[Intel-gfx] [PATCH 1/2] drm/i915: Fallback to single PAGE_SIZE segments for DMA remapping

[Tvrtko Ursulin]

On 19/12/2016 12:43, Chris Wilson wrote:
> If we at first do not succeed with attempting to remap our physical
> pages using a coalesced scattergather list, try again with one
> scattergather entry per page. This should help with swiotlb as it uses a
> limited buffer size and only searches for contiguous chunks within its
> buffer aligned up to the next boundary - i.e. we may prematurely cause a
> failure as we are unable to utilize the unused space between large
> chunks and trigger an error such as:
>
> 	 i915 0000:00:02.0: swiotlb buffer is full (sz: 1630208 bytes)
>
> Reported-by: Juergen Gross <jgross at suse.com>
> Fixes: 871dfbd67d4e ("drm/i915: Allow compaction upto SWIOTLB max segment size")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Imre Deak <imre.deak at intel.com>
> Cc: <drm-intel-fixes at lists.freedesktop.org>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 26 ++++++++++++++++++++++----
>  1 file changed, 22 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 412f3513f269..4e263df2afc3 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2326,7 +2326,8 @@ static struct sg_table *
>  i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
>  {
>  	struct drm_i915_private *dev_priv = to_i915(obj->base.dev);
> -	int page_count, i;
> +	const unsigned long page_count = obj->base.size / PAGE_SIZE;
> +	unsigned long i;
>  	struct address_space *mapping;
>  	struct sg_table *st;
>  	struct scatterlist *sg;
> @@ -2352,7 +2353,7 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
>  	if (st == NULL)
>  		return ERR_PTR(-ENOMEM);
>
> -	page_count = obj->base.size / PAGE_SIZE;
> +rebuild_st:
>  	if (sg_alloc_table(st, page_count, GFP_KERNEL)) {
>  		kfree(st);
>  		return ERR_PTR(-ENOMEM);
> @@ -2411,8 +2412,25 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
>  	i915_sg_trim(st);
>
>  	ret = i915_gem_gtt_prepare_pages(obj, st);
> -	if (ret)
> -		goto err_pages;
> +	if (ret) {
> +		/* DMA remapping failed? One possible cause is that
> +		 * it could not reserve enough large entries, asking
> +		 * for PAGE_SIZE chunks instead may be helpful.
> +		 */
> +		if (max_segment > PAGE_SIZE) {
> +			for_each_sgt_page(page, sgt_iter, st)
> +				put_page(page);
> +			sg_free_table(st);
> +
> +			max_segment = PAGE_SIZE;
> +			goto rebuild_st;
> +		} else {
> +			dev_warn(&dev_priv->drm.pdev->dev,
> +				 "Failed to DMA remap %lu pages\n",
> +				 page_count);
> +			goto err_pages;
> +		}
> +	}
>
>  	if (i915_gem_object_needs_bit17_swizzle(obj))
>  		i915_gem_object_do_bit_17_swizzle(obj, st);
>

How much is the cost of freeing and re-acquiring pages in the fall back 
case? It could be avoidable by using the table and adding something like 
sgt = i915_sg_copy(sgt, table_max_segment). But it depends on how likely 
is this path to be hit on swiotlb platforms. I have no idea. Our 
datasets are much bigger than the swiotlb space - if that is true on 
such platforms?

Regards,

Tvrtko