linux-kernel - Re: [PATCH] iommu: fix ARM_SMMU vs QCOM

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Date:   Mon, 11 Oct 2021 11:56:54 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Thierry Reding <treding@...dia.com>,
        Andy Gross <agross@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH] iommu: fix ARM_SMMU vs QCOM_SCM compilation

On Mon, Oct 11, 2021 at 11:10 AM Dmitry Baryshkov
<dmitry.baryshkov@...aro.org> wrote:
>
> On Mon, 11 Oct 2021 at 09:09, Arnd Bergmann <arnd@...db.de> wrote:
> >
> > On Mon, Oct 11, 2021 at 6:11 AM Dmitry Baryshkov
> > <dmitry.baryshkov@...aro.org> wrote:
> > > On Sun, 10 Oct 2021 at 20:42, Arnd Bergmann <arnd@...db.de> wrote:
> > >
> > > The patch seems correct, but it becomes overcomplicated. What about:
> > > - restoring QCOM_SCM stubs
> >
> > The stubs are what has led to the previous bugs in this area to often
> > go unnoticed for too long, as illustrated by your suggestion
> >
> > > - making ARM_SMMU select QCOM_SCM if ARM_SMMU_QCOM
> >
> > I assume you meant "select QCOM_SCM if ARCH_QCOM",
> > after we stop using ARM_SMMU_QCOM?
> >
> > > This would have almost the same result as with your patch, but without
> > > extra ARM_SMMU_QCOM Kconfig symbol.
> >
> > The "almost" is the problem: consider the case of
> >
> > CONFIG_ARM=y
> > CONFIG_COMPILE_TEST=y
> > CONFIG_ARCH_QCOM=n
> > CONFIG_ARM_SMMU=y
> > CONFIG_DRM_MSM=m
> > CONFIG_QCOM_SCM=m (selected by DRM_MSM)
> >
> > The stubs here lead to ARM_SMMU linking against the QCOM_SCM
> > driver from built-in code, which fails because QCOM_SCM itself
> > is a loadable module.
>
> I see. The idealist in me wishes to change my suggestion to
> 'select QCOM_SCM if ARCH_QCOM || COMPILE_TEST'
> but I have the subtle feeling that this also might fail somehow.

I think that would actually work, but it has the nasty side-effect
that simply flipping 'CONFIG_COMPILE_TEST' changes what
the kernel does, rather than just hiding or unhiding additional
options.

> > We can move the "select QCOM_SCM" in the ARM_SMMU_QCOM
> > symbol if we make that a tristate though, if you want to separate it
> > a little more.
>
> This would complicate things a bit, as we would no longer be able to
> use 'arm-smmu-$(CONFIG_ARM_SMMU_QCOM) +=' construct.

I'm fairly sure we could still use that, Kbuild is smart enough
to include both 'file-m +=' and 'file-y += ' in 'file.ko', see
scripts/Makefile.lib:

# If $(foo-objs), $(foo-y), $(foo-m), or $(foo-) exists, foo.o is a
composite object
multi-obj-y := $(call multi-search, $(obj-y), .o, -objs -y)
multi-obj-m := $(call multi-search, $(obj-m), .o, -objs -y -m)
multi-obj-ym := $(multi-obj-y) $(multi-obj-m)

# Replace multi-part objects by their individual parts,
# including built-in.a from subdirectories
real-obj-y := $(call real-search, $(obj-y), .o, -objs -y)
real-obj-m := $(call real-search, $(obj-m), .o, -objs -y -m)

What doesn't work is having a built-in driver in a directory that is
guarded with a =m symbol, or including a =m object into a =y
module.

        Arnd