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

[Jiri Slaby]

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…

thanks,
--
js
suse labs