Re: [PATCH 05/14] isdn: isdnloop: suppress a gcc-7 warning

[Kevin Easton]

On Fri, Jul 14, 2017 at 12:37:05PM +0200, Arnd Bergmann wrote:
> On Fri, Jul 14, 2017 at 12:08 PM, Joe Perches <joe@xxxxxxxxxxx> wrote:
> > On Fri, 2017-07-14 at 11:25 +0200, Arnd Bergmann wrote:
> >> We test whether a bit is set in a mask here, which is correct
> >> but gcc warns about it as it thinks it might be confusing:
> >>
> >> drivers/isdn/isdnloop/isdnloop.c:412:37: error: ?: using integer constants in boolean context, the expression will always evaluate to 'true' [-Werror=int-in-bool-context]

...

> > Perhaps this is a logic defect and should be:
> >
> >                 if (!(card->flags & ((channel) ? ISDNLOOP_FLAGS_B2ACTIVE : ISDNLOOP_FLAGS_B1ACTIVE)))
> 
> Yes, good catch. I had thought about it for a bit whether that would be
> the answer, but come to the wrong conclusion on my own.
> 
> Note that the version you suggested will still have the warning, so I think
> it needs to be

It shouldn't - the warning is for using an integer *constant* in boolean
context, but the result of & isn't a constant and should be fine.

!(flags & mask) is a very common idiom.

    - Kevin