From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933918AbbFJQcE (ORCPT ); Wed, 10 Jun 2015 12:32:04 -0400 Received: from pandora.arm.linux.org.uk ([78.32.30.218]:50894 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933892AbbFJQbk (ORCPT ); Wed, 10 Jun 2015 12:31:40 -0400 Date: Wed, 10 Jun 2015 17:31:21 +0100 From: Russell King - ARM Linux To: Dan Williams Cc: Joerg Roedel , Jens Axboe , Michal Simek , Greg Kroah-Hartman , "linux-kernel@vger.kernel.org" , Julia Lawall , "dmaengine@vger.kernel.org" , David Woodhouse , Christoph Hellwig , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH 1/2] scatterlist: use sg_phys() Message-ID: <20150610163121.GP7557@n2100.arm.linux.org.uk> References: <20150609162659.21910.41681.stgit@dwillia2-desk3.amr.corp.intel.com> <20150609162710.21910.57295.stgit@dwillia2-desk3.amr.corp.intel.com> <20150610093252.GA20384@8bytes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 10, 2015 at 09:00:31AM -0700, Dan Williams wrote: > On Wed, Jun 10, 2015 at 2:32 AM, Joerg Roedel wrote: > > On Tue, Jun 09, 2015 at 12:27:10PM -0400, Dan Williams wrote: > >> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c > >> index 7e7583ddd607..9f6ff6671f01 100644 > >> --- a/arch/arm/mm/dma-mapping.c > >> +++ b/arch/arm/mm/dma-mapping.c > >> @@ -1502,7 +1502,7 @@ static int __map_sg_chunk(struct device *dev, struct scatterlist *sg, > >> return -ENOMEM; > >> > >> for (count = 0, s = sg; count < (size >> PAGE_SHIFT); s = sg_next(s)) { > >> - phys_addr_t phys = page_to_phys(sg_page(s)); > >> + phys_addr_t phys = sg_phys(s) - s->offset; > > > > So sg_phys() turns out to be 'page_to_phys(sg_page(s)) + s->offset', > > which makes the above statement to: > > > > page_to_phys(sg_page(s)) + s->offset - s->offset; > > > > The compiler will probably optimize that away, but it still doesn't look > > like an improvement. > > The goal is to eventually stop leaking struct page deep into the i/o > stack. Anything that relies on being able to retrieve a struct page > out of an sg entry needs to be converted. I think we need a new > helper for this case "sg_phys_aligned()?". Why? The aim of the code is not to detect whether the address is aligned to a page (if it were, it'd be testing for a zero s->offset, or it would be testing for an s->offset being a multiple of the page size. Let's first understand the code that's being modified (which seems to be something which isn't done very much today - people seem to just like changing code for the hell of it.) for (count = 0, s = sg; count < (size >> PAGE_SHIFT); s = sg_next(s)) { phys_addr_t phys = page_to_phys(sg_page(s)); unsigned int len = PAGE_ALIGN(s->offset + s->length); if (!is_coherent && !dma_get_attr(DMA_ATTR_SKIP_CPU_SYNC, attrs)) __dma_page_cpu_to_dev(sg_page(s), s->offset, s->length, dir); prot = __dma_direction_to_prot(dir); ret = iommu_map(mapping->domain, iova, phys, len, prot); if (ret < 0) goto fail; count += len >> PAGE_SHIFT; iova += len; } What it's doing is trying to map each scatterlist entry via an IOMMU. Unsurprisingly, IOMMUs are page based - you can't map a partial IOMMU page. However, what says that the IOMMU page size is the same as the host CPU page size - it may not be... so the above code is wrong for a completely different reason. What we _should_ be doing is finding out what the IOMMU minimum page size is, and using that in conjunction with the sg_phys() of the scatterlist entry to determine the start and length of the mapping such that the IOMMU mapping covers the range described by the scatterlist entry. (iommu_map() takes arguments which must be aligned to the IOMMU minimum page size.) However, what we can also see from the above is that we have other code here using sg_page() - which is a necessity to be able to perform the required DMA cache maintanence to ensure that the data is visible to the DMA device. We need to kmap_atomic() these in order to flush them, and there's no other way other than struct page to represent a highmem page. So, I think your intent to want to remove struct page from the I/O subsystem