linux-kernel - Re: [PATCH 09/13] cpufreq: acpi-cpufreq: Remove unused ID structs

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Date:   Wed, 15 Jul 2020 13:27:28 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Lee Jones <lee.jones@...aro.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Andy Grover <andrew.grover@...el.com>,
        Paul Diefenbaugh <paul.s.diefenbaugh@...el.com>,
        Dominik Brodowski <linux@...do.de>,
        Denis Sadykov <denis.m.sadykov@...el.com>
Subject: Re: [PATCH 09/13] cpufreq: acpi-cpufreq: Remove unused ID structs

On Wed, Jul 15, 2020 at 5:27 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 15-07-20, 08:54, Viresh Kumar wrote:
> > On 14-07-20, 22:03, Lee Jones wrote:
> > > On Tue, 14 Jul 2020, Rafael J. Wysocki wrote:
> > >
> > > > On Tue, Jul 14, 2020 at 4:51 PM Lee Jones <lee.jones@...aro.org> wrote:
> > > > >
> > > > > Can't see them being used anywhere and the compiler doesn't complain
> > > > > that they're missing, so ...
> > > >
> > > > Aren't they needed for automatic module loading in certain configurations?
> > >
> > > Any idea how that works, or where the code is for that?
> >
> > The MODULE_DEVICE_TABLE() thingy creates a map of vendor-id,
> > product-id that the kernel keeps after boot (and so there is no static
> > reference of it for the compiler), later when a device is hotplugged
> > into the kernel it refers to the map to find the related driver for it
> > and loads it if it isn't already loaded.
> >
> > This has some of it, search for MODULE_DEVICE_TABLE() in it.
> > Documentation/driver-api/usb/hotplug.rst
>
> And you just need to add __maybe_unused to them to suppress the
> warning.

Wouldn't that cause the compiler to optimize them away if it doesn't
see any users?