From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935725AbcCQLGz (ORCPT ); Thu, 17 Mar 2016 07:06:55 -0400 Received: from szxga03-in.huawei.com ([119.145.14.66]:6653 "EHLO szxga03-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932903AbcCQLGw (ORCPT ); Thu, 17 Mar 2016 07:06:52 -0400 Subject: Re: [PATCH 1/1] arm64/dma-mapping: remove an unnecessary conversion To: Catalin Marinas References: <1458007931-14432-1-git-send-email-thunder.leizhen@huawei.com> <20160315153757.GF12311@e104818-lin.cambridge.arm.com> <56E8BD51.6010008@huawei.com> CC: Will Deacon , linux-kernel , Zefan Li , Xinwei Hu , Tianhong Ding , Hanjun Guo , linux-arm-kernel From: "Leizhen (ThunderTown)" Message-ID: <56EA8FB3.2050505@huawei.com> Date: Thu, 17 Mar 2016 19:06:27 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 MIME-Version: 1.0 In-Reply-To: <56E8BD51.6010008@huawei.com> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.177.23.164] X-CFilter-Loop: Reflected X-Mirapoint-Virus-RAPID-Raw: score=unknown(0), refid=str=0001.0A020206.56EA8FC3.0194,ss=1,re=0.000,recu=0.000,reip=0.000,cl=1,cld=1,fgs=0, ip=0.0.0.0, so=2013-05-26 15:14:31, dmn=2013-03-21 17:37:32 X-Mirapoint-Loop-Id: 3d2066dab2c82cf7fe0d0f58817ef384 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2016/3/16 9:56, Leizhen (ThunderTown) wrote: > > > On 2016/3/15 23:37, Catalin Marinas wrote: >> On Tue, Mar 15, 2016 at 10:12:11AM +0800, Zhen Lei wrote: >>> 1. In swiotlb_alloc_coherent, the branch of __get_free_pages. Directly >>> return vaddr on success, and pass vaddr to free_pages on failure. >>> 2. So, we can directly transparent pass vaddr from __dma_free to >>> swiotlb_free_coherent, keep consistent with swiotlb_alloc_coherent. >>> >>> This patch have no functional change, >> >> I don't think so. >> >>> but can obtain a bit performance improvement. >> >> Have you actually measured it? > I have not run any performance testing, but reduced a line of code. So I said "a bit". > >> >>> diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c >>> index a6e757c..b2f2834 100644 >>> --- a/arch/arm64/mm/dma-mapping.c >>> +++ b/arch/arm64/mm/dma-mapping.c >>> @@ -187,8 +187,6 @@ static void __dma_free(struct device *dev, size_t size, >>> void *vaddr, dma_addr_t dma_handle, >>> struct dma_attrs *attrs) >>> { >>> - void *swiotlb_addr = phys_to_virt(dma_to_phys(dev, dma_handle)); >>> - >>> size = PAGE_ALIGN(size); >>> >>> if (!is_device_dma_coherent(dev)) { >>> @@ -196,7 +194,7 @@ static void __dma_free(struct device *dev, size_t size, >>> return; >>> vunmap(vaddr); >>> } >>> - __dma_free_coherent(dev, size, swiotlb_addr, dma_handle, attrs); >>> + __dma_free_coherent(dev, size, vaddr, dma_handle, attrs); >>> } >> >> What happens when !is_device_dma_coherent(dev)? (hint: read two lines >> above __dma_free_coherent). Do you afraid "vaddr" maybe modified by these statement? First, it could not be __free_from_pool. Otherwise, the function vunmap(which after it) can not work well. Then, it count not be vunmap too, the parameter is defined as "const void *". In the call chain: __dma_free_coherent-->__dma_free_coherent-->swiotlb_free_coherent, only swiotlb_free_coherent finally use "vaddr". >> > The whole function of __dma_free as below: (nobody use swiotlb_addr except __dma_free_coherent) > static void __dma_free(struct device *dev, size_t size, > void *vaddr, dma_addr_t dma_handle, > struct dma_attrs *attrs) > { > void *swiotlb_addr = phys_to_virt(dma_to_phys(dev, dma_handle)); > > size = PAGE_ALIGN(size); > > if (!is_device_dma_coherent(dev)) { > if (__free_from_pool(vaddr, size)) > return; > vunmap(vaddr); > } > __dma_free_coherent(dev, size, swiotlb_addr, dma_handle, attrs); > } >