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: <CALAqxLW13=cvTX3ghskb9uG_YoVh7kvp8UQGUB8mVDGYXHWpVQ@mail.gmail.com>
Date:   Thu, 29 Oct 2020 18:02:14 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Will Deacon <will@...nel.org>, Maulik Shah <mkshah@...eaurora.org>,
        Jason Cooper <jason@...edaemon.net>,
        Saravana Kannan <saravanak@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Lina Iyer <ilina@...eaurora.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        iommu@...ts.linux-foundation.org, Andy Gross <agross@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH v2 5/5] firmware: QCOM_SCM: Allow qcom_scm driver to be
 loadable as a permenent module

On Wed, Oct 28, 2020 at 7:51 AM Robin Murphy <robin.murphy@....com> wrote:
> Hmm, perhaps I'm missing something here, but even if the config options
> *do* line up, what prevents arm-smmu probing before qcom-scm and
> dereferencing NULL in qcom_scm_qsmmu500_wait_safe_toggle() before __scm
> is initialised?

Oh man, this spun me on a "wait, but how does it all work!" trip. :)

So in the non-module case, the qcom_scm driver is a subsys_initcall
and the arm-smmu is a module_platform_driver, so the ordering works
out.

In the module case, the arm-smmu code isn't loaded until the qcom_scm
driver finishes probing due to the symbol dependency handling.

To double check this, I added a big msleep at the top of the
qcom_scm_probe to try to open the race window you described, but the
arm_smmu_device_probe() doesn't run until after qcom_scm_probe
completes.

So at least as a built in / built in, or a module/module case its ok.
And in the case where arm-smmu is a module and qcom_scm is built in
that's ok too.

Its just the case my patch is trying to prevent is where arm-smmu is
built in, but qcom_scm is a module that it can't work (due to build
errors in missing symbols,  or if we tried to use function pointers to
plug in the qcom_scm - the lack of initialization ordering).

Hopefully that addresses your concern? Let me know if I'm still
missing something.

thanks
-john