Build failure: -Wno-unused-const-variable DNE on old GCC

[Michael Ellerman]

On Thu, 2016-01-07 at 18:07 -0800, Brian Norris wrote:
> On Fri, Jan 08, 2016 at 12:31:54PM +1100, Ian Munsie wrote:
> > Excerpts from Brian Norris's message of 2016-01-08 10:02:25 +1100:
> > > >  - It forces cxl developers to a higher standard. cxl has already had
> > > >    more than it's fair share of incredibly difficult to debug issues,
> > > >    so any way we can reduce the risk of errors going in makes our lives
> > > >    (and our end-users lives) better.
> > > 
> > > One problem with this point: not all warnings are under the purview of
> > > cxl developers. For instance, if I turn up warning verbosity (W=1), then
> 
> (BTW, I think most of these files are currently clean with W=1, but not W=2)

> > > the *header* files start producing plenty of warnings. Should this break
> > > the build? Your code didn't change, and you can't fix those errors.
> > 
> > That's a good point, but the specific warnings that we suppressed in the
> > new compiler are in drivers/misc/cxl/cxl.h, which is an internal header
> > that should only ever be included by the cxl driver. We do have some
> > headers elsewhere which are included by other drivers, the generic ppc
> > architecture code and userspace, but these are all warning free and
> > won't be affected by the -Werror when included from elsewhere.
> 
> I was referring to when extra warnings are turned on, not just the
> default. So this fails spectacularly:
> 
>   $ export ARCH=powerpc
>   $ make ppc64_defconfig
>   $ make drivers/misc/cxl/api.o W=2
>     CC [M]  drivers/misc/cxl/api.o
>   In file included from include/linux/bitops.h:36:0,
>                    from include/linux/kernel.h:10,
>                    from include/linux/list.h:8,
>                    from include/linux/pci.h:25,
>                    from drivers/misc/cxl/api.c:10:
>   ./arch/powerpc/include/asm/bitops.h:226:94: error: declaration of 'ffs' shadows a built-in function [-Werror=shadow]

>
> I doubt you plan to fix all those. So making -Werror configurable seems like
> the only way forward, then. (Glad you agreed!)

Yep, I'm happy to make Werror configurable.

cxl can probably just use the existing PPC_WERROR.

cheers