From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753644AbeDKOdW (ORCPT ); Wed, 11 Apr 2018 10:33:22 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:38646 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753347AbeDKOdS (ORCPT ); Wed, 11 Apr 2018 10:33:18 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 49914603AF Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=none smtp.mailfrom=okaya@codeaurora.org Subject: Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for scatter-gather buffers To: Robin Murphy , amd-gfx@lists.freedesktop.org, timur@codeaurora.org, sulrich@codeaurora.org Cc: 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 , =?UTF-8?Q?Christian_K=c3=b6nig?= , linux-arm-kernel@lists.infradead.org, iommu@lists.linux-foundation.org, hch@lst.de References: <1523394001-4615-1-git-send-email-okaya@codeaurora.org> <32b82296-bba5-b5f1-266b-45c1ed66da94@arm.com> From: Sinan Kaya Message-ID: <1a1d0069-c8d3-cc0d-cf1c-8e90e4a2b0d7@codeaurora.org> Date: Wed, 11 Apr 2018 10:33:14 -0400 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <32b82296-bba5-b5f1-266b-45c1ed66da94@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/11/2018 8:03 AM, 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. Sure, I'll take a better fix if there is one. > >> 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). > > As a short-term fix, at least do something like what i915 does and constrain the table allocation to the desired segment size as well, so things remain self-consistent. But still never claim that faking a hardware constraint as a workaround for a driver shortcoming is "the right thing to do" ;) You are asking for something like this from here, right? https://elixir.bootlin.com/linux/v4.16.1/source/drivers/gpu/drm/i915/i915_gem_dmabuf.c#L58 ret = sg_alloc_table(st, obj->mm.pages->nents, GFP_KERNEL); if (ret) goto err_free; src = obj->mm.pages->sgl; dst = st->sgl; for (i = 0; i < obj->mm.pages->nents; i++) { sg_set_page(dst, sg_page(src), src->length, 0); dst = sg_next(dst); src = sg_next(src); } This seems to allocate the scatter gather list and fill it in manually before passing it to dma_map_sg(). I'll give it a try. Just double checking. > > Robin. > >> Signed-off-by: Sinan Kaya -- Sinan Kaya Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.