From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E85A0C2D0C0 for ; Mon, 23 Dec 2019 11:29:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C643F20828 for ; Mon, 23 Dec 2019 11:29:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726783AbfLWL3d (ORCPT ); Mon, 23 Dec 2019 06:29:33 -0500 Received: from foss.arm.com ([217.140.110.172]:43030 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726679AbfLWL3c (ORCPT ); Mon, 23 Dec 2019 06:29:32 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9F7EC328; Mon, 23 Dec 2019 03:29:31 -0800 (PST) Received: from [192.168.1.123] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EE2493F68F; Mon, 23 Dec 2019 03:29:25 -0800 (PST) Subject: Re: [PATCH 0/8] Convert the intel iommu driver to the dma-iommu api To: Jani Nikula , Tom Murphy , iommu@lists.linux-foundation.org Cc: Joonas Lahtinen , Rodrigo Vivi , David Airlie , Daniel Vetter , Joerg Roedel , Will Deacon , Marek Szyprowski , Kukjin Kim , Krzysztof Kozlowski , David Woodhouse , Lu Baolu , Andy Gross , Bjorn Andersson , Matthias Brugger , Rob Clark , Heiko Stuebner , Gerald Schaefer , Thierry Reding , Jonathan Hunter , Jean-Philippe Brucker , Alex Williamson , Cornelia Huck , Julien Grall , Marc Zyngier , Eric Auger , Thomas Gleixner , intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-samsung-soc@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mediatek@lists.infradead.org, linux-rockchip@lists.infradead.org, linux-s390@vger.kernel.org, linux-tegra@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org References: <20191221150402.13868-1-murphyt7@tcd.ie> <87blrzwcn8.fsf@intel.com> From: Robin Murphy Message-ID: <432d306c-fe9f-75b2-f0f7-27698f1467ad@arm.com> Date: Mon, 23 Dec 2019 11:29:17 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: <87blrzwcn8.fsf@intel.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 8bit Sender: linux-samsung-soc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-samsung-soc@vger.kernel.org On 2019-12-23 10:37 am, Jani Nikula wrote: > On Sat, 21 Dec 2019, Tom Murphy wrote: >> This patchset converts the intel iommu driver to the dma-iommu api. >> >> While converting the driver I exposed a bug in the intel i915 driver >> which causes a huge amount of artifacts on the screen of my >> laptop. You can see a picture of it here: >> https://github.com/pippy360/kernelPatches/blob/master/IMG_20191219_225922.jpg >> >> This issue is most likely in the i915 driver and is most likely caused >> by the driver not respecting the return value of the >> dma_map_ops::map_sg function. You can see the driver ignoring the >> return value here: >> https://github.com/torvalds/linux/blob/7e0165b2f1a912a06e381e91f0f4e495f4ac3736/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c#L51 >> >> Previously this didn’t cause issues because the intel map_sg always >> returned the same number of elements as the input scatter gather list >> but with the change to this dma-iommu api this is no longer the >> case. I wasn’t able to track the bug down to a specific line of code >> unfortunately. >> >> Could someone from the intel team look at this? > > Let me get this straight. There is current API that on success always > returns the same number of elements as the input scatter gather > list. You propose to change the API so that this is no longer the case? No, the API for dma_map_sg() has always been that it may return fewer DMA segments than nents - see Documentation/DMA-API.txt (and otherwise, the return value would surely be a simple success/fail condition). Relying on a particular implementation behaviour has never been strictly correct, even if it does happen to be a very common behaviour. > A quick check of various dma_map_sg() calls in the kernel seems to > indicate checking for 0 for errors and then ignoring the non-zero return > is a common pattern. Are you sure it's okay to make the change you're > proposing? Various code uses tricks like just iterating the mapped list until the first segment with zero sg_dma_len(). Others may well simply have bugs. Robin. > Anyway, due to the time of year and all, I'd like to ask you to file a > bug against i915 at [1] so this is not forgotten, and please let's not > merge the changes before this is resolved. > > > Thanks, > Jani. > > > [1] https://gitlab.freedesktop.org/drm/intel/issues/new > >