linux-kernel - Re: [PATCH 34/36] tty: serial: pmac_zilog: Make disposable variable __always

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Date:   Thu, 5 Nov 2020 08:36:26 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Paul Mackerras <paulus@...ba.org>, linux-serial@...r.kernel.org
Subject: Re: [PATCH 34/36] tty: serial: pmac_zilog: Make disposable variable
 __always_unused

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.

> And even if it was, would (void)!read_zsdata(port) fix it?

That's hideous. :D

*Much* better to just use '__always_used' in that use-case.

> > > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > Cc: Jiri Slaby <jirislaby@...nel.org>
> > > Cc: Michael Ellerman <mpe@...erman.id.au>
> > > Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > > Cc: Paul Mackerras <paulus@...ba.org>
> > > Cc: linux-serial@...r.kernel.org
> > > Cc: linuxppc-dev@...ts.ozlabs.org
> > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > > ---
> > >   drivers/tty/serial/pmac_zilog.h | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/tty/serial/pmac_zilog.h
> > > b/drivers/tty/serial/pmac_zilog.h
> > > index bb874e76810e0..968aec7c1cf82 100644
> > > --- a/drivers/tty/serial/pmac_zilog.h
> > > +++ b/drivers/tty/serial/pmac_zilog.h
> > > @@ -362,7 +362,7 @@ static inline void zssync(struct uart_pmac_port
> > > *port)
> > >   /* Misc macros */
> > >   #define ZS_CLEARERR(port)    (write_zsreg(port, 0, ERR_RES))
> > > -#define ZS_CLEARFIFO(port)   do { volatile unsigned char garbage; \
> > > +#define ZS_CLEARFIFO(port)   do { volatile unsigned char
> > > __always_unused garbage; \
> > >                        garbage = read_zsdata(port); \
> > >                        garbage = read_zsdata(port); \
> > >                        garbage = read_zsdata(port); \
> > > 
> 
> thanks,

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