Re: [PATCH v5 05/38] drm: prime: use sgtable iterators in drm_prime_sg_to_page_addr_arrays()

[Alex Goins]

Tested-by: Alex Goins <agoins@xxxxxxxxxx>

This change fixes a regression with drm_prime_sg_to_page_addr_arrays() and
AMDGPU in v5.9.

Commit 39913934 similarly revamped AMDGPU to use sgtable helper functions. When
it changed from dma_map_sg_attrs() to dma_map_sgtable(), as a side effect it
started correctly updating sgt->nents to the return value of dma_map_sg_attrs().
However, drm_prime_sg_to_page_addr_arrays() incorrectly uses sgt->nents to
iterate over pages, rather than sgt->orig_nents, resulting in it now returning
the incorrect number of pages on AMDGPU.

I had written a patch that changes drm_prime_sg_to_page_addr_arrays() to use
for_each_sgtable_sg() instead of for_each_sg(), iterating using sgt->orig_nents:

-       for_each_sg(sgt->sgl, sg, sgt->nents, count) {
+       for_each_sgtable_sg(sgt, sg, count) {

This patch takes it further, but still has the effect of fixing the number of
pages that drm_prime_sg_to_page_addr_arrays() returns. Something like this
should be included in v5.9 to prevent a regression with AMDGPU.

Thanks,
Alex

On Wed, 13 May 2020, Marek Szyprowski wrote:

> Replace the current hand-crafted code for extracting pages and DMA
> addresses from the given scatterlist by the much more robust
> code based on the generic scatterlist iterators and recently
> introduced sg_table-based wrappers. The resulting code is simple and
> easy to understand, so the comment describing the old code is no
> longer needed.
> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> ---
> For more information, see '[PATCH v5 00/38] DRM: fix struct sg_table nents
> vs. orig_nents misuse' thread:
> https://lore.kernel.org/linux-iommu/20200513132114.6046-1-m.szyprowski@xxxxxxxxxxx/T/
> ---
>  drivers/gpu/drm/drm_prime.c | 47 ++++++++++++++-------------------------------
>  1 file changed, 14 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 1d2e5fe..dfdf4d4 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -985,45 +985,26 @@ struct drm_gem_object *drm_gem_prime_import(struct drm_device *dev,
>  int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page **pages,
>  				     dma_addr_t *addrs, int max_entries)
>  {
> -	unsigned count;
> -	struct scatterlist *sg;
> -	struct page *page;
> -	u32 page_len, page_index;
> -	dma_addr_t addr;
> -	u32 dma_len, dma_index;
> +	struct sg_dma_page_iter dma_iter;
> +	struct sg_page_iter page_iter;
> +	struct page **p = pages;
> +	dma_addr_t *a = addrs;
>  
> -	/*
> -	 * Scatterlist elements contains both pages and DMA addresses, but
> -	 * one shoud not assume 1:1 relation between them. The sg->length is
> -	 * the size of the physical memory chunk described by the sg->page,
> -	 * while sg_dma_len(sg) is the size of the DMA (IO virtual) chunk
> -	 * described by the sg_dma_address(sg).
> -	 */
> -	page_index = 0;
> -	dma_index = 0;
> -	for_each_sg(sgt->sgl, sg, sgt->nents, count) {
> -		page_len = sg->length;
> -		page = sg_page(sg);
> -		dma_len = sg_dma_len(sg);
> -		addr = sg_dma_address(sg);
> -
> -		while (pages && page_len > 0) {
> -			if (WARN_ON(page_index >= max_entries))
> +	if (pages) {
> +		for_each_sgtable_page(sgt, &page_iter, 0) {
> +			if (p - pages >= max_entries)
>  				return -1;
> -			pages[page_index] = page;
> -			page++;
> -			page_len -= PAGE_SIZE;
> -			page_index++;
> +			*p++ = sg_page_iter_page(&page_iter);
>  		}
> -		while (addrs && dma_len > 0) {
> -			if (WARN_ON(dma_index >= max_entries))
> +	}
> +	if (addrs) {
> +		for_each_sgtable_dma_page(sgt, &dma_iter, 0) {
> +			if (a - addrs >= max_entries)
>  				return -1;
> -			addrs[dma_index] = addr;
> -			addr += PAGE_SIZE;
> -			dma_len -= PAGE_SIZE;
> -			dma_index++;
> +			*a++ = sg_page_iter_dma_address(&dma_iter);
>  		}
>  	}
> +
>  	return 0;
>  }
>  EXPORT_SYMBOL(drm_prime_sg_to_page_addr_arrays);
> -- 
> 1.9.1
> 
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