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

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Message-ID: <20200713204133.GA3731@willie-the-truck>
Date:   Mon, 13 Jul 2020 21:41:34 +0100
From:   Will Deacon <will@...nel.org>
To:     John Stultz <john.stultz@...aro.org>
Cc:     lkml <linux-kernel@...r.kernel.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>,
        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>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        iommu@...ts.linux-foundation.org, linux-gpio@...r.kernel.org
Subject: Re: [PATCH v2 5/5] firmware: QCOM_SCM: Allow qcom_scm driver to be
 loadable as a permenent module

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@...nel.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@...nel.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.

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.

Will