From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f199.google.com (mail-pf0-f199.google.com [209.85.192.199]) by kanga.kvack.org (Postfix) with ESMTP id 183496B0038 for ; Wed, 9 Nov 2016 04:27:34 -0500 (EST) Received: by mail-pf0-f199.google.com with SMTP id l66so92150079pfl.7 for ; Wed, 09 Nov 2016 01:27:34 -0800 (PST) Received: from foss.arm.com (foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id i8si41208972pgp.114.2016.11.09.01.27.33 for ; Wed, 09 Nov 2016 01:27:33 -0800 (PST) Date: Wed, 9 Nov 2016 09:27:26 +0000 From: Brian Starkey 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> Sender: owner-linux-mm@kvack.org List-ID: To: Jaewon Kim Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org 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 > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org