linux-kernel - Re: drivers/gpio/gpio-exar.c:52 exar_offset_to_sel_addr() warn: replace divide condition 'pin / 8' with 'pin >= 8'

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Date:   Tue, 27 Dec 2022 20:46:13 +0300
From:   Dan Carpenter <error27@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     oe-kbuild@...ts.linux.dev, Qingtao Cao <qingtao.cao.au@...il.com>,
        lkp@...el.com, oe-kbuild-all@...ts.linux.dev,
        linux-kernel@...r.kernel.org, Bartosz Golaszewski <brgl@...ev.pl>
Subject: Re: drivers/gpio/gpio-exar.c:52 exar_offset_to_sel_addr() warn:
 replace divide condition 'pin / 8' with 'pin >= 8'

On Sun, Dec 25, 2022 at 12:50:46PM +0100, Andy Shevchenko wrote:
> 
> 
> Lähetetty iPhonesta
> 
> > Andy Shevchenko <andy.shevchenko@...il.com> kirjoitti 25.12.2022 kello 12.45:
> > 
> > 
> > 
> > Lähetetty iPhonesta
> > 
> >>> Dan Carpenter <error27@...il.com> kirjoitti 24.12.2022 kello 20.30:
> >>> 
> >>> On Sat, Dec 24, 2022 at 05:19:27PM +0100, Andy Shevchenko wrote:
> >>> 
> >>> 
> >>> Lähetetty iPhonesta
> >>> 
> >>>> Dan Carpenter <error27@...il.com> kirjoitti 23.12.2022 kello 11.54:
> >>>> 
> >>>> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> >>>> head:   f9ff5644bcc04221bae56f922122f2b7f5d24d62
> >>>> commit: 5134272f9f3f71d4e1f3aa15cb09321af49b3646 gpio: exar: access MPIO registers on cascaded chips
> >>>> config: ia64-randconfig-m031-20221218
> >>>> compiler: ia64-linux-gcc (GCC) 12.1.0
> >>>> 
> >>>> If you fix the issue, kindly add following tag where applicable
> >>>> | Reported-by: kernel test robot <lkp@...el.com>
> >>>> | Reported-by: Dan Carpenter <error27@...il.com>
> >>>> 
> >>>> smatch warnings:
> >>>> drivers/gpio/gpio-exar.c:52 exar_offset_to_sel_addr() warn: replace divide condition 'pin / 8' with 'pin >= 8'
> >>>> drivers/gpio/gpio-exar.c:62 exar_offset_to_lvl_addr() warn: replace divide condition 'pin / 8' with 'pin >= 8'
> >>>> 
> >>> 
> >>> 
> >>> 
> >>> I don’t think this is a good advice. If we want to limit that, we need
> >>> to check also upper limit. But. The GPIO framework does that. So,
> >>> changing / to >= is bogus.
> >> 
> >> 
> >> How is checking pin / 8 not mathematically equivalent to pin >= 8?
> > 
> > The point is that semantically the / is better in case this code will ever support more than two banks of pins.
> 
> On top of that it’s paired with pin % 8.
> 

I noticed that, but it's a common bug though that a lot of people
accidentally write if (pin / 8) when if ((pin % 8) == 0) is intended.

For example:

drivers/rtc/rtc-m48t59.c
   132          M48T59_WRITE((bin2bcd(tm->tm_mon + 1) & 0x1F), M48T59_MONTH);
   133          M48T59_WRITE(bin2bcd(year % 100), M48T59_YEAR);
   134  
   135          if (pdata->type == M48T59RTC_TYPE_M48T59 && (year / 100))
                                                             ^^^^^^^^^^
This code is pretty clearly an example of where people accidentally uses
/ to mean "divides cleanly".  (I have not patched or reported this code,
btw so if anyone wants an easy patch to send it's available).

   136                  val = (M48T59_WDAY_CEB | M48T59_WDAY_CB);
   137          val |= (bin2bcd(tm->tm_wday) & 0x07);
   138          M48T59_WRITE(val, M48T59_WDAY);

regards,
dan carpenter