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@xxxxxxx>
Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
---
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@xxxxxxxxxxx/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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