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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Date:   Fri, 10 Jul 2020 08:54:12 +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 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?

Will