linux-kernel - Re: [PATCH v7 3/3] gpio: pca953x: fix address calculation for pcal