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

[John Stultz <john.stultz@linaro.org>]

On Mon, Nov 16, 2020 at 7:59 AM Thierry Reding <thierry.reding@gmail.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