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=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 C5BE3C07E9C for ; Fri, 9 Jul 2021 13:57:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AA9E4613B6 for ; Fri, 9 Jul 2021 13:57:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231880AbhGIN7m (ORCPT ); Fri, 9 Jul 2021 09:59:42 -0400 Received: from foss.arm.com ([217.140.110.172]:53552 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231454AbhGIN7m (ORCPT ); Fri, 9 Jul 2021 09:59:42 -0400 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 7BF5FED1; Fri, 9 Jul 2021 06:56:58 -0700 (PDT) Received: from [10.57.35.192] (unknown [10.57.35.192]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 000A63F694; Fri, 9 Jul 2021 06:56:52 -0700 (PDT) Subject: Re: [PATCH v2 0/3] iommu: Enable non-strict DMA on QCom SD/MMC To: Joerg Roedel , Doug Anderson Cc: Ulf Hansson , Linux Doc Mailing List , Peter Zijlstra , linux-pci@vger.kernel.org, Konrad Dybcio , Thierry Reding , Joel Fernandes , Rajat Jain , Will Deacon , Rob Clark , Saravana Kannan , Jonathan Corbet , quic_c_gdjako@quicinc.com, Linux ARM , Viresh Kumar , Veerabhadrarao Badiganti , "Paul E. McKenney" , linux-arm-msm , Bjorn Helgaas , Sonny Rao , Vlastimil Babka , Randy Dunlap , Linux MMC List , Adrian Hunter , LKML , "list@263.net:IOMMU DRIVERS" , Joerg Roedel , iommu@lists.linux-foundation.org, Andrew Morton , "Maciej W. Rozycki" References: <20210624171759.4125094-1-dianders@chromium.org> From: Robin Murphy Message-ID: Date: Fri, 9 Jul 2021 14:56:47 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-arm-msm@vger.kernel.org On 2021-07-08 09:08, Joerg Roedel wrote: > On Wed, Jul 07, 2021 at 01:00:13PM -0700, Doug Anderson wrote: >> a) Nothing is inherently broken with my current approach. >> >> b) My current approach doesn't make anybody terribly upset even if >> nobody is totally in love with it. > > Well, no, sorry :) > > I don't think it is a good idea to allow drivers to opt-out of the > strict-setting. This is a platform or user decision, and the driver > should accept whatever it gets. > > So the real question is still why strict is the default setting and how > to change that. Or document for the users that want performance how to > change the setting, so that they can decide. As I mentioned before, conceptually I think this very much belongs in sysfs as a user decision. We essentially have 4 levels of "strictness": 1: DMA domain with bounce pages 2: DMA domain 3: DMA domain with flush queue 4: Identity domain The "make this device go faster because I trust it" use-case is why we have the sysfs interface to switch between 2 and 4, so it's entirely logical to have the intermediate option as well for when 3 is "faster" enough while still giving a bit more peace of mind than full-on bypass. Making it a platform-specific decision that's hidden in a driver - arm-smmu-qcom can be considered a dumping ground of detailed platform knowledge ;) - happens to work as a reasonable compromise for this specific case, but I concur that it could be viewed as setting a precedent for other cases which we definitely aren't as reasonable. I've been thinking about the sysfs thing some more, and since it's Friday afternoon and I can't concentrate on what I'm supposed to be doing anyway, let's see how far I can get by Monday... Robin.