Re: [PATCH 34/36] tty: serial: pmac_zilog: Make disposable variable __always_unused

Lee Jones
2020-11-05T09:00:16Z

On Thu, 05 Nov 2020, Jiri Slaby wrote:

> On 05. 11. 20, 9:36, Lee Jones wrote:
> > On Thu, 05 Nov 2020, Jiri Slaby wrote:
> >
> > > On 05. 11. 20, 8:04, Christophe Leroy wrote:
> > > >
> > > >
> > > > Le 04/11/2020 à 20:35, Lee Jones a écrit :
> > > > > Fixes the following W=1 kernel build warning(s):
> > > > >
> > > > >   drivers/tty/serial/pmac_zilog.h:365:58: warning: variable
> > > > > ‘garbage’ set but not used [-Wunused-but-set-variable]
> > > >
> > > > Explain how you are fixing this warning.
> > > >
> > > > Setting  __always_unused is usually not the good solution for fixing
> > > > this warning, but here I guess this is likely the good solution. But it
> > > > should be explained why.
> >
> > There are normally 3 ways to fix this warning;
> >
> > - Start using/checking the variable/result
> > - Remove the variable
> > - Mark it as __{always,maybe}_unused
> >
> > The later just tells the compiler that not checking the resultant
> > value is intentional. There are some functions (as Jiri mentions
> > below) which are marked as '__must_check' which *require* a dummy
> > (garbage) variable to be used.
> >
> > > Or, why is the "garbage =" needed in the first place? read_zsdata is not
> > > defined with __warn_unused_result__.
> >
> > I used '__always_used' here for fear of breaking something.
> >
> > However, if it's safe to remove it, then all the better.
>
> Yes please -- this "garbage" is one of the examples of volatile misuses. If
> readb didn't work on volatile pointer, marking the return variable as
> volatile wouldn't save it.
>
> > > And even if it was, would (void)!read_zsdata(port) fix it?
> >
> > That's hideous. :D
>
> Sure, marking reads as must_check would be insane.
>
> > *Much* better to just use '__always_used' in that use-case.
>
> Then using a dummy variable to fool must_check must mean must_check is used
> incorrectly, no :)? But there are always exceptions…

Agreed on all points.

Will fix.

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