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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Message-ID: <Y6dTX2wx/SXlswGC@kadam>
Date:   Sat, 24 Dec 2022 22:30:39 +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 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?

I don't understand this code at all.  The divide is inscrutable  Is it
storing something in in the lower 3 bits and something in bit 4?  In
that case it might be nicer to just check (pin & BIT(4)).

regards,
dan carpenter

> 
> 
> > vim +52 drivers/gpio/gpio-exar.c
> > 
> > 696868d0a79c21 Bartosz Golaszewski 2020-09-30  47  static unsigned int
> > 696868d0a79c21 Bartosz Golaszewski 2020-09-30  48  exar_offset_to_sel_addr(struct exar_gpio_chip *exar_gpio, unsigned int offset)
> > 696868d0a79c21 Bartosz Golaszewski 2020-09-30  49  {
> > 5134272f9f3f71 Qingtao Cao         2022-09-02  50      unsigned int pin = exar_gpio->first_pin + (offset % 16);
> > 5134272f9f3f71 Qingtao Cao         2022-09-02  51      unsigned int cascaded = offset / 16;
> > 5134272f9f3f71 Qingtao Cao         2022-09-02 @52      unsigned int addr = pin / 8 ? EXAR_OFFSET_MPIOSEL_HI : EXAR_OFFSET_MPIOSEL_LO;
> > 
> > Why not change this to pin > 8 instead.  Comparisons are faster than
> > divide ops.  Way more readable too.
> > 
> > 5134272f9f3f71 Qingtao Cao         2022-09-02  53  
> > 5134272f9f3f71 Qingtao Cao         2022-09-02  54      return addr + (cascaded ? exar_gpio->cascaded_offset : 0);
> > 696868d0a79c21 Bartosz Golaszewski 2020-09-30  55  }
> > 696868d0a79c21 Bartosz Golaszewski 2020-09-30  56  
> > 696868d0a79c21 Bartosz Golaszewski 2020-09-30  57  static unsigned int
> > 696868d0a79c21 Bartosz Golaszewski 2020-09-30  58  exar_offset_to_lvl_addr(struct exar_gpio_chip *exar_gpio, unsigned int offset)
> > 696868d0a79c21 Bartosz Golaszewski 2020-09-30  59  {
> > 5134272f9f3f71 Qingtao Cao         2022-09-02  60      unsigned int pin = exar_gpio->first_pin + (offset % 16);
> > 5134272f9f3f71 Qingtao Cao         2022-09-02  61      unsigned int cascaded = offset / 16;
> > 5134272f9f3f71 Qingtao Cao         2022-09-02 @62      unsigned int addr = pin / 8 ? EXAR_OFFSET_MPIOLVL_HI : EXAR_OFFSET_MPIOLVL_LO;
> > 5134272f9f3f71 Qingtao Cao         2022-09-02  63  
> > 5134272f9f3f71 Qingtao Cao         2022-09-02  64      return addr + (cascaded ? exar_gpio->cascaded_offset : 0);
> > 696868d0a79c21 Bartosz Golaszewski 2020-09-30  65  }
> > 
> > -- 
> > 0-DAY CI Kernel Test Service
> > https://01.org/lkp
> >