[PATCH] drm: modify pages_to_sg prime helper to create optimized SG table

[Aaron Plattner]

On 01/28/2013 05:38 AM, Rahul Sharma wrote:
> It fixes the issue arises due to passing 'nr_pages' in place of 'nents' to
> sg_alloc_table. When ARM_HAS_SG_CHAIN is disabled, it is causing failure in
> creating SG table for the buffers having more than 204 physical pages i.e.
> equal to SG_MAX_SINGLE_ALLOC.
>
> When using sg_alloc_table_from_pages interface, in place of sg_alloc_table,
> page list will be passes to get each contiguous section which is represented
> by a single entry in the table. For a Contiguous Buffer, number of entries
> should be equal to 1.
>
> Following check is causing the failure which is not applicable for Non-Contig
> buffers:
>
> 	if (WARN_ON_ONCE(nents > max_ents))
> 		return -EINVAL;
>
> Above patch is well tested for EXYNOS4 and EXYNOS5 for with/wihtout IOMMU
> supprot. NOUVEAU and RADEON platforms also depends on drm_prime_pages_to_sg
> helper function.
>
> This set is base on "exynos-drm-fixes" branch at
> http://git.kernel.org/?p=linux/kernel/git/daeinki/drm-exynos.git
>
> Signed-off-by: Rahul Sharma <rahul.sharma at samsung.com>

Reviewed-by: Aaron Plattner <aplattner at nvidia.com>

I also verified that this reduces my 2025-entry sg_table to 6 entries, so

Tested-by: Aaron Plattner <aplattner at nvidia.com>

-- 
Aaron

> ---
>   drivers/gpu/drm/drm_prime.c | 8 ++------
>   1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 7f12573..072ee08 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -217,21 +217,17 @@ int drm_prime_fd_to_handle_ioctl(struct drm_device *dev, void *data,
>   struct sg_table *drm_prime_pages_to_sg(struct page **pages, int nr_pages)
>   {
>   	struct sg_table *sg = NULL;
> -	struct scatterlist *iter;
> -	int i;
>   	int ret;
>
>   	sg = kmalloc(sizeof(struct sg_table), GFP_KERNEL);
>   	if (!sg)
>   		goto out;
>
> -	ret = sg_alloc_table(sg, nr_pages, GFP_KERNEL);
> +	ret = sg_alloc_table_from_pages(sg, pages, nr_pages, 0,
> +				nr_pages << PAGE_SHIFT, GFP_KERNEL);
>   	if (ret)
>   		goto out;
>
> -	for_each_sg(sg->sgl, iter, nr_pages, i)
> -		sg_set_page(iter, pages[i], PAGE_SIZE, 0);
> -
>   	return sg;
>   out:
>   	kfree(sg);
>