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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Mon, 11 Oct 2021 08:08:44 +0200
Message-ID: <CAK8P3a3JwQP1b0KeLRN0UCMmzFn3+gY2oexYUwGyt2bOqC0P4A@mail.gmail.com>
Subject: Re: [PATCH] iommu: fix ARM_SMMU vs QCOM_SCM compilation
To:     Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Kalle Valo <kvalo@codeaurora.org>,
        Thierry Reding <treding@nvidia.com>,
        Andy Gross <agross@kernel.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        "open list:IOMMU DRIVERS" <iommu@lists.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>
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On Mon, Oct 11, 2021 at 6:11 AM Dmitry Baryshkov
<dmitry.baryshkov@linaro.org> wrote:
> On Sun, 10 Oct 2021 at 20:42, Arnd Bergmann <arnd@arndb.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.

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.

        Arnd