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

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Message-ID: <20201110174316.GA12192@shinobu>
Date:   Tue, 10 Nov 2020 12:43:16 -0500
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Syed Nayyar Waris <syednwaris@...il.com>
Cc:     Michal Simek <michal.simek@...inx.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        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>,
        Bartosz Golaszewski <bgolaszewski@...libre.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 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@...il.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

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