From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753870AbeDTHNT (ORCPT ); Fri, 20 Apr 2018 03:13:19 -0400 Received: from mail-wr0-f171.google.com ([209.85.128.171]:45854 "EHLO mail-wr0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753529AbeDTHNQ (ORCPT ); Fri, 20 Apr 2018 03:13:16 -0400 X-Google-Smtp-Source: AIpwx4/PVgsm2t2jrBsk9PtbCOaZk1Fo/KNjY3AYi/02GZ9o1ahsc6TNUlzT/TBndygDj+FlYceHJw== Date: Fri, 20 Apr 2018 09:13:12 +0200 From: Daniel Vetter To: Christoph Hellwig Cc: Daniel Vetter , Jerome Glisse , Christian =?iso-8859-1?Q?K=F6nig?= , "moderated list:DMA BUFFER SHARING FRAMEWORK" , "open list:DMA BUFFER SHARING FRAMEWORK" , dri-devel , amd-gfx list , Linux Kernel Mailing List , Logan Gunthorpe , Dan Williams Subject: Re: [PATCH 4/8] dma-buf: add peer2peer flag Message-ID: <20180420071312.GF31310@phenom.ffwll.local> Mail-Followup-To: Christoph Hellwig , Jerome Glisse , Christian =?iso-8859-1?Q?K=F6nig?= , "moderated list:DMA BUFFER SHARING FRAMEWORK" , "open list:DMA BUFFER SHARING FRAMEWORK" , dri-devel , amd-gfx list , Linux Kernel Mailing List , Logan Gunthorpe , Dan Williams References: <20180325110000.2238-1-christian.koenig@amd.com> <20180325110000.2238-4-christian.koenig@amd.com> <20180329065753.GD3881@phenom.ffwll.local> <8b823458-8bdc-3217-572b-509a28aae742@gmail.com> <20180403090909.GN3881@phenom.ffwll.local> <20180403170645.GB5935@redhat.com> <20180403180832.GZ3881@phenom.ffwll.local> <20180416123937.GA9073@infradead.org> <20180419081657.GA16735@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180419081657.GA16735@infradead.org> X-Operating-System: Linux phenom 4.15.0-1-amd64 User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 19, 2018 at 01:16:57AM -0700, Christoph Hellwig wrote: > On Mon, Apr 16, 2018 at 03:38:56PM +0200, Daniel Vetter wrote: > > We've broken that assumption in i915 years ago. Not struct page backed > > gpu memory is very real. > > > > Of course we'll never feed such a strange sg table to a driver which > > doesn't understand it, but allowing sg_page == NULL works perfectly > > fine. At least for gpu drivers. > > For GPU drivers on x86 with no dma coherency problems, sure. But not > all the world is x86. We already have problems due to dmabugs use > of the awkward get_sgtable interface (see the common on > arm_dma_get_sgtable that I fully agree with), and doing this for memory > that doesn't have a struct page at all will make things even worse. x86 dma isn't coherent either, if you're a GPU :-) Flushing gpu caches tends to be too expensive, so there's pci-e support and chipset support to forgo it. Plus drivers flushing caches themselves. The dma_get_sgtable thing is indeed fun, right solution would probably be to push the dma-buf export down into the dma layer. The comment for arm_dma_get_sgtable is also not a realy concern, because dma-buf also abstracts away the flushing (or well is supposed to), so there really shouldn't be anyone calling the streaming apis on the returned sg table. That's why dma-buf gives you an sg table that's mapped already. > > If that's not acceptable then I guess we could go over the entire tree > > and frob all the gpu related code to switch over to a new struct > > sg_table_might_not_be_struct_page_backed, including all the other > > functions we added over the past few years to iterate over sg tables. > > But seems slightly silly, given that sg tables seem to do exactly what > > we need. > > It isn't silly. We will have to do some surgery like that anyway > because the current APIs don't work. So relax, sit back and come up > with an API that solves the existing issues and serves us well in > the future. So we should just implement a copy of sg table for dma-buf, since I still think it does exactly what we need for gpus? Yes there's a bit a layering violation insofar that drivers really shouldn't each have their own copy of "how do I convert a piece of dma memory into dma-buf", but that doesn't render the interface a bad idea. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch