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

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