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SubjectRe: [PATCH v7 3/3] gpio: pca953x: fix address calculation for pcal6524
On Wed 2018-06-06 07:33:32, H. Nikolaus Schaller wrote:
> Hi,
>
> > Am 05.06.2018 um 22:39 schrieb Pavel Machek <pavel@ucw.cz>:
> >
> > On Tue 2018-06-05 18:37:21, Andy Shevchenko wrote:
> >> On Wed, May 23, 2018 at 5:06 PM, Pavel Machek <pavel@ucw.cz> wrote:
> >>> On Thu 2018-05-17 06:59:49, H. Nikolaus Schaller wrote:
> >>>> The register constants are so far defined in a way that they fit
> >>>> for the pcal9555a when shifted by the number of banks, i.e. are
> >>>> multiplied by 2 in the accessor function.
> >>>>
> >>>> Now, the pcal6524 has 3 banks which means the relative offset
> >>>> is multiplied by 4 for the standard registers.
> >>>>
> >>>> Simply applying the bit shift to the extended registers gives
> >>>> a wrong result, since the base offset is already included in
> >>>> the offset.
> >>>>
> >>>> Therefore, we have to add code to the 24 bit accessor functions
> >>>> that adjusts the register number for these exended registers.
> >>>>
> >>>> The formula finally used was developed and proposed by
> >>>> Andy Shevchenko <andy.shevchenko@gmail.com>.
> >>
> >>>> int bank_shift = fls((chip->gpio_chip.ngpio - 1) / BANK_SZ);
> >>>> + int addr = (reg & PCAL_GPIO_MASK) << bank_shift;
> >>>> + int pinctrl = (reg & PCAL_PINCTRL_MASK) << 1;
> >>
> >>> Is this reasonable to do on each register access? Compiler will not be
> >>> able to optimize out fls and shifts, right?
> >>
> >> On modern CPUs fls() is one assembly command. OTOH, any proposal to do
> >> this better?
> >>
> >> What I can see is that bank_shift is invariant to the function, and
> >> maybe cached.
> >
> > Yes, I thought that caching bank_shift might be good idea. I thought
> > it was constant for given chip...
>
> Yes, it is an f(chip), but the question that comes to my mind is if
> optimization is worth any effort. This is an accessor method over

It will also be less ugly. Copy&pasted complex exprepsion all over the
driver is not nice.
Pavel

--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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