Re: [PATCH v6 01/36] drm: prime: add common helper to check scatterlist contiguity

[Robin Murphy <robin.murphy@arm.com>]

On 2020-06-18 16:39, Marek Szyprowski wrote:
> It is a common operation done by DRM drivers to check the contiguity
> of the DMA-mapped buffer described by a scatterlist in the
> sg_table object. Let's add a common helper for this operation.
> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> ---
>   drivers/gpu/drm/drm_gem_cma_helper.c | 23 +++--------------------
>   drivers/gpu/drm/drm_prime.c          | 26 ++++++++++++++++++++++++++
>   include/drm/drm_prime.h              |  2 ++
>   3 files changed, 31 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
> index 06a5b9ee1fe0..41566a15dabd 100644
> --- a/drivers/gpu/drm/drm_gem_cma_helper.c
> +++ b/drivers/gpu/drm/drm_gem_cma_helper.c
> @@ -471,26 +471,9 @@ drm_gem_cma_prime_import_sg_table(struct drm_device *dev,
>   {
>   	struct drm_gem_cma_object *cma_obj;
>   
> -	if (sgt->nents != 1) {
> -		/* check if the entries in the sg_table are contiguous */
> -		dma_addr_t next_addr = sg_dma_address(sgt->sgl);
> -		struct scatterlist *s;
> -		unsigned int i;
> -
> -		for_each_sg(sgt->sgl, s, sgt->nents, i) {
> -			/*
> -			 * sg_dma_address(s) is only valid for entries
> -			 * that have sg_dma_len(s) != 0
> -			 */
> -			if (!sg_dma_len(s))
> -				continue;
> -
> -			if (sg_dma_address(s) != next_addr)
> -				return ERR_PTR(-EINVAL);
> -
> -			next_addr = sg_dma_address(s) + sg_dma_len(s);
> -		}
> -	}
> +	/* check if the entries in the sg_table are contiguous */
> +	if (drm_prime_get_contiguous_size(sgt) < attach->dmabuf->size)
> +		return ERR_PTR(-EINVAL);
>   
>   	/* Create a CMA GEM buffer. */
>   	cma_obj = __drm_gem_cma_create(dev, attach->dmabuf->size);
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index bbfc713bfdc3..0784969894c1 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -825,6 +825,32 @@ struct sg_table *drm_prime_pages_to_sg(struct page **pages, unsigned int nr_page
>   }
>   EXPORT_SYMBOL(drm_prime_pages_to_sg);
>   
> +/**
> + * drm_prime_get_contiguous_size - returns the contiguous size of the buffer
> + * @sgt: sg_table describing the buffer to check
> + *
> + * This helper calculates the contiguous size in the DMA address space
> + * of the the buffer described by the provided sg_table.
> + *
> + * This is useful for implementing
> + * &drm_gem_object_funcs.gem_prime_import_sg_table.
> + */
> +unsigned long drm_prime_get_contiguous_size(struct sg_table *sgt)
> +{
> +	dma_addr_t expected = sg_dma_address(sgt->sgl);
> +	struct sg_dma_page_iter dma_iter;
> +	unsigned long size = 0;
> +
> +	for_each_sgtable_dma_page(sgt, &dma_iter, 0) {
> +		if (sg_page_iter_dma_address(&dma_iter) != expected)
> +			break;
> +		expected += PAGE_SIZE;
> +		size += PAGE_SIZE;

Hmm, in many cases this is likely to be far less efficient than simply 
using for_each_sgtable_dma() and sg_dma_len() equivalent to the original 
implementation, and there doesn't seem to be any good reason for that. 
Plus AFAICS it could potentially let false-positives through if someone 
were to pass in a table with non-page-aligned lengths (I assume that's 
expected never to happen, but still...)

Robin.

> +	}
> +	return size;
> +}
> +EXPORT_SYMBOL(drm_prime_get_contiguous_size);
> +
>   /**
>    * drm_gem_prime_export - helper library implementation of the export callback
>    * @obj: GEM object to export
> diff --git a/include/drm/drm_prime.h b/include/drm/drm_prime.h
> index 9af7422b44cf..47ef11614627 100644
> --- a/include/drm/drm_prime.h
> +++ b/include/drm/drm_prime.h
> @@ -92,6 +92,8 @@ struct sg_table *drm_prime_pages_to_sg(struct page **pages, unsigned int nr_page
>   struct dma_buf *drm_gem_prime_export(struct drm_gem_object *obj,
>   				     int flags);
>   
> +unsigned long drm_prime_get_contiguous_size(struct sg_table *sgt);
> +
>   /* helper functions for importing */
>   struct drm_gem_object *drm_gem_prime_import_dev(struct drm_device *dev,
>   						struct dma_buf *dma_buf,
> 
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