swiotlb-xen: use the same foreign page check everywhere

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 <hch@lst.de>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

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