Subject: [PATCH 06/21] powerpc: dma-mapping: minimize for_cpu flushing

From: Arnd Bergmann <[email-masked]>
Date: Mon, 27 Mar 2023 14:13:02 +0200

From: Arnd Bergmann <[email-masked]>

The powerpc dma_sync_*_for_cpu() variants do more flushes than on other
architectures. Reduce it to what everyone else does:

 - No flush is needed after data has been sent to a device

 - When data has been received from a device, the cache only needs to
   be invalidated to clear out cache lines that were speculatively
   prefetched.

In particular, the second flushing of partial cache lines of bidirectional
buffers is actively harmful -- if a single cache line is written by both
the CPU and the device, flushing it again does not maintain coherency
but instead overwrite the data that was just received from the device.

Signed-off-by: Arnd Bergmann <[email-masked]>
---
 arch/powerpc/mm/dma-noncoherent.c | 18 ++++--------------
 1 file changed, 4 insertions(+), 14 deletions(-)

diff --git a/arch/powerpc/mm/dma-noncoherent.c b/arch/powerpc/mm/dma-noncoherent.c
index f10869d27de5..e108cacf877f 100644
--- a/arch/powerpc/mm/dma-noncoherent.c
+++ b/arch/powerpc/mm/dma-noncoherent.c
@@ -132,21 +132,11 @@ void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size,
 	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);
+	case DMA_TO_DEVICE:
 		break;
-	case DMA_BIDIRECTIONAL:	/* writeback and invalidate */
-		__dma_phys_op(start, end, DMA_CACHE_FLUSH);
+	case DMA_FROM_DEVICE:
+	case DMA_BIDIRECTIONAL:
+		__dma_phys_op(start, end, DMA_CACHE_INVAL);
 		break;
 	}
 }
-- 
2.39.2