Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753297AbcKIJ2e (ORCPT ); Wed, 9 Nov 2016 04:28:34 -0500 Received: from foss.arm.com ([217.140.101.70]:48572 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752336AbcKIJ2b (ORCPT ); Wed, 9 Nov 2016 04:28:31 -0500 Date: Wed, 9 Nov 2016 09:27:26 +0000 From: Brian Starkey To: Jaewon Kim Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] [RFC] drivers: dma-coherent: use MEMREMAP_WB instead of MEMREMAP_WC Message-ID: <20161109092726.GA6009@e106950-lin.cambridge.arm.com> References: <1478682609-26477-1-git-send-email-jaewon31.kim@samsung.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <1478682609-26477-1-git-send-email-jaewon31.kim@samsung.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1529 Lines: 44 Hi Jaewon, On Wed, Nov 09, 2016 at 06:10:09PM +0900, Jaewon Kim wrote: >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 > This doesn't look like a good idea to me. The point of coherent memory is to have it non-cached, however WB will make writes hit the cache. Writing to the cache is of course faster than writing to RAM, but that's not what we want to do here. -Brian >Signed-off-by: Jaewon Kim >--- > 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 >