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

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Date:   Mon, 11 Oct 2021 12:09:21 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     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, 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.

>
> 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.

-- 
With best wishes
Dmitry