From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-6.0 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 1E58C7D04D for ; Tue, 16 Apr 2019 15:21:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727004AbfDPPVH (ORCPT ); Tue, 16 Apr 2019 11:21:07 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:57710 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726645AbfDPPVH (ORCPT ); Tue, 16 Apr 2019 11:21:07 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E670680D; Tue, 16 Apr 2019 08:21:06 -0700 (PDT) Received: from fuggles.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7E3B23F59C; Tue, 16 Apr 2019 08:21:02 -0700 (PDT) Date: Tue, 16 Apr 2019 16:21:00 +0100 From: Will Deacon To: Robin Murphy Cc: John Garry , Zhen Lei , Jean-Philippe Brucker , Joerg Roedel , Jonathan Corbet , linux-doc , Sebastian Ott , Gerald Schaefer , Martin Schwidefsky , Heiko Carstens , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Tony Luck , Fenghua Yu , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H . Peter Anvin" , David Woodhouse , iommu , linux-kernel , linux-s390 , linuxppc-dev , x86 , linux-ia64 , Hanjun Guo Subject: Re: [PATCH v5 1/6] iommu: add generic boot option iommu.dma_mode Message-ID: <20190416152100.GB4187@fuggles.cambridge.arm.com> References: <20190409125308.18304-1-thunder.leizhen@huawei.com> <20190409125308.18304-2-thunder.leizhen@huawei.com> <010d3cbd-ef74-ad21-c735-0af8b18955e6@huawei.com> <222946ee-adcc-1311-82a7-6afc9ffbc846@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <222946ee-adcc-1311-82a7-6afc9ffbc846@arm.com> User-Agent: Mutt/1.11.1+86 (6f28e57d73f2) () Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Fri, Apr 12, 2019 at 02:11:31PM +0100, Robin Murphy wrote: > On 12/04/2019 11:26, John Garry wrote: > > On 09/04/2019 13:53, Zhen Lei wrote: > > > +static int __init iommu_dma_mode_setup(char *str) > > > +{ > > > +    if (!str) > > > +        goto fail; > > > + > > > +    if (!strncmp(str, "passthrough", 11)) > > > +        iommu_default_dma_mode = IOMMU_DMA_MODE_PASSTHROUGH; > > > +    else if (!strncmp(str, "lazy", 4)) > > > +        iommu_default_dma_mode = IOMMU_DMA_MODE_LAZY; > > > +    else if (!strncmp(str, "strict", 6)) > > > +        iommu_default_dma_mode = IOMMU_DMA_MODE_STRICT; > > > +    else > > > +        goto fail; > > > + > > > +    pr_info("Force dma mode to be %d\n", iommu_default_dma_mode); > > > > What happens if the cmdline option iommu.dma_mode is passed multiple > > times? We get mutliple - possibily conflicting - prints, right? > > Indeed; we ended up removing such prints for the existing options here, > specifically because multiple messages seemed more likely to be confusing > than useful. > > > And do we need to have backwards compatibility, such that the setting > > for iommu.strict or iommu.passthrough trumps iommu.dma_mode, regardless > > of order? > > As above I think it would be preferable to just keep using the existing > options anyway. The current behaviour works out as: > > iommu.passthrough | Y | N > iommu.strict | x | Y N > ------------------|-------------|---------|-------- > MODE | PASSTHROUGH | STRICT | LAZY > > which seems intuitive enough that a specific dma_mode option doesn't add > much value, and would more likely just overcomplicate things for users as > well as our implementation. Agreed. We can't remove the existing options, and they do the job perfectly well so I don't see the need to add more options on top. Will