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    SubjectRe: [PATCH v4 02/38] scatterlist: add generic wrappers for iterating over sgtable objects
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    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@arm.com>

    > Signed-off-by: Marek Szyprowski <m.szyprowski@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
    > *
    >

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