From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wg0-f49.google.com (mail-wg0-f49.google.com [74.125.82.49]) by kanga.kvack.org (Postfix) with ESMTP id 7120B6B0038 for ; Tue, 3 Feb 2015 07:59:30 -0500 (EST) Received: by mail-wg0-f49.google.com with SMTP id k14so44422719wgh.8 for ; Tue, 03 Feb 2015 04:59:29 -0800 (PST) Received: from mail-we0-x230.google.com (mail-we0-x230.google.com. [2a00:1450:400c:c03::230]) by mx.google.com with ESMTPS id gp3si31035397wib.60.2015.02.03.04.59.28 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 03 Feb 2015 04:59:28 -0800 (PST) Received: by mail-we0-f176.google.com with SMTP id w62so44757533wes.7 for ; Tue, 03 Feb 2015 04:59:27 -0800 (PST) Date: Tue, 3 Feb 2015 14:00:49 +0100 From: Daniel Vetter Subject: Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms Message-ID: <20150203130049.GN14009@phenom.ffwll.local> References: <20150129143908.GA26493@n2100.arm.linux.org.uk> <20150129154718.GB26493@n2100.arm.linux.org.uk> <20150129192610.GE26493@n2100.arm.linux.org.uk> <20150202165405.GX14009@phenom.ffwll.local> <20150203074856.GF14009@phenom.ffwll.local> <20150203122813.GN8656@n2100.arm.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150203122813.GN8656@n2100.arm.linux.org.uk> Sender: owner-linux-mm@kvack.org List-ID: To: Russell King - ARM Linux Cc: Daniel Vetter , Rob Clark , Sumit Semwal , LKML , "linux-media@vger.kernel.org" , DRI mailing list , Linaro MM SIG Mailman List , "linux-arm-kernel@lists.infradead.org" , "linux-mm@kvack.org" , Linaro Kernel Mailman List , Tomasz Stanislawski , Robin Murphy , Marek Szyprowski On Tue, Feb 03, 2015 at 12:28:14PM +0000, Russell King - ARM Linux wrote: > On Tue, Feb 03, 2015 at 08:48:56AM +0100, 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. > > What I've found with *my* etnaviv drm implementation (not Christian's - I > found it impossible to work with Christian, especially with the endless > "msm doesn't do it that way, so we shouldn't" responses and his attitude > towards cherry-picking my development work [*]) is that it's much easier to > keep the GPU MMU local to the GPU and under the control of the DRM MM code, > rather than attaching the IOMMU to the DMA API and handling it that way. > > There are several reasons for that: > > 1. DRM has a better idea about when the memory needs to be mapped to the > GPU, and it can more effectively manage the GPU MMU. > > 2. The GPU MMU may have TLBs which can only be flushed via a command in > the GPU command stream, so it's fundamentally necessary for the MMU to > be managed by the GPU driver so that it knows when (and how) to insert > the flushes. 3. Switching between different address spaces (for per gpu context isolation) often requires intricate knowledge about the gpu and close coordination. Well maybe just a part of 2 really, but an important one. Fully agree and tbh I'm not sure whether the current push in arm to expose all gpu mmus as iommus is solid. Even for pasid (per-context iommu tables) which is a big official standard there's still a lot of open questions about how to do it properly. And it requires strict hw support so that the hw always knows which pasid it should use for a given dma access. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch -- 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