linux-kernel - Re: [PATCH v6 3/3] firmware: QCOM_SCM: Allow qcom_scm driver to be loadable as a permenent module

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Date:   Mon, 16 Nov 2020 11:40:55 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <maz@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Vinod Koul <vkoul@...nel.org>,
        Kalle Valo <kvalo@...eaurora.org>,
        Maulik Shah <mkshah@...eaurora.org>,
        Lina Iyer <ilina@...eaurora.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Todd Kjos <tkjos@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jon Hunter <jonathanh@...dia.com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH v6 3/3] firmware: QCOM_SCM: Allow qcom_scm driver to be
 loadable as a permenent module

On Mon, Nov 16, 2020 at 7:59 AM Thierry Reding <thierry.reding@...il.com> wrote:
>
> On Fri, Nov 06, 2020 at 04:27:10AM +0000, John Stultz wrote:
> > diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> > index 04878caf6da49..c64d7a2b65134 100644
> > --- a/drivers/iommu/Kconfig
> > +++ b/drivers/iommu/Kconfig
> > @@ -248,6 +248,7 @@ config SPAPR_TCE_IOMMU
> >  config ARM_SMMU
> >       tristate "ARM Ltd. System MMU (SMMU) Support"
> >       depends on ARM64 || ARM || (COMPILE_TEST && !GENERIC_ATOMIC64)
> > +     depends on QCOM_SCM || !QCOM_SCM #if QCOM_SCM=m this can't be =y
> >       select IOMMU_API
> >       select IOMMU_IO_PGTABLE_LPAE
> >       select ARM_DMA_USE_IOMMU if ARM
>
> This, in conjunction with deferred probe timeout, causes mayhem on
> Tegra186. The problem, as far as I can tell, is that there are various
> devices that are hooked up to the ARM SMMU, but if ARM SMMU ends up
> being built as a loadable module, then those devices will initialize
> without IOMMU support (because deferred probe will timeout before the
> ARM SMMU module can be loaded from the root filesystem).
>
> Unfortunately, the ARM SMMU module will eventually end up being loaded
> once the root filesystem has been mounted (for example via SDHCI or
> Ethernet, both with using just plain, non-IOMMU-backed DMA API) and then
> initialize, configuring as "fault by default", which then results from a
> slew of SMMU faults from all the devices that have previously configured
> themselves without IOMMU support.

Oof. My apologies for the trouble. Thanks so much for the report.

Out of curiosity, does booting with deferred_probe_timeout=30 avoid
the issue for you?

thanks
-john