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=-16.9 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,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 0A419C433F5 for ; Wed, 15 Sep 2021 20:13:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D16D160E54 for ; Wed, 15 Sep 2021 20:13:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231690AbhIOUOX (ORCPT ); Wed, 15 Sep 2021 16:14:23 -0400 Received: from foss.arm.com ([217.140.110.172]:58648 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231487AbhIOUOW (ORCPT ); Wed, 15 Sep 2021 16:14:22 -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 BCD926D; Wed, 15 Sep 2021 13:13:02 -0700 (PDT) Received: from [10.57.15.112] (unknown [10.57.15.112]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5518B3F59C; Wed, 15 Sep 2021 13:13:01 -0700 (PDT) Subject: Re: [PATCH] ACPI/IORT: Add 'smmu=off' command line option To: Yao Hongbo Cc: zhangliguang@linux.alibaba.com, alikernel-developer@linux.alibaba.com, will@kernel.org, lorenzo.pieralisi@arm.com, guohanjun@huawei.com, sudeep.holla@arm.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org References: <20210915120046.62936-1-yaohongbo@linux.alibaba.com> From: Robin Murphy Message-ID: Date: Wed, 15 Sep 2021 21:12:55 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 MIME-Version: 1.0 In-Reply-To: <20210915120046.62936-1-yaohongbo@linux.alibaba.com> 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-kernel@vger.kernel.org On 2021-09-15 13:00, Yao Hongbo wrote: > Add a generic command line option to disable arm smmu drivers. > iommu.passthrough can only bypass the IOMMU for DMA, but > sometimes we need to ignore all available SMMUs. Please elaborate on "sometimes" - what's the use-case for this which can't already be achieved by other means like module_blacklist, switching kernel images, ACPI table overrides, and so on? > This patch is only used for acpi on arm64. And yet the documentation implies that it works for all arm64 systems :/ And furthermore, why? If it's genuinely useful to disable SMMUs on ACPI-based systems, surely it must be equally useful to disable SMMUs on DT-based systems? > Signed-off-by: Yao Hongbo > --- > Documentation/admin-guide/kernel-parameters.txt | 4 ++++ > drivers/acpi/arm64/iort.c | 18 +++++++++++++++++- > 2 files changed, 21 insertions(+), 1 deletion(-) > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > index 91ba391f..6cffd91 100644 > --- a/Documentation/admin-guide/kernel-parameters.txt > +++ b/Documentation/admin-guide/kernel-parameters.txt > @@ -5198,6 +5198,10 @@ > smart2= [HW] > Format: [,[,...,]] > > + smmu= [ARM64] > + Format: {off} > + off: Disable arm smmu driver. There are at least two Arm SMMU drivers; as a user I might be wondering about the ambiguity there. > + > smsc-ircc2.nopnp [HW] Don't use PNP to discover SMC devices > smsc-ircc2.ircc_cfg= [HW] Device configuration I/O port > smsc-ircc2.ircc_sir= [HW] SIR base I/O port > diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c > index 3b23fb7..70f92e7 100644 > --- a/drivers/acpi/arm64/iort.c > +++ b/drivers/acpi/arm64/iort.c > @@ -40,6 +40,22 @@ struct iort_fwnode { > static LIST_HEAD(iort_fwnode_list); > static DEFINE_SPINLOCK(iort_fwnode_lock); > > +static bool acpi_smmu_disabled; > + > +static int __init acpi_smmu_parse(char *str) > +{ > + if (!str) > + return -EINVAL; > + > + if (!strncmp(str, "off", 3)) { > + acpi_smmu_disabled = true; > + pr_info("SMMU disabled\n"); > + } > + > + return 0; > +} > +__setup("smmu=", acpi_smmu_parse); > + > /** > * iort_set_fwnode() - Create iort_fwnode and use it to register > * iommu data in the iort_fwnode_list > @@ -1596,7 +1612,7 @@ static void __init iort_init_platform_devices(void) > iort_enable_acs(iort_node); > > ops = iort_get_dev_cfg(iort_node); > - if (ops) { > + if (ops && !acpi_smmu_disabled) { This will also effectively disable PMCGs, which is an undocumented side-effect, and not necessarily desirable - PMCG functionality is largely orthogonal, and may potentially be used to monitor traffic even when the associated SMMU instance is disabled. TBH there's really nothing SMMU-specific about this at all. I know I've had a number of debugging situations where it would have been handy to have a way to prevent a specific built-in driver from binding automatically during boot, so if you really have got a compelling reason to need it for SMMU, then you could still implement it generically in a way that everyone could benefit from. Thanks, Robin. > fwnode = acpi_alloc_fwnode_static(); > if (!fwnode) > return; >