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

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Message-ID: <20200715123854.GF3165313@dell>
Date:   Wed, 15 Jul 2020 13:38:54 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Denis Sadykov <denis.m.sadykov@...el.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paul Diefenbaugh <paul.s.diefenbaugh@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Dominik Brodowski <linux@...do.de>
Subject: Re: [PATCH 09/13] cpufreq: acpi-cpufreq: Remove unused ID structs

On Wed, 15 Jul 2020, Robin Murphy wrote:

> On 2020-07-15 13:16, Lee Jones wrote:
> > On Wed, 15 Jul 2020, Rafael J. Wysocki wrote:
> > 
> > > On Wed, Jul 15, 2020 at 1:50 PM Lee Jones <lee.jones@...aro.org> wrote:
> > > > 
> > > > 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.
> > > 
> > > I'm not sure why that would be the case?
> > 
> > Nor me.  In fact, take a look at my latest email.  I think I just
> > proved out that it's not broken.  The warning is valid and
> > MODULE_DEVICE_TABLE() appears to work just as it should.
> 
> I won't claim to be an expert at all, but...
> 
> For !MODULE, MODULE_DEVICE_TABLE() expands to nothing, so after
> preprocessing the static variable is literally unreferenced.
> 
> Otherwise, MODULE_DEVICE_TABLE() emits an extern declaration of another
> variable which is defined via the "alias" attribute to refer to the symbol
> of the static variable. Thus the compiler presumably has to treat it as
> potentially accessible from other compilation units such that it can't be
> optimised away.

Right.  That's essentially how I'm reading it.

> > > > Removing boiler-plate is good, but not at the expense of obfuscation.
> > > 
> > > I'm not following you here to be honest.
> > 
> > Never mind.  It's no longer important.
> > 
> > > BTW, I'm wondering if removing the "static" modifier from the
> > > definitions of the structures in question makes the warnings you want
> > > to get rid of go away.
> > 
> > I'm sure that it would.  But that just alludes to the fact that the
> > tables may be in use elsewhere, which in the case of !MODULE is
> > untrue.  That's probably more of a hack than using __maybe_unused.
> 
> Right, that just ends up with someone sending another patch changing it back
> to shut up "variable foo was not declared, should it be static?" warnings
> from Sparse ;)

Exactly.

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