Re: [PATCH v2 5/5] firmware: QCOM_SCM: Allow qcom_scm driver to be loadable as a permenent module

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

On Fri, Jul 10, 2020 at 12:54 AM Will Deacon <will@kernel.org> wrote:
> On Thu, Jul 09, 2020 at 08:28:45PM -0700, John Stultz wrote:
> > On Thu, Jul 2, 2020 at 7:18 AM Will Deacon <will@kernel.org> wrote:
> > > On Thu, Jun 25, 2020 at 12:10:39AM +0000, John Stultz wrote:
> > > > diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> > > > index b510f67dfa49..714893535dd2 100644
> > > > --- a/drivers/iommu/Kconfig
> > > > +++ b/drivers/iommu/Kconfig
> > > > @@ -381,6 +381,7 @@ config SPAPR_TCE_IOMMU
> > > >  config ARM_SMMU
> > > >       tristate "ARM Ltd. System MMU (SMMU) Support"
> > > >       depends on (ARM64 || ARM || (COMPILE_TEST && !GENERIC_ATOMIC64)) && MMU
> > > > +     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 looks like a giant hack. Is there another way to handle this?
> >
> > Sorry for the slow response here.
> >
> > So, I agree the syntax looks strange (requiring a comment obviously
> > isn't a good sign), but it's a fairly common way to ensure drivers
> > don't get built in if they optionally depend on another driver that
> > can be built as a module.
> >   See "RFKILL || !RFKILL", "EXTCON || !EXTCON", or "USB_GADGET ||
> > !USB_GADGET" in various Kconfig files.
> >
> > I'm open to using a different method, and in a different thread you
> > suggested using something like symbol_get(). I need to look into it
> > more, but that approach looks even more messy and prone to runtime
> > failures. Blocking the unwanted case at build time seems a bit cleaner
> > to me, even if the syntax is odd.
>
> Maybe just split it out then, so that the ARM_SMMU entry doesn't have this,
> as that driver _really_ doesn't care about SoC details like this. In other
> words, add a new entry along the lines of:
>
>         config ARM_SMMU_QCOM_IMPL
>         default y
>         #if QCOM_SCM=m this can't be =y
>         depends on ARM_SMMU & (QCOM_SCM || !QCOM_SCM)
>
> and then have arm-smmu.h provide a static inline qcom_smmu_impl_init()
> which returns -ENODEV if CONFIG_ARM_SMMU_QCOM_IMPL=n and hack the Makefile
> so that we don't bother to compile arm-smmu-qcom.o in that case.
>
> Would that work?

I think this proposal still has problems with the directionality of the call.

The arm-smmu-impl.o calls to arm-smmu-qcom.o which calls qcom_scm.o
So if qcom_scm.o is part of a module, the calling code in
arm-smmu-qcom.o also needs to be a module, which means CONFIG_ARM_SMMU
needs to be a module.

I know you said the arm-smmu driver doesn't care about SoC details,
but the trouble is that currently the arm-smmu driver does directly
call the qcom-scm code. So it is a real dependency. However, if
QCOM_SCM is not configured, it calls stubs and that's ok.  In that
way, the "depends on QCOM_SCM || !QCOM_SCM" line actually makes sense.
It looks terrible because we're used to boolean logic, but it's
ternary.

Maybe can have the ARM_SMMU_QCOM_IMPL approach you suggest above, but
that just holds the issue out at arms length, because we're still
going to need to have:
  depends on ARM_SMMU_QCOM_IMPL || !ARM_SMMU_QCOM_IMPL
in the ARM_SMMU definition, which I suspect you're wanting to avoid.

Otherwise the only thing I can think of is a deeper reworking of the
arm-smmu-impl code so that the arm-smmu-qcom code probes itself and
registers its hooks with the arm-smmu core.
That way the arm-smmu driver would not directly call any SoC specific
code (and thus have no dependencies outward). But it's probably a fair
amount of churn vs the extra depends string.

thanks
-john