From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f178.google.com (mail-wi0-f178.google.com [209.85.212.178]) by kanga.kvack.org (Postfix) with ESMTP id 2C1406B0038 for ; Tue, 3 Feb 2015 09:18:17 -0500 (EST) Received: by mail-wi0-f178.google.com with SMTP id bs8so22095623wib.5 for ; Tue, 03 Feb 2015 06:18:16 -0800 (PST) Received: from mout.kundenserver.de (mout.kundenserver.de. [212.227.17.24]) by mx.google.com with ESMTPS id a20si31470555wiw.25.2015.02.03.06.18.14 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 03 Feb 2015 06:18:15 -0800 (PST) From: Arnd Bergmann Subject: Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms Date: Tue, 03 Feb 2015 15:17:27 +0100 Message-ID: <4689826.8DDCrX2ZhK@wuerfel> In-Reply-To: References: <1422347154-15258-1-git-send-email-sumit.semwal@linaro.org> <20150203074856.GF14009@phenom.ffwll.local> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: owner-linux-mm@kvack.org List-ID: To: linux-arm-kernel@lists.infradead.org Cc: Rob Clark , Russell King - ARM Linux , Sumit Semwal , LKML , "linux-media@vger.kernel.org" , DRI mailing list , Linaro MM SIG Mailman List , "linux-mm@kvack.org" , Linaro Kernel Mailman List , Tomasz Stanislawski , Robin Murphy , Marek Szyprowski , Daniel Vetter On Tuesday 03 February 2015 09:04:03 Rob Clark wrote: > On Tue, Feb 3, 2015 at 2:48 AM, Daniel Vetter wrote: > > On Mon, Feb 02, 2015 at 03:30:21PM -0500, Rob Clark wrote: > >> On Mon, Feb 2, 2015 at 11:54 AM, Daniel Vetter wrote: > >> >> My initial thought is for dma-buf to not try to prevent something than > >> >> an exporter can actually do.. I think the scenario you describe could > >> >> be handled by two sg-lists, if the exporter was clever enough. > >> > > >> > That's already needed, each attachment has it's own sg-list. After all > >> > there's no array of dma_addr_t in the sg tables, so you can't use one sg > >> > for more than one mapping. And due to different iommu different devices > >> > can easily end up with different addresses. > >> > >> > >> Well, to be fair it may not be explicitly stated, but currently one > >> should assume the dma_addr_t's in the dmabuf sglist are bogus. With > >> gpu's that implement per-process/context page tables, I'm not really > >> sure that there is a sane way to actually do anything else.. > > > > Hm, what does per-process/context page tables have to do here? At least on > > i915 we have a two levels of page tables: > > - first level for vm/device isolation, used through dma api > > - 2nd level for per-gpu-context isolation and context switching, handled > > internally. > > > > Since atm the dma api doesn't have any context of contexts or different > > pagetables, I don't see who you could use that at all. > > Since I'm stuck w/ an iommu, instead of built in mmu, my plan was to > drop use of dma-mapping entirely (incl the current call to dma_map_sg, > which I just need until we can use drm_cflush on arm), and > attach/detach iommu domains directly to implement context switches. > At that point, dma_addr_t really has no sensible meaning for me. I think what you see here is a quite common hardware setup and we really lack the right abstraction for it at the moment. Everybody seems to work around it with a mix of the dma-mapping API and the iommu API. These are doing different things, and even though the dma-mapping API can be implemented on top of the iommu API, they are not really compatible. The drm_clflush helpers don't seem like the right solution to me, because all other devices outside of drm will face the same issue, and I suspect we should fill the missing gaps in the API in a more generic way. Arnd -- 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