xen_dma_map_page uses a different and more complicated check for foreign
pages than the other three cache maintainance helpers.  Switch it to the
simpler pfn_valid method a well, and document the scheme with a single
improved comment in xen_dma_map_page.

Signed-off-by: Christoph Hellwig <h...@lst.de>
Reviewed-by: Stefano Stabellini <sstabell...@kernel.org>
---
 include/xen/arm/page-coherent.h | 31 +++++++++----------------------
 1 file changed, 9 insertions(+), 22 deletions(-)

diff --git a/include/xen/arm/page-coherent.h b/include/xen/arm/page-coherent.h
index a840d6949a87..a8d9c0678c27 100644
--- a/include/xen/arm/page-coherent.h
+++ b/include/xen/arm/page-coherent.h
@@ -53,23 +53,17 @@ static inline void xen_dma_map_page(struct device *hwdev, 
struct page *page,
             dma_addr_t dev_addr, unsigned long offset, size_t size,
             enum dma_data_direction dir, unsigned long attrs)
 {
-       unsigned long page_pfn = page_to_xen_pfn(page);
-       unsigned long dev_pfn = XEN_PFN_DOWN(dev_addr);
-       unsigned long compound_pages =
-               (1<<compound_order(page)) * XEN_PFN_PER_PAGE;
-       bool local = (page_pfn <= dev_pfn) &&
-               (dev_pfn - page_pfn < compound_pages);
+       unsigned long pfn = PFN_DOWN(dev_addr);
 
        /*
-        * Dom0 is mapped 1:1, while the Linux page can span across
-        * multiple Xen pages, it's not possible for it to contain a
-        * mix of local and foreign Xen pages. So if the first xen_pfn
-        * == mfn the page is local otherwise it's a foreign page
-        * grant-mapped in dom0. If the page is local we can safely
-        * call the native dma_ops function, otherwise we call the xen
-        * specific function.
+        * Dom0 is mapped 1:1, and while the Linux page can span across multiple
+        * Xen pages, it is not possible for it to contain a mix of local and
+        * foreign Xen pages.  Calling pfn_valid on a foreign mfn will always
+        * return false, so if pfn_valid returns true the pages is local and we
+        * can use the native dma-direct functions, otherwise we call the Xen
+        * specific version.
         */
-       if (local)
+       if (pfn_valid(pfn))
                dma_direct_map_page(hwdev, page, offset, size, dir, attrs);
        else
                __xen_dma_map_page(hwdev, page, dev_addr, offset, size, dir, 
attrs);
@@ -79,14 +73,7 @@ static inline void xen_dma_unmap_page(struct device *hwdev, 
dma_addr_t handle,
                size_t size, enum dma_data_direction dir, unsigned long attrs)
 {
        unsigned long pfn = PFN_DOWN(handle);
-       /*
-        * Dom0 is mapped 1:1, while the Linux page can be spanned accross
-        * multiple Xen page, it's not possible to have a mix of local and
-        * foreign Xen page. Dom0 is mapped 1:1, so calling pfn_valid on a
-        * foreign mfn will always return false. If the page is local we can
-        * safely call the native dma_ops function, otherwise we call the xen
-        * specific function.
-        */
+
        if (pfn_valid(pfn))
                dma_direct_unmap_page(hwdev, handle, size, dir, attrs);
        else
-- 
2.20.1

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