is a false hope, unless you want to end up rewriting lots of architecture code, and you can come up with an alternative method to represent highmem pages. -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net. From mboxrd@z Thu Jan 1 00:00:00 1970 From: linux@arm.linux.org.uk (Russell King - ARM Linux) Date: Wed, 10 Jun 2015 17:31:21 +0100 Subject: [PATCH 1/2] scatterlist: use sg_phys() In-Reply-To: References: <20150609162659.21910.41681.stgit@dwillia2-desk3.amr.corp.intel.com> <20150609162710.21910.57295.stgit@dwillia2-desk3.amr.corp.intel.com> <20150610093252.GA20384@8bytes.org> Message-ID: <20150610163121.GP7557@n2100.arm.linux.org.uk> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Wed, Jun 10, 2015 at 09:00:31AM -0700, Dan Williams wrote: > On Wed, Jun 10, 2015 at 2:32 AM, Joerg Roedel wrote: > > On Tue, Jun 09, 2015 at 12:27:10PM -0400, Dan Williams wrote: > >> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c > >> index 7e7583ddd607..9f6ff6671f01 100644 > >> --- a/arch/arm/mm/dma-mapping.c > >> +++ b/arch/arm/mm/dma-mapping.c > >> @@ -1502,7 +1502,7 @@ static int __map_sg_chunk(struct device *dev, struct scatterlist *sg, > >> return -ENOMEM; > >> > >> for (count = 0, s = sg; count < (size >> PAGE_SHIFT); s = sg_next(s)) { > >> - phys_addr_t phys = page_to_phys(sg_page(s)); > >> + phys_addr_t phys = sg_phys(s) - s->offset; > > > > So sg_phys() turns out to be 'page_to_phys(sg_page(s)) + s->offset', > > which makes the above statement to: > > > > page_to_phys(sg_page(s)) + s->offset - s->offset; > > > > The compiler will probably optimize that away, but it still doesn't look > > like an improvement. > > The goal is to eventually stop leaking struct page deep into the i/o > stack. Anything that relies on being able to retrieve a struct page > out of an sg entry needs to be converted. I think we need a new > helper for this case "sg_phys_aligned()?". Why? The aim of the code is not to detect whether the address is aligned to a page (if it were, it'd be testing for a zero s->offset, or it would be testing for an s->offset being a multiple of the page size. Let's first understand the code that's being modified (which seems to be something which isn't done very much today - people seem to just like changing code for the hell of it.) for (count = 0, s = sg; count < (size >> PAGE_SHIFT); s = sg_next(s)) { phys_addr_t phys = page_to_phys(sg_page(s)); unsigned int len = PAGE_ALIGN(s->offset + s->length); if (!is_coherent && !dma_get_attr(DMA_ATTR_SKIP_CPU_SYNC, attrs)) __dma_page_cpu_to_dev(sg_page(s), s->offset, s->length, dir); prot = __dma_direction_to_prot(dir); ret = iommu_map(mapping->domain, iova, phys, len, prot); if (ret < 0) goto fail; count += len >> PAGE_SHIFT; iova += len; } What it's doing is trying to map each scatterlist entry via an IOMMU. Unsurprisingly, IOMMUs are page based - you can't map a partial IOMMU page. However, what says that the IOMMU page size is the same as the host CPU page size - it may not be... so the above code is wrong for a completely different reason. What we _should_ be doing is finding out what the IOMMU minimum page size is, and using that in conjunction with the sg_phys() of the scatterlist entry to determine the start and length of the mapping such that the IOMMU mapping covers the range described by the scatterlist entry. (iommu_map() takes arguments which must be aligned to the IOMMU minimum page size.) However, what we can also see from the above is that we have other code here using sg_page() - which is a necessity to be able to perform the required DMA cache maintanence to ensure that the data is visible to the DMA device. We need to kmap_atomic() these in order to flush them, and there's no other way other than struct page to represent a highmem page. So, I think your intent to want to remove struct page from the I/O subsystem is a false hope, unless you want to end up rewriting lots of architecture code, and you can come up with an alternative method to represent highmem pages. -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net.