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

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Message-ID: <CAKv+Gu8xY-RvJKTKfv-e0qonL4oECDBK+ppxztvh9BgdpG8CUg@mail.gmail.com>
Date:   Sun, 22 Apr 2018 11:57:51 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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 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?