Hi Will,

On 7/28/2014 12:11 PM, Will Deacon wrote:

Hi Olav,

On Mon, Jul 28, 2014 at 07:38:51PM +0100, Olav Haugan wrote:

quoted

Mapping and unmapping are more often than not in the critical path.
map_sg and unmap_sg allows IOMMU driver implementations to optimize
the process of mapping and unmapping buffers into the IOMMU page tables.

Instead of mapping a buffer one page at a time and requiring potentially
expensive TLB operations for each page, this function allows the driver
to map all pages in one go and defer TLB maintenance until after all
pages have been mapped.

Additionally, the mapping operation would be faster in general since
clients does not have to keep calling map API over and over again for
each physically contiguous chunk of memory that needs to be mapped to a
virtually contiguous region.

Signed-off-by: Olav Haugan <redacted>
---
 drivers/iommu/iommu.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/iommu.h | 28 ++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 1698360..cd65511 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c

@@ -1088,6 +1088,54 @@ size_t iommu_unmap(struct iommu_domain *domain, unsigned long iova, size_t size)
 }
 EXPORT_SYMBOL_GPL(iommu_unmap);
 
+int iommu_map_sg(struct iommu_domain *domain, unsigned long iova,
+			struct scatterlist *sg, unsigned int nents,
+			int prot, unsigned long flags)
+{
+	int ret = 0;
+	unsigned long offset = 0;
+
+	BUG_ON(iova & (~PAGE_MASK));
+
+	if (unlikely(domain->ops->map_sg == NULL)) {
+		unsigned int i;
+		struct scatterlist *s;
+
+		for_each_sg(sg, s, nents, i) {
+			phys_addr_t phys = page_to_phys(sg_page(s));
+			u32 page_len = PAGE_ALIGN(s->offset + s->length);

Hmm, this is a pretty horrible place where CPU page size (from the sg list)
meets the IOMMU and I think we need to do something better to avoid spurious
failures. In other words, the sg list should be iterated in such a way that
we always pass a multiple of a supported iommu page size to iommu_map.

All the code using PAGE_MASK and PAGE_ALIGN needn't match what is supported
by the IOMMU hardware.

I am not sure what you mean. How can we iterate over the sg list in a
different way to ensure we pass a multiple of a supported iommu page
size? Each entry in the sg list are physically discontinuous from each
other. If the page is too big iommu_map will take care of it for us. It
already finds the biggest supported page size and splits up the calls to
domain->ops->map(). Also, whoever allocates memory for use by IOMMU
needs to be aware of what the supported minimum size is or else they
would get mapping failures anyway.

(The code in __map_sg_chunk in arch/arm/mm/dma-mapping.c does the same
thing btw.)

Thanks,

Olav

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