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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Fri, 20 Nov 2020 14:26:35 +0100
Message-ID: <CAK8P3a07ohL40kSgj1EJ-EuQk5HUGyVGXE1acsg50PiKf7j=mA@mail.gmail.com>
Subject: Re: [PATCH v12 4/4] gpio: xilinx: Utilize generic bitmap_get_value
 and _set_value
To:     Syed Nayyar Waris <syednwaris@gmail.com>
Cc:     William Breathitt Gray <vilhelm.gray@gmail.com>,
        Michal Simek <michal.simek@xilinx.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Linus Walleij <linus.walleij@linaro.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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On Fri, Nov 13, 2020 at 5:52 PM Syed Nayyar Waris <syednwaris@gmail.com> wrote:
> On Wed, Nov 11, 2020 at 3:30 AM Syed Nayyar Waris <syednwaris@gmail.com> wrote:
> > On Tue, Nov 10, 2020 at 12:43:16PM -0500, William Breathitt Gray wrote:
> > > On Tue, Nov 10, 2020 at 10:52:42PM +0530, Syed Nayyar Waris wrote:
> > > > On Tue, Nov 10, 2020 at 6:05 PM William Breathitt Gray
> > > > <vilhelm.gray@gmail.com> wrote:
> > > > >
> > > > > On Tue, Nov 10, 2020 at 11:02:43AM +0100, Michal Simek wrote:
> > > > > >
> > > > > >
> > > > > > On 09. 11. 20 18:31, William Breathitt Gray wrote:
> > > > > > > On Mon, Nov 09, 2020 at 07:22:20PM +0200, Andy Shevchenko wrote:
> > > > > > >> On Mon, Nov 09, 2020 at 12:11:40PM -0500, William Breathitt Gray wrote:
> > > > > > >>> 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:
> > > > > > >>
> > > > > > >> ...
> > > > > > >>
> > > > > > >>>>  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.
> > > > > > >>
> > > > > > >> length here is in longs. I guess this is the point of entire patch.
> > > > > > >
> > > > > > > Ah yes, this should become 'const unsigned long nbits' and represent the
> > > > > > > length of the bitmap in bits and not longs.
> > > >
> > > > Hi William, Andy and All,
> > > >
> > > > Thank You for reviewing. I was looking into the review comments and I
> > > > have a question on the above.
> > > >
> > > > Actually, in bitmap_set_value(), the intended comparison is to be made
> > > > between 'index + 1' and 'length' (which is now renamed as 'nbits').
> > > > That is, the comparison would look-like as follows:
> > > > if (index + 1 >= nbits)
> > > >
> > > > The 'index' is getting populated with BIT_WORD(start).
> > > > The 'index' variable in above is the actual index of the bitmap array,
> > > > while in previous mail it is suggested to use 'nbits' which represent
> > > > the length of the bitmap in bits and not longs.
> > > >
> > > > Isn't it comparing two different things? index of array (not the
> > > > bit-wise-length) on left hand side and nbits (bit-wise-length) on
> > > > right hand side?
> > > >
> > > > Have I misunderstood something? If yes, request to clarify.
> > > >
> > > > Or do I have to first divide 'nbits' by BITS_PER_LONG and then compare
> > > > it with 'index + 1'? Something like this?
> > > >
> > > > Regards
> > > > Syed Nayyar Waris
> > >
> > > The array elements of the bitmap memory region are abstracted away for
> > > the covenience of the users of the bitmap_* functions; the driver
> > > authors are able to treat their bitmaps as just a set of contiguous bits
> > > and not worry about where the division between array elements happen.
> > >
> > > So to match the interface of the other bitmap_* functions, you should
> > > take in nbits and figure out the actual array length by dividing by
> > > BITS_PER_LONG inside bitmap_set_value(). Then you can use your
> > > conditional check (index + 1 >= length) like you have been doing so far.
> > >
> > > William Breathitt Gray
> >
> > Hi Arnd,
> >
> > Sharing a new version of bitmap_set_value(). Let me know if it looks
> > good and whether it suppresses the compiler warning.
> >
> > The below patch is created against the v12 version of bitmap_set_value().
> >
> > -static inline void bitmap_set_value(unsigned long *map,
> > -                                    unsigned long value,
> > -                                    unsigned long start, unsigned long nbits)
> > +static inline void bitmap_set_value(unsigned long *map, unsigned long nbits,
> > +                                   unsigned long value, unsigned long value_width,
> > +                                   unsigned long start)
> >  {
> > -        const size_t index = BIT_WORD(start);
> > +        const unsigned long index = BIT_WORD(start);
> > +        const unsigned long length = BIT_WORD(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;
> >
> > -        value &= GENMASK(nbits - 1, 0);
> > +        value &= GENMASK(value_width - 1, 0);
> >
> > -        if (space >= nbits) {
> > -                map[index] &= ~(GENMASK(nbits - 1, 0) << offset);
> > +        if (space >= value_width) {
> > +                map[index] &= ~(GENMASK(value_width - 1, 0) << offset);
> >                  map[index] |= value << offset;
> >          } else {
> >                  map[index + 0] &= ~BITMAP_FIRST_WORD_MASK(start);
> >                  map[index + 0] |= value << offset;
> > -                map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
> > +
> > +               if (index + 1 >= length)
> > +                       __builtin_unreachable();
> > +
> > +                map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + value_width);
> >                  map[index + 1] |= value >> space;
> >          }
> >  }
> >
> >
>
> Hi Arnd,
>
> What do you think of the above solution ( new version of
> bitmap_set_value() )? Does it look good?

Sorry for the late reply and thanks for continuing to look at solutions.

I don't really like the idea of having the __builtin_unreachable() in
there, since that would lead to even worse undefined behavior
(jumping to a random instruction) than the previous one (writing
to a random location) when invalid data gets passed.

Isn't passing the length of the bitmap sufficient to suppress the
warning (sorry I did not try myself)? If not, maybe this could
be a "BUG_ON(index + 1 >= length)" instead of the
__builtin_unreachable(). That way it would at least crash
in a well-defined way.

     Arnd