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

Jiri Slaby
2020-11-05T08:55:27Z

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
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