linux-kernel - [PATCH] [RFC] drivers: dma-coherent: use MEMREMAP_WB instead of MEMREMAP

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Date:   Wed, 09 Nov 2016 18:10:09 +0900
From:   Jaewon Kim <jaewon31.kim@...sung.com>
To:     brian.starkey@....com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        jaewon31.kim@...sung.com
Subject: [PATCH] [RFC] drivers: dma-coherent: use MEMREMAP_WB instead of
 MEMREMAP_WC

Commit 6b03ae0d42bf (drivers: dma-coherent: use MEMREMAP_WC for DMA_MEMORY_MA)
added MEMREMAP_WC for DMA_MEMORY_MAP. If, however, CPU cache can be used on
DMA_MEMORY_MAP, I think MEMREMAP_WC can be changed to MEMREMAP_WB. On my local
ARM device, memset in dma_alloc_from_coherent sometimes takes much longer with
MEMREMAP_WC compared to MEMREMAP_WB.

Test results on AArch64 by allocating 4MB with putting trace_printk right
before and after memset.
	MEMREMAP_WC : 11.0ms, 5.7ms, 4.2ms, 4.9ms, 5.4ms, 4.3ms, 3.5ms
	MEMREMAP_WB : 0.7ms, 0.6ms, 0.6ms, 0.6ms, 0.6ms, 0.5ms, 0.4 ms

Signed-off-by: Jaewon Kim <jaewon31.kim@...sung.com>
---
 drivers/base/dma-coherent.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/base/dma-coherent.c b/drivers/base/dma-coherent.c
index 640a7e6..0512a1d 100644
--- a/drivers/base/dma-coherent.c
+++ b/drivers/base/dma-coherent.c
@@ -33,7 +33,7 @@ static bool dma_init_coherent_memory(
 		goto out;
 
 	if (flags & DMA_MEMORY_MAP)
-		mem_base = memremap(phys_addr, size, MEMREMAP_WC);
+		mem_base = memremap(phys_addr, size, MEMREMAP_WB);
 	else
 		mem_base = ioremap(phys_addr, size);
 	if (!mem_base)
-- 
1.9.1