[PATCH v4 02/38] scatterlist: add generic wrappers for iterating over sgtable objects

[Robin Murphy]

On 2020-05-12 10:00 am, Marek Szyprowski wrote:
> struct sg_table is a common structure used for describing a memory
> buffer. It consists of a scatterlist with memory pages and DMA addresses
> (sgl entry), as well as the number of scatterlist entries: CPU pages
> (orig_nents entry) and DMA mapped pages (nents entry).
> 
> It turned out that it was a common mistake to misuse nents and orig_nents
> entries, calling the scatterlist iterating functions with a wrong number
> of the entries.
> 
> To avoid such issues, lets introduce a common wrappers operating directly
> on the struct sg_table objects, which take care of the proper use of
> the nents and orig_nents entries.
> 
> While touching this, lets clarify some ambiguities in the comments for
> the existing for_each helpers.

Reviewed-by: Robin Murphy <robin.murphy at arm.com>

> Signed-off-by: Marek Szyprowski <m.szyprowski at samsung.com>
> ---
> For more information, see '[PATCH v4 00/38] DRM: fix struct sg_table nents
> vs. orig_nents misuse' thread:
> https://lore.kernel.org/dri-devel/20200512085710.14688-1-m.szyprowski@samsung.com/T/
> ---
>   include/linux/scatterlist.h | 50 ++++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 47 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 6eec50f..4f922af 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -151,6 +151,20 @@ static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
>   #define for_each_sg(sglist, sg, nr, __i)	\
>   	for (__i = 0, sg = (sglist); __i < (nr); __i++, sg = sg_next(sg))
>   
> +/*
> + * Loop over each sg element in the given sg_table object.
> + */
> +#define for_each_sgtable_sg(sgt, sg, i)		\
> +	for_each_sg(sgt->sgl, sg, sgt->orig_nents, i)
> +
> +/*
> + * Loop over each sg element in the given *DMA mapped* sg_table object.
> + * Please use sg_dma_address(sg) and sg_dma_len(sg) to extract DMA addresses
> + * of the each element.
> + */
> +#define for_each_sgtable_dma_sg(sgt, sg, i)	\
> +	for_each_sg(sgt->sgl, sg, sgt->nents, i)
> +
>   /**
>    * sg_chain - Chain two sglists together
>    * @prv:	First scatterlist
> @@ -401,9 +415,10 @@ static inline struct page *sg_page_iter_page(struct sg_page_iter *piter)
>    * @sglist:	sglist to iterate over
>    * @piter:	page iterator to hold current page, sg, sg_pgoffset
>    * @nents:	maximum number of sg entries to iterate over
> - * @pgoffset:	starting page offset
> + * @pgoffset:	starting page offset (in pages)
>    *
>    * Callers may use sg_page_iter_page() to get each page pointer.
> + * In each loop it operates on PAGE_SIZE unit.
>    */
>   #define for_each_sg_page(sglist, piter, nents, pgoffset)		   \
>   	for (__sg_page_iter_start((piter), (sglist), (nents), (pgoffset)); \
> @@ -412,18 +427,47 @@ static inline struct page *sg_page_iter_page(struct sg_page_iter *piter)
>   /**
>    * for_each_sg_dma_page - iterate over the pages of the given sg list
>    * @sglist:	sglist to iterate over
> - * @dma_iter:	page iterator to hold current page
> + * @dma_iter:	DMA page iterator to hold current page
>    * @dma_nents:	maximum number of sg entries to iterate over, this is the value
>    *              returned from dma_map_sg
> - * @pgoffset:	starting page offset
> + * @pgoffset:	starting page offset (in pages)
>    *
>    * Callers may use sg_page_iter_dma_address() to get each page's DMA address.
> + * In each loop it operates on PAGE_SIZE unit.
>    */
>   #define for_each_sg_dma_page(sglist, dma_iter, dma_nents, pgoffset)            \
>   	for (__sg_page_iter_start(&(dma_iter)->base, sglist, dma_nents,        \
>   				  pgoffset);                                   \
>   	     __sg_page_iter_dma_next(dma_iter);)
>   
> +/**
> + * for_each_sgtable_page - iterate over all pages in the sg_table object
> + * @sgt:	sg_table object to iterate over
> + * @piter:	page iterator to hold current page
> + * @pgoffset:	starting page offset (in pages)
> + *
> + * Iterates over the all memory pages in the buffer described by
> + * a scatterlist stored in the given sg_table object.
> + * See also for_each_sg_page(). In each loop it operates on PAGE_SIZE unit.
> + */
> +#define for_each_sgtable_page(sgt, piter, pgoffset)	\
> +	for_each_sg_page(sgt->sgl, piter, sgt->orig_nents, pgoffset)
> +
> +/**
> + * for_each_sgtable_dma_page - iterate over the DMA mapped sg_table object
> + * @sgt:	sg_table object to iterate over
> + * @dma_iter:	DMA page iterator to hold current page
> + * @pgoffset:	starting page offset (in pages)
> + *
> + * Iterates over the all DMA mapped pages in the buffer described by
> + * a scatterlist stored in the given sg_table object.
> + * See also for_each_sg_dma_page(). In each loop it operates on PAGE_SIZE
> + * unit.
> + */
> +#define for_each_sgtable_dma_page(sgt, dma_iter, pgoffset)	\
> +	for_each_sg_dma_page(sgt->sgl, dma_iter, sgt->nents, pgoffset)
> +
> +
>   /*
>    * Mapping sg iterator
>    *
>