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

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Date:   Fri, 14 Jul 2017 12:37:05 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Joe Perches <joe@...ches.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Karsten Keil <isdn@...ux-pingi.de>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Guenter Roeck <linux@...ck-us.net>,
        IDE-ML <linux-ide@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH 05/14] isdn: isdnloop: suppress a gcc-7 warning

On Fri, Jul 14, 2017 at 12:08 PM, Joe Perches <joe@...ches.com> 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]
>>
>> This replaces the negation of an integer with an equivalent
>> comparison to zero, which gets rid of the warning.
> []
>> diff --git a/drivers/isdn/isdnloop/isdnloop.c b/drivers/isdn/isdnloop/isdnloop.c
> []
>> @@ -409,7 +409,7 @@ isdnloop_sendbuf(int channel, struct sk_buff *skb, isdnloop_card *card)
>>               return -EINVAL;
>>       }
>>       if (len) {
>> -             if (!(card->flags & (channel) ? ISDNLOOP_FLAGS_B2ACTIVE : ISDNLOOP_FLAGS_B1ACTIVE))
>> +             if ((card->flags & (channel) ? ISDNLOOP_FLAGS_B2ACTIVE : ISDNLOOP_FLAGS_B1ACTIVE) == 0)
>>                       return 0;
>>               if (card->sndcount[channel] > ISDNLOOP_MAX_SQUEUE)
>>                       return 0;
>
> The if as written can not be zero.
>
> drivers/isdn/isdnloop/isdnloop.h:#define ISDNLOOP_FLAGS_B1ACTIVE 1      /* B-Channel-1 is open           */
> drivers/isdn/isdnloop/isdnloop.h:#define ISDNLOOP_FLAGS_B2ACTIVE 2      /* B-Channel-2 is open           */
>
> 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

                 if (card->flags &
                    ((channel) ? ISDNLOOP_FLAGS_B2ACTIVE :
ISDNLOOP_FLAGS_B1ACTIVE)
                     == 0)

or something like that, probably having a temporary flag variable would be best:

          int flag =  channel ? ISDNLOOP_FLAGS_B2ACTIVE :
ISDNLOOP_FLAGS_B1ACTIVE;
          if ((card->flags & flag) == 0)
                       return 0;

         Arnd