Re: [PATCH] mfd: kempld-core: Mark kempld-acpi_table as __maybe_unused

[Lee Jones <lee.jones@linaro.org>]

On Mon, 05 Oct 2020, Michael Brunner wrote:

> On Fri, 2020-10-02 at 08:01 +0100, Lee Jones wrote:
> > On Thu, 01 Oct 2020, Michael Brunner wrote:
> > 
> > > The Intel 0-DAY CI Kernel Test Service reports an unused variable
> > > warning when compiling with clang for PowerPC:
> > > 
> > > > > drivers/mfd/kempld-core.c:556:36: warning: unused variable
> > > > > 'kempld_acpi_table' [-Wunused-const-variable]
> > >    static const struct acpi_device_id kempld_acpi_table[] = {
> > > 
> > > The issue can be fixed by marking kempld_acpi_table as
> > > __maybe_unused.
> > > 
> > > Fixes: e8299c7313af ("[PATCH] mfd: Add ACPI support to Kontron PLD
> > > driver")
> > > 
> > > Reported-by: kernel test robot <lkp@intel.com>
> > > Signed-off-by: Michael Brunner <michael.brunner@kontron.com>
> > > ---
> > >  drivers/mfd/kempld-core.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/mfd/kempld-core.c b/drivers/mfd/kempld-core.c
> > > index 1dfe556df038..273481dfaad4 100644
> > > --- a/drivers/mfd/kempld-core.c
> > > +++ b/drivers/mfd/kempld-core.c
> > > @@ -553,7 +553,7 @@ static int kempld_remove(struct platform_device
> > > *pdev)
> > >  	return 0;
> > >  }
> > >  
> > > -static const struct acpi_device_id kempld_acpi_table[] = {
> > > +static const struct acpi_device_id __maybe_unused
> > > kempld_acpi_table[] = {
> > >  	{ "KEM0001", (kernel_ulong_t)&kempld_platform_data_generic },
> > >  	{}
> > >  };
> > 
> > This is not the right fix.  Better just to compile it out completely
> > in these circumstances.  I already have a fix for this in soak.
> 
> Ok - thank you for the other fix you submitted!
> 
> But just out of curiosity - in process/coding-style.rst is written that
> __maybe_unused should be preferred over wrapping in preprocessor
> conditionals, if a function or variable may potentially go unused in a
> particular configuration. So why is my patch not the right one here? At
> least in my tests it seemed to solve the issue.

It's a bone of contention for sure.  In these kinds of scenarios
(i.e. ACPI and OF tables) it is way more common to wrap them:

$ git grep -B3 'acpi_device_id\|of_device_id' | grep 'CONFIG_ACPI\|CONFIG_OF' | wc -l
596
$ git grep -B3 'acpi_device_id\|of_device_id' | grep __maybe_unused | wc -l
63

Parsing them out completely, also has the benefit of saving space,
reducing the size of the finalised binary.

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