From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752786AbeDLNvH (ORCPT ); Thu, 12 Apr 2018 09:51:07 -0400 Received: from foss.arm.com ([217.140.101.70]:33516 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752721AbeDLNvF (ORCPT ); Thu, 12 Apr 2018 09:51:05 -0400 Subject: Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for scatter-gather buffers To: =?UTF-8?Q?Christian_K=c3=b6nig?= , Christoph Hellwig Cc: Sinan Kaya , amd-gfx@lists.freedesktop.org, timur@codeaurora.org, sulrich@codeaurora.org, Tom St Denis , "David (ChunMing) Zhou" , Emily Deng , David Airlie , linux-arm-msm@vger.kernel.org, Felix Kuehling , open list , "open list:DRM DRIVERS" , David Panariti , Jim Qu , Huang Rui , Roger He , Monk Liu , Feifei Xu , Alex Deucher , linux-arm-kernel@lists.infradead.org, iommu@lists.linux-foundation.org References: <1523394001-4615-1-git-send-email-okaya@codeaurora.org> <32b82296-bba5-b5f1-266b-45c1ed66da94@arm.com> <20180412062600.GB30499@lst.de> <6802bd0c-7da6-659a-7e34-8f05d7c6422c@amd.com> From: Robin Murphy Message-ID: <7a3e7459-2965-54c8-ff0c-6edb3a6aecb5@arm.com> Date: Thu, 12 Apr 2018 14:51:00 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <6802bd0c-7da6-659a-7e34-8f05d7c6422c@amd.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/04/18 10:42, Christian König wrote: > Am 12.04.2018 um 08:26 schrieb Christoph Hellwig: >> On Wed, Apr 11, 2018 at 01:03:59PM +0100, Robin Murphy wrote: >>> On 10/04/18 21:59, Sinan Kaya wrote: >>>> Code is expecing to observe the same number of buffers returned from >>>> dma_map_sg() function compared to sg_alloc_table_from_pages(). This >>>> doesn't hold true universally especially for systems with IOMMU. >>> So why not fix said code? It's clearly not a real hardware >>> limitation, and >>> the map_sg() APIs have potentially returned fewer than nents since >>> forever, >>> so there's really no excuse. >> Yes, relying on dma_map_sg returning the same number of entries as passed >> it is completely bogus. > > I agree that the common DRM functions should be able to deal with both, > but we should let the driver side decide if it wants multiple addresses > combined or not. > >> >>>> IOMMU driver tries to combine buffers into a single DMA address as much >>>> as it can. The right thing is to tell the DMA layer how much combining >>>> IOMMU can do. >>> Disagree; this is a dodgy hack, since you'll now end up passing >>> scatterlists into dma_map_sg() which already violate max_seg_size to >>> begin >>> with, and I think a conscientious DMA API implementation would be at >>> rights >>> to fail the mapping for that reason (I know arm64 happens not to, but >>> that >>> was a deliberate design decision to make my life easier at the time). >> Agreed. > > Sounds like my understanding of max_seg_size is incorrect, what exactly > should that describe? The segment size and boundary mask are there to represent a device's hardware scatter-gather capabilities - a lot of things have limits on the size and alignment of the data pointed to by a single descriptor (e.g. an xHCI TRB, where the data buffer must not cross a 64KB boundary) - although they can also be relevant to non-scatter-gather devices if they just have limits on how big/aligned a single DMA transfer can be. Robin.