Re: [Intel-gfx] [PATCH] drm: Remove SCATTERLIST_MAX_SEGMENT

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Oct 28, 2020 at 04:15:26PM -0300, Jason Gunthorpe wrote:
> Since commit 9a40401cfa13 ("lib/scatterlist: Do not limit max_segment to
> PAGE_ALIGNED values") the max_segment input to sg_alloc_table_from_pages()
> does not have to be any special value. The new algorithm will always
> create something less than what the user provides. Thus eliminate this
> confusing constant.
> 
> - vmwgfx should use the HW capability, not mix in the OS page size for
>   calling dma_set_max_seg_size()
> 
> - i915 uses i915_sg_segment_size() both for sg_alloc_table_from_pages
>   and for some open coded sgl construction. This doesn't change the value
>   since rounddown(size, UINT_MAX) == SCATTERLIST_MAX_SEGMENT
> 
> - drm_prime_pages_to_sg uses it as a default if max_segment is zero,
>   UINT_MAX is fine to use directly.
> 
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Thomas Hellstrom <thellstrom@vmware.com>
> Cc: Qian Cai <cai@lca.pw>
> Cc: "Ursulin, Tvrtko" <tvrtko.ursulin@intel.com>
> Suggested-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

lgtm. Do you want to push this through some other queue, or should I put
this into drm trees? Prefer 5.10 or 5.11?

If you want to merge this Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
-Daniel

> ---
>  drivers/gpu/drm/drm_prime.c             | 4 ++--
>  drivers/gpu/drm/i915/i915_scatterlist.h | 2 +-
>  drivers/gpu/drm/vmwgfx/vmwgfx_drv.c     | 3 +--
>  include/linux/scatterlist.h             | 6 ------
>  tools/testing/scatterlist/main.c        | 2 +-
>  5 files changed, 5 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index d6808f678db541..c3693e5e8b74b0 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -816,8 +816,8 @@ struct sg_table *drm_prime_pages_to_sg(struct drm_device *dev,
>  
>  	if (dev)
>  		max_segment = dma_max_mapping_size(dev->dev);
> -	if (max_segment == 0 || max_segment > SCATTERLIST_MAX_SEGMENT)
> -		max_segment = SCATTERLIST_MAX_SEGMENT;
> +	if (max_segment == 0)
> +		max_segment = UINT_MAX;
>  	sge = __sg_alloc_table_from_pages(sg, pages, nr_pages, 0,
>  					  nr_pages << PAGE_SHIFT,
>  					  max_segment,
> diff --git a/drivers/gpu/drm/i915/i915_scatterlist.h b/drivers/gpu/drm/i915/i915_scatterlist.h
> index b7b59328cb76ab..883dd8d09d6bf2 100644
> --- a/drivers/gpu/drm/i915/i915_scatterlist.h
> +++ b/drivers/gpu/drm/i915/i915_scatterlist.h
> @@ -112,7 +112,7 @@ static inline unsigned int i915_sg_segment_size(void)
>  	unsigned int size = swiotlb_max_segment();
>  
>  	if (size == 0)
> -		return SCATTERLIST_MAX_SEGMENT;
> +		size = UINT_MAX;
>  
>  	size = rounddown(size, PAGE_SIZE);
>  	/* swiotlb_max_segment_size can return 1 byte when it means one page. */
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
> index 31e3e5c9f36223..c1817f1a3006e0 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
> @@ -792,8 +792,7 @@ static int vmw_driver_load(struct drm_device *dev, unsigned long chipset)
>  	if (unlikely(ret != 0))
>  		goto out_err0;
>  
> -	dma_set_max_seg_size(dev->dev, min_t(unsigned int, U32_MAX & PAGE_MASK,
> -					     SCATTERLIST_MAX_SEGMENT));
> +	dma_set_max_seg_size(dev->dev, U32_MAX);
>  
>  	if (dev_priv->capabilities & SVGA_CAP_GMR2) {
>  		DRM_INFO("Max GMR ids is %u\n",
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 36c47e7e66a203..6f70572b2938be 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -18,12 +18,6 @@ struct scatterlist {
>  #endif
>  };
>  
> -/*
> - * Since the above length field is an unsigned int, below we define the maximum
> - * length in bytes that can be stored in one scatterlist entry.
> - */
> -#define SCATTERLIST_MAX_SEGMENT (UINT_MAX & PAGE_MASK)
> -
>  /*
>   * These macros should be used after a dma_map_sg call has been done
>   * to get bus addresses of each of the SG entries and their lengths.
> diff --git a/tools/testing/scatterlist/main.c b/tools/testing/scatterlist/main.c
> index b2c7e9f7b8d3dc..d264bf853034bd 100644
> --- a/tools/testing/scatterlist/main.c
> +++ b/tools/testing/scatterlist/main.c
> @@ -50,7 +50,7 @@ static void fail(struct test *test, struct sg_table *st, const char *cond)
>  
>  int main(void)
>  {
> -	const unsigned int sgmax = SCATTERLIST_MAX_SEGMENT;
> +	const unsigned int sgmax = UINT_MAX;
>  	struct test *test, tests[] = {
>  		{ -EINVAL, 1, pfn(0), PAGE_SIZE, PAGE_SIZE + 1, 1 },
>  		{ -EINVAL, 1, pfn(0), PAGE_SIZE, 0, 1 },
> -- 
> 2.28.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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