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Date:   Thu, 5 Nov 2020 09:00:07 +0000
From:   Lee Jones <lee.jones@linaro.org>
To:     Jiri Slaby <jirislaby@kernel.org>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        Paul Mackerras <paulus@samba.org>, linux-serial@vger.kernel.org
Subject: Re: [PATCH 34/36] tty: serial: pmac_zilog: Make disposable variable
 __always_unused
Message-ID: <20201105090007.GE4488@dell>
References: <20201104193549.4026187-1-lee.jones@linaro.org>
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 <445a6440-b4c8-4536-891b-0cefc78e5f57@csgroup.eu>
 <e027b620-56f8-7d8b-84ff-54839f94a4c7@kernel.org>
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 <a6b63789-1315-cec1-9575-0d858a6da1d5@kernel.org>
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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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