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

[John Stultz <john.stultz@xxxxxxxxxx>]

On Mon, Jul 13, 2020 at 1:41 PM Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Fri, Jul 10, 2020 at 03:21:53PM -0700, John Stultz wrote:
> > On Fri, Jul 10, 2020 at 12:54 AM Will Deacon <will@xxxxxxxxxx> 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@xxxxxxxxxx> 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.
>
> Yes, it looks ugly, but the part I really have issues with is that building
> QCOM_SCM=m and ARM_SMMU=y is perfectly fine if you don't run on an SoC
> with the qcom implementation. I don't see why we need to enforce things
> here beyond making sure that all selectable permutations _build_ and
> fail gracefully at runtime on the qcom SoC if SCM isn't available.

Ok. Thanks, that context/rationale makes sense to me now!  I'll dig in
and see if I can figure out the runtime get_symbol handling you
suggested for the scm callout.

Thanks again for the feedback!
-john