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

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Date:   Tue, 10 Nov 2020 07:35:38 -0500
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Michal Simek <michal.simek@...inx.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Syed Nayyar Waris <syednwaris@...il.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 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.
> > 
> >> But to me sounds like it would be better to have simply bitmap_set_value64() /
> >> bitmap_set_value32() with proper optimization done and forget about variadic
> >> ones for now.
> > 
> > The gpio-xilinx driver can have arbitrary sizes for width[0] and
> > width[1], so unfortunately that means we don't know the start position
> > nor the width of the value beforehand.
> 
> Start position should be all the time zero. You can't configure this IP
> to start from bit 2. Width can vary but start is IMHO all the time from
> 0 bit.
> 
> Thanks,
> Michal

Hi Michal,

I'm referring to the mask creation, not the data bus transfer; see the
implementation of the xgpio_set_multiple() function in linux-next for
reference:
<https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/gpio/gpio-xilinx.c?h=akpm>.

To generate the old mask we call the following:

	bitmap_set_value(old, state[0], 0, width[0]);
	bitmap_set_value(old, state[1], width[0], width[1]);

Here, width[0] and width[1] can vary, which makes the exact values of
the start and nbits parameters unknown beforehand (although we do know
they are within the bitmap boundary).

Regardless, this is not an issue because we know the bitmap_set_value()
is supposed to be called with valid values. We just need a way to hint
to GCC that this is the case, without increasing the latency of the
function -- which I think is possible if we use __builtin_unreachable()
for the conditional path checking the index against the length of the
bitmap.

William Breathitt Gray

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