linux-kernel - Re: [PATCH] firmware: QCOM_SCM: Allow qcom_scm driver to be loadable as a permenent module

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Date:   Mon, 19 Jul 2021 12:00:05 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Andy Gross <agross@...nel.org>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <maz@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Vinod Koul <vkoul@...nel.org>,
        Kalle Valo <kvalo@...eaurora.org>,
        Maulik Shah <mkshah@...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>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH] firmware: QCOM_SCM: Allow qcom_scm driver to be loadable
 as a permenent module

On Fri, Jul 16, 2021 at 10:01 PM Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
> On Tue 06 Jul 23:53 CDT 2021, John Stultz wrote:
> > Allow the qcom_scm driver to be loadable as a permenent module.
> >
> > This still uses the "depends on QCOM_SCM || !QCOM_SCM" bit to
> > ensure that drivers that call into the qcom_scm driver are
> > also built as modules. While not ideal in some cases its the
> > only safe way I can find to avoid build errors without having
> > those drivers select QCOM_SCM and have to force it on (as
> > QCOM_SCM=n can be valid for those drivers).
> >
> > Reviving this now that Saravana's fw_devlink defaults to on,
> > which should avoid loading troubles seen before.
> >
>
> Are you (in this last paragraph) saying that all those who have been
> burnt by fw_devlink during the last months and therefor run with it
> disabled will have a less fun experience once this is merged?
>

I guess potentially. So way back when this was originally submitted,
some folks had trouble booting if it was set as a module due to it
loading due to the deferred_probe_timeout expiring.
My attempts to change the default timeout value to be larger ran into
trouble, but Saravana's fw_devlink does manage to resolve things
properly for this case.

But if folks are having issues w/ fw_devlink, and have it disabled,
and set QCOM_SCM=m they could still trip over the issue with the
timeout firing before it is loaded (especially if they are loading
modules from late mounted storage rather than ramdisk).

> (I'm picking this up, but I don't fancy the idea that some people are
> turning the boot process into a lottery)

Me neither, and I definitely think the deferred_probe_timeout logic is
way too fragile, which is why I'm eager for fw_devlink as it's a much
less racy approach to handling module loading dependencies.  So if you
want to hold on this, while any remaining fw_devlink issues get
sorted, that's fine.  But I'd also not cast too much ire at
fw_devlink, as the global probe timeout approach for handling optional
links isn't great, and we need a better solution.

thanks
-john