On Mon, Oct 10, 2016 at 11:27:00PM +0100, Chris Wilson wrote:
> commit 1625e7e549c5 ("drm/i915: make compact dma scatter lists creation
> work with SWIOTLB backend") took a heavy handed approach to undo the
> scatterlist compaction in the face of SWIOTLB. (The compaction hit a bug
> whereby we tried to pass a segment larger than SWIOTLB could handle.) We
> can be a little more intelligent and try compacting the scatterlist up
> to the maximum SWIOTLB segment size (when using SWIOTLB).
> 

Won't this cause a bigger usage of the SWIOTLB bounce buffer ?

> v2: Tidy sg_mark_end() and cpp
> 
> Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> CC: Imre Deak <imre.d...@intel.com>
> CC: Daniel Vetter <daniel.vet...@ffwll.ch>
> Cc: Konrad Rzeszutek Wilk <konrad.w...@oracle.com>
> Cc: Tvrtko Ursulin <tvrtko.ursu...@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 30 ++++++++++++++++++------------
>  1 file changed, 18 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index dff8d05d80ee..50fd611926cb 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2201,6 +2201,15 @@ unlock:
>       mutex_unlock(&obj->mm.lock);
>  }
>  
> +static unsigned long swiotlb_max_size(void)
> +{
> +#if IS_ENABLED(CONFIG_SWIOTLB)
> +     return swiotlb_nr_tbl() << IO_TLB_SHIFT;
> +#else
> +     return 0;
> +#endif
> +}
> +
>  static struct sg_table *
>  i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
>  {
> @@ -2212,6 +2221,7 @@ i915_gem_object_get_pages_gtt(struct 
> drm_i915_gem_object *obj)
>       struct sgt_iter sgt_iter;
>       struct page *page;
>       unsigned long last_pfn = 0;     /* suppress gcc warning */
> +     unsigned long max_segment;
>       int ret;
>       gfp_t gfp;
>  
> @@ -2222,6 +2232,10 @@ i915_gem_object_get_pages_gtt(struct 
> drm_i915_gem_object *obj)
>       GEM_BUG_ON(obj->base.read_domains & I915_GEM_GPU_DOMAINS);
>       GEM_BUG_ON(obj->base.write_domain & I915_GEM_GPU_DOMAINS);
>  
> +     max_segment = swiotlb_max_size();
> +     if (!max_segment)
> +             max_segment = obj->base.size;
> +
>       st = kmalloc(sizeof(*st), GFP_KERNEL);
>       if (st == NULL)
>               return ERR_PTR(-ENOMEM);
> @@ -2263,15 +2277,9 @@ i915_gem_object_get_pages_gtt(struct 
> drm_i915_gem_object *obj)
>                               goto err_pages;
>                       }
>               }
> -#ifdef CONFIG_SWIOTLB
> -             if (swiotlb_nr_tbl()) {
> -                     st->nents++;
> -                     sg_set_page(sg, page, PAGE_SIZE, 0);
> -                     sg = sg_next(sg);
> -                     continue;
> -             }
> -#endif
> -             if (!i || page_to_pfn(page) != last_pfn + 1) {
> +             if (!i ||
> +                 sg->length >= max_segment ||
> +                 page_to_pfn(page) != last_pfn + 1) {
>                       if (i)
>                               sg = sg_next(sg);
>                       st->nents++;
> @@ -2284,9 +2292,7 @@ i915_gem_object_get_pages_gtt(struct 
> drm_i915_gem_object *obj)
>               /* Check that the i965g/gm workaround works. */
>               WARN_ON((gfp & __GFP_DMA32) && (last_pfn >= 0x00100000UL));
>       }
> -#ifdef CONFIG_SWIOTLB
> -     if (!swiotlb_nr_tbl())
> -#endif
> +     if (sg) /* loop terminated early; short sg table */
>               sg_mark_end(sg);
>  
>       ret = i915_gem_gtt_prepare_pages(obj, st);
> -- 
> 2.9.3
> 
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