[PATCH v5 03/38] iommu: add generic helper for mapping sgtable objects

[Marek Szyprowski]

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 mapping functions with a wrong number of entries.

To avoid such issues, lets introduce a common wrapper operating directly
on the struct sg_table objects, which take care of the proper use of
the nents and orig_nents entries.

Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Acked-by: Joerg Roedel <jroedel@xxxxxxx>
Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx> 
---
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/
---
 include/linux/iommu.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 7cfd2dd..5bed18a 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -478,6 +478,22 @@ extern size_t iommu_map_sg_atomic(struct iommu_domain *domain,
 extern void iommu_set_fault_handler(struct iommu_domain *domain,
 			iommu_fault_handler_t handler, void *token);
 
+/**
+ * iommu_map_sgtable - Map the given buffer to the IOMMU domain
+ * @domain:	The IOMMU domain to perform the mapping
+ * @iova:	The start address to map the buffer
+ * @sgt:	The sg_table object describing the buffer
+ * @prot:	IOMMU protection bits
+ *
+ * Creates a mapping at @iova for the buffer described by a scatterlist
+ * stored in the given sg_table object in the provided IOMMU domain.
+ */
+static inline size_t iommu_map_sgtable(struct iommu_domain *domain,
+			unsigned long iova, struct sg_table *sgt, int prot)
+{
+	return iommu_map_sg(domain, iova, sgt->sgl, sgt->orig_nents, prot);
+}
+
 extern void iommu_get_resv_regions(struct device *dev, struct list_head *list);
 extern void iommu_put_resv_regions(struct device *dev, struct list_head *list);
 extern void generic_iommu_put_resv_regions(struct device *dev,
-- 
1.9.1