[PATCH/RFC 4/7] powerpc/mm: handle highmem pages in arch_dma_prep_coherent()

[Marek Szyprowski <m.szyprowski@samsung.com>]

arch_dma_prep_coherent() performs the cache maintenance on the address
returned by page_address(). On 32-bit powerpc with CONFIG_HIGHMEM that
is NULL for a highmem page which is not kmapped, so the flush would
operate on the wrong virtual range and leave the dirty lines of the
buffer in place.

The generic DMA code may pass highmem pages to this hook:
dma_direct_alloc() handles a highmem page returned by
dma_alloc_contiguous() by forcing the remap path, which calls
arch_dma_prep_coherent() on that page before creating the uncached
mapping. This cannot happen on powerpc today only because it does not
select HAVE_DMA_CONTIGUOUS, so dma-direct never gets a highmem page
there, but nothing in the hook's contract rules it out.

Use __dma_sync_page() with DMA_BIDIRECTIONAL, which the
arch_sync_dma_for_*() hooks already use and which kmaps highmem pages
one at a time. Without CONFIG_HIGHMEM it does the same
flush_dcache_range() on the linear mapping as before.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
---
 arch/powerpc/mm/dma-noncoherent.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/arch/powerpc/mm/dma-noncoherent.c b/arch/powerpc/mm/dma-noncoherent.c
index 30260b5d146d..e646256365d5 100644
--- a/arch/powerpc/mm/dma-noncoherent.c
+++ b/arch/powerpc/mm/dma-noncoherent.c
@@ -118,7 +118,5 @@ void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size,
 
 void arch_dma_prep_coherent(struct page *page, size_t size)
 {
-	unsigned long kaddr = (unsigned long)page_address(page);
-
-	flush_dcache_range(kaddr, kaddr + size);
+	__dma_sync_page(page_to_phys(page), size, DMA_BIDIRECTIONAL);
 }
-- 
2.43.0