linux-kernel - Re: [PATCH] ACPI / bus: ignore rather than fail bus driver registrations on non-ACPI boot

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Date:   Mon, 23 Apr 2018 09:39:32 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>, Mark Brown <broonie@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bill Fletcher <bill.fletcher@...aro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ACPI / bus: ignore rather than fail bus driver registrations on non-ACPI boot

On Sunday, April 22, 2018 2:34:17 PM CEST Ard Biesheuvel wrote:
> On 22 April 2018 at 11:57, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> > On 22 April 2018 at 11:27, Rafael J. Wysocki <rafael@...nel.org> wrote:
> >> On Thu, Apr 19, 2018 at 6:58 PM, Ard Biesheuvel
> >> <ard.biesheuvel@...aro.org> wrote:
> >>> When building ACPI bus drivers such as button.ko into the core kernel,
> >>> other drivers that depend on its symbols are loadable even when booting
> >>> with ACPI disabled. For instance, nouveau.ko has a link time dependency
> >>> on acpi_lid_open() on ACPI capable kernels, and calls it regardless of
> >>> whether the system booted via ACPI.
> >>>
> >>> However, when building button.ko as a module, it will refuse to load if
> >>> the system did not boot in ACPI mode, which subsequently prevents the
> >>> nouveau driver from loading as well, resulting in broken graphics.
> >>>
> >>> Given that returning an error from an initcall() is ignored for drivers
> >>> that are built into the kernel,
> >>
> >> Which makes sense, because they are present in the kernel anyway.
> >>
> >>> let's align the module case with this,
> >>> and not return an error when registering an ACPI bus driver on a system
> >>> that did not boot via ACPI.
> >>
> >> But why is loading a module that's never going to be used actually OK?
> >>
> >> Isn't this a problem with the assumptions made by the nouveau driver
> >> that need not be met depending on what configuration the kernel is run
> >> in?
> >>
> >> Honestly, it doesn't appear quite right to try to change the rest of
> >> the kernel to follow the nouveau's expectations.
> >>
> >
> > I don't disagree here, I am just unsure whether other options are any better.
> >
> > I think the alternative is to make acpi_lid_open() a non-modular
> > function of the ACPI core that invokes the button ACPI bus driver if
> > it was loaded, and always returns false otherwise. Would that work for
> > you?
> 
> BTW not only nouveau invokes acpi_lid_open(), i915 does it as well.

Clearly, the design is somewhat ad-hoc here.

It looks like using module_acpi_driver() in button.c is a mistake given the
dependencies.  The module initialization should ignore the
acpi_bus_register_driver() failure in there, but there's no reason for the
other ACPI driver modules to be affected by that.

And if you change that, please add a comment referring to the dependencies
in question.