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      | 34 ++++++++++++--------
>   5 files changed, 29 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 c698f95af15fe..629acb403a2c9 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, get_order(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 f42ca1179f373..11125c32dd35d 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 4f861782c3e85..a4aa9500fa179 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;
> @@ -538,7 +538,7 @@ 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 d4398948f0162..f34e01a7fefb9 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 9ddb3e743a3e5..b0a1db44f8950 100644
> --- a/drivers/gpu/drm/i915/i915_scatterlist.h
> +++ b/drivers/gpu/drm/i915/i915_scatterlist.h
> @@ -9,7 +9,8 @@
>   
>   #include <linux/pfn.h>
>   #include <linux/scatterlist.h>
> -#include <linux/swiotlb.h>
> +#include <linux/dma-mapping.h>
> +#include <xen/xen.h>
>   
>   #include "i915_gem.h"
>   
> @@ -127,19 +128,26 @@ 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));
> +
> +	/*
> +	 * For Xen PV guests pages aren't contiguous in DMA (machine) address
> +	 * space.  The DMA API takes care of that both in dma_alloc_* (by
> +	 * calling into the hypervisor to make the pages contiguous) and in
> +	 * dma_map_* (by bounce buffering).  But i915 abuses ignores the
> +	 * coherency aspects of the DMA API and thus can't cope with bounce
> +	 * buffering actually happening, so add a hack here to force small
> +	 * allocations and mappings when running in PV mode on Xen.
> +	 *
> +	 * Note this will still break if bounce buffering is required for other
> +	 * reasons, like confidential computing hypervisors or PCIe root ports
> +	 * with addressing limitations.
> +	 */

As a side note, I think this is something Thomas was pointing out in the 
previous round.

> +	if (xen_pv_domain())
> +		max = PAGE_SIZE;

I wonder if it would be cleaner to just fork XEN vs rest in a single 
conditional, since all the other computations are pointless in the 
native case. I mean like:

if (xen)
   return PAGE_SIZE
else
   max = min_t(..dma_max_mapping_size..
   return round_down..

I do see the XEN branch will compile out anyway on !CONFIG_XEN so it 
only really matters there - up to you.

Most importantly for me, this time round CI did not report any strange 
failures which were previously interfering with this patch. So it could 
really be it was bugs in the Intel IOMMU driver which were causing the 
problems back then, as Bob has been suspecting.

Therefore I am happy to merge this as is. I'll just queue up one more 
test run just in case. I'll let you know when done, and assuming the 
best - thank you both for the cleanup!

Regards,

Tvrtko

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