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=-6.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,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 15F88C07E99 for ; Fri, 9 Jul 2021 13:58:47 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id DD9AF613B6 for ; Fri, 9 Jul 2021 13:58:46 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DD9AF613B6 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date:Message-ID:From: References:Cc:To:Subject:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=usj6JzThruoq/akwsRLZ2BxPzkxc/Xerxezqk3/pC68=; b=ju0sPbAsKnE6gaNGloyNO0/5n9 wOHeu71naijtFRk5gtOh4f4DGl+IFenhXn6p3oWSXrhbsGigTXtaiUy0UfefnGO47ecDZrFivQcRj tcwrPFrC5WRWKNT6/ZCvscMWuJgE15SYlTkvkh+AZTWCUjJUzzXBFLI3PF7AgusFx9pMxdpaHsrm1 WF0Io6vKDEREzQf1tYmozVJFyefTcsgeC/EyS429mGsrNxRyQqlh04sCUmVi/kuAYZeXqoWny7hUh Y4iL+iHXN3px1NK2pX6jX0HDfhtGC3H4KCrIAVdOYnDrAOKk491FV6ezmRozJpxPDPxB/BgaCGHd6 V+mBzDtg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m1r06-001slA-DJ; Fri, 09 Jul 2021 13:57:06 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m1r02-001skH-G8 for linux-arm-kernel@lists.infradead.org; Fri, 09 Jul 2021 13:57:04 +0000 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-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210709_065702_691002_4602EBF0 X-CRM114-Status: GOOD ( 24.46 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.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. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel