On Mon, 17 Nov 2014, Stefano Stabellini wrote:
> 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.
Given the lack of interest from the community, I am going to drop this
idea.
> 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,
>
