linux-kernel - Re: [PATCH v12 4/4] gpio: xilinx: Utilize generic bitmap_get_value and _set

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Message-ID: <20201109171140.GA14045@shinobu>
Date:   Mon, 9 Nov 2020 12:11:40 -0500
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Syed Nayyar Waris <syednwaris@...il.com>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michal Simek <michal.simek@...inx.com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v12 4/4] gpio: xilinx: Utilize generic bitmap_get_value
 and _set_value

On Mon, Nov 09, 2020 at 10:15:29PM +0530, Syed Nayyar Waris wrote:
> On Mon, Nov 09, 2020 at 03:41:53PM +0100, Arnd Bergmann wrote:
> > On Mon, Nov 9, 2020 at 2:41 PM William Breathitt Gray
> > <vilhelm.gray@...il.com> wrote:
> > > On Mon, Nov 09, 2020 at 06:04:11PM +0530, Syed Nayyar Waris wrote:
> > >
> > > One of my concerns is that we're incurring the latency two additional
> > > conditional checks just to suppress a compiler warning about a case that
> > > wouldn't occur in the actual use of bitmap_set_value(). I'm hoping
> > > there's a way for us to suppress these warnings without adding onto the
> > > latency of this function; given that bitmap_set_value() is intended to
> > > be used in loops, conditionals here could significantly increase latency
> > > in drivers.
> > 
> > At least for this caller, the size check would be a compile-time
> > constant that can be eliminated.
> > 
> > > I wonder if array_index_nospec() might have the side effect of
> > > suppressing these warnings for us. For example, would this work:
> > >
> > > static inline void bitmap_set_value(unsigned long *map,
> > >                                     unsigned long value,
> > >                                     unsigned long start, unsigned long nbits)
> > > {
> > >         const unsigned long offset = start % BITS_PER_LONG;
> > >         const unsigned long ceiling = round_up(start + 1, BITS_PER_LONG);
> > >         const unsigned long space = ceiling - start;
> > >         size_t index = BIT_WORD(start);
> > >
> > >         value &= GENMASK(nbits - 1, 0);
> > >
> > >         if (space >= nbits) {
> > >                 index = array_index_nospec(index, index + 1);
> > >
> > >                 map[index] &= ~(GENMASK(nbits - 1, 0) << offset);
> > >                 map[index] |= value << offset;
> > >         } else {
> > >                 index = array_index_nospec(index, index + 2);
> > >
> > >                 map[index + 0] &= ~BITMAP_FIRST_WORD_MASK(start);
> > >                 map[index + 0] |= value << offset;
> > >                 map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
> > >                 map[index + 1] |= value >> space;
> > >         }
> > > }
> > >
> > > Or is this going to produce the same warning because we're not using an
> > > explicit check against the map array size?
> > 
> > https://godbolt.org/z/fxnsG9
> > 
> > It still warns about the 'map[index + 1]' access: from all I can tell,
> > gcc mainly complains because it cannot rule out that 'space < nbits',
> > and then it knows the size of 'DECLARE_BITMAP(old, 64)' and finds
> > that if 'index + 0' is correct, then 'index + 1' overflows that array.
> > 
> >       Arnd
> 
> Hi Arnd,
> 
> As suggested by William, sharing another solution to suppress the 
> compiler warning. Please let me know your views on the below fix. Thanks.
> 
> If its alright, I shall submit a (new) v13 patchset soon. Let me know.
> 
> @@ -1,5 +1,5 @@
>  static inline void bitmap_set_value(unsigned long *map,
> -                                    unsigned long value,
> +                                    unsigned long value, const size_t length,
>                                      unsigned long start, unsigned long nbits)
>  {
>          const size_t index = BIT_WORD(start);
> @@ -15,6 +15,10 @@ static inline void bitmap_set_value(unsigned long *map,
>          } else {
>                  map[index + 0] &= ~BITMAP_FIRST_WORD_MASK(start);
>                  map[index + 0] |= value << offset;
> +
> +               if (index + 1 >= length)
> +                       __builtin_unreachable();
> +
>                  map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
>                  map[index + 1] |= value >> space;
>          }

Hi Syed,

Let's rename 'length' to 'nbits' as Arnd suggested, and rename 'nbits'
to value_width.

William Breathitt Gray

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