[RFC] add a struct page* parameter to dma_map_ops.unmap_page

[Stefano Stabellini]

Hi all,
I am writing this email to ask for your advice.

On architectures where dma addresses are different from physical
addresses, it can be difficult to retrieve the physical address of a
page from its dma address.

Specifically this is the case for Xen on arm and arm64 but I think that
other architectures might have the same issue.

Knowing the physical address is necessary to be able to issue any
required cache maintenance operations when unmap_page,
sync_single_for_cpu and sync_single_for_device are called.

Adding a struct page* parameter to unmap_page, sync_single_for_cpu and
sync_single_for_device would make Linux dma handling on Xen on arm and
arm64 much easier and quicker.

I think that other drivers have similar problems, such as the Intel
IOMMU driver having to call find_iova and walking down an rbtree to get
the physical address in its implementation of unmap_page.

Callers have the struct page* in their hands already from the previous
map_page call so it shouldn't be an issue for them.  A problem does
exist however: there are about 280 callers of dma_unmap_page and
pci_unmap_page. We have even more callers of the dma_sync_single_for_*
functions.



Is such a change even conceivable? How would one go about it?

I think that Xen would not be the only one to gain from it, but I would
like to have a confirmation from others: given the magnitude of the
changes involved I would actually prefer to avoid them unless multiple
drivers/archs/subsystems could really benefit from them.

Cheers,

Stefano


diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index d5d3881..158a765 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -31,8 +31,9 @@ struct dma_map_ops {
 			       unsigned long offset, size_t size,
 			       enum dma_data_direction dir,
 			       struct dma_attrs *attrs);
-	void (*unmap_page)(struct device *dev, dma_addr_t dma_handle,
-			   size_t size, enum dma_data_direction dir,
+	void (*unmap_page)(struct device *dev, struct page *page,
+			   dma_addr_t dma_handle, size_t size,
+			   enum dma_data_direction dir,
 			   struct dma_attrs *attrs);
 	int (*map_sg)(struct device *dev, struct scatterlist *sg,
 		      int nents, enum dma_data_direction dir,
@@ -41,10 +42,10 @@ struct dma_map_ops {
 			 struct scatterlist *sg, int nents,
 			 enum dma_data_direction dir,
 			 struct dma_attrs *attrs);
-	void (*sync_single_for_cpu)(struct device *dev,
+	void (*sync_single_for_cpu)(struct device *dev, struct page *page,
 				    dma_addr_t dma_handle, size_t size,
 				    enum dma_data_direction dir);
-	void (*sync_single_for_device)(struct device *dev,
+	void (*sync_single_for_device)(struct device *dev, struct page *page,
 				       dma_addr_t dma_handle, size_t size,
 				       enum dma_data_direction dir);
 	void (*sync_sg_for_cpu)(struct device *dev,