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

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Date:   Wed, 15 Jul 2020 12:50:29 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.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, 15 Jul 2020, Rafael J. Wysocki wrote:

> On Wed, Jul 15, 2020 at 1:34 PM Lee Jones <lee.jones@...aro.org> wrote:
> >
> > On Wed, 15 Jul 2020, Rafael J. Wysocki wrote:
> >
> > > 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?
> >
> > It looks like they're only unused when !MODULE,
> 
> OK
> 
> > in which case optimising them away would be the correct thing to do, no?

It would be good if someone with a little more knowledge could provide
a second opinion though.  I would think (hope) that the compiler would
be smart enough to see when its actually in use.  After all, it is the
compiler that places the information into the device table.

If that is not the case, then the MODULE_DEVICE_TABLE() magic is
broken and will need fixing.  Removing boiler-plate is good, but not
at the expense of obfuscation.

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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