[Intel-gfx] [PATCH] drm/i915: Use the existing pages when retrying to DMA map

[Chris Wilson]

On Tue, Dec 20, 2016 at 01:42:47PM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> Rather than freeing and re-allocating the pages when DMA mapping
> in large chunks fails, we can just rebuild the sg table with no
> coalescing.

You are freeing and reallocating the pages - I thought you meant the
sg_table. The cost for recovering the shmemfs after allocation/swapin
already performed in the first pass should be a look up in the radix tree.
Not worrisome.
 
> Also change back the page counter to unsigned int because that
> is what the sg API supports.

Rather go the other way and start reporting an error when we overflow.
Being strict all page_counts should be u64. :|

> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> ---
> Only compile tested!
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 40 ++++++++++++++++++++++++++++++----------
>  1 file changed, 30 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 5275f6248ce3..e73f9f5a5d23 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2340,12 +2340,36 @@ static void i915_sg_trim(struct sg_table *orig_st)
>  	*orig_st = new_st;
>  }
>  
> +static void i915_sg_uncoalesce(struct sg_table *orig_st, unsigned long nents)
> +{
> +	struct sg_table new_st;
> +	struct scatterlist *new_sg;
> +	struct sgt_iter sgt_iter;
> +	struct page *page;
> +
> +	if (sg_alloc_table(&new_st, nents, GFP_KERNEL))
> +		return;

I was hoping for a way to use the pages already allocated. Feels
possible, just reversing the chain and walking backwards will be fun.

> +	new_sg = new_st.sgl;
> +	for_each_sgt_page(page, sgt_iter, orig_st) {
> +		sg_set_page(new_sg, page, PAGE_SIZE, 0);
> +		/* called before being DMA mapped, no need to copy sg->dma_* */
> +		new_sg = sg_next(new_sg);
> +	}
> +
> +	GEM_BUG_ON(new_sg); /* Should walk exactly nents and hit the end */
> +
> +	sg_free_table(orig_st);
> +
> +	*orig_st = new_st;
> +}

-- 
Chris Wilson, Intel Open Source Technology Centre