[PATCH 07/21] powerpc: dma-mapping: always clean cache in _for_device() op

[Arnd Bergmann]

From: Arnd Bergmann <arnd at arndb.de>

The powerpc implementation of arch_sync_dma_for_device() is unique in that
it sometimes performs a full flush for the arch_sync_dma_for_device(paddr,
size, DMA_FROM_DEVICE) operation when the address is unaligned, but
otherwise invalidates the caches.

Since the _for_cpu() counterpart has to invalidate the cache already
in order to avoid stale data from prefetching, this operation only really
needs to ensure that there are no dirty cache lines, which can be done
using either invalidation or cleaning the cache, but not necessarily
both.

Most architectures traditionally go for invalidation here, but as
Will Deacon points out, this can leak old data to user space if
a DMA is started but the device ends up not actually filling the
entire buffer, see the link below.

The same argument applies to DMA_BIDIRECTIONAL transfers. Using
a cache-clean operation is the safe choice here, followed by
invalidating the cache after the DMA to get rid of stale data
that was prefetched before the completion of the DMA.

Link: https://lore.kernel.org/all/20220606152150.GA31568@willie-the-truck/
Signed-off-by: Arnd Bergmann <arnd at arndb.de>
---
 arch/powerpc/mm/dma-noncoherent.c | 21 +--------------------
 1 file changed, 1 insertion(+), 20 deletions(-)

diff --git a/arch/powerpc/mm/dma-noncoherent.c b/arch/powerpc/mm/dma-noncoherent.c
index e108cacf877f..00e59a4faa2b 100644
--- a/arch/powerpc/mm/dma-noncoherent.c
+++ b/arch/powerpc/mm/dma-noncoherent.c
@@ -104,26 +104,7 @@ static void __dma_phys_op(phys_addr_t paddr, size_t size, enum dma_cache_op op)
 void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
 		enum dma_data_direction dir)
 {
-	switch (direction) {
-	case DMA_NONE:
-		BUG();
-	case DMA_FROM_DEVICE:
-		/*
-		 * invalidate only when cache-line aligned otherwise there is
-		 * the potential for discarding uncommitted data from the cache
-		 */
-		if ((start | end) & (L1_CACHE_BYTES - 1))
-			__dma_phys_op(start, end, DMA_CACHE_FLUSH);
-		else
-			__dma_phys_op(start, end, DMA_CACHE_INVAL);
-		break;
-	case DMA_TO_DEVICE:		/* writeback only */
-		__dma_phys_op(start, end, DMA_CACHE_CLEAN);
-		break;
-	case DMA_BIDIRECTIONAL:	/* writeback and invalidate */
-		__dma_phys_op(start, end, DMA_CACHE_FLUSH);
-		break;
-	}
+	__dma_phys_op(start, end, DMA_CACHE_CLEAN);
 }
 
 void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size,
-- 
2.39.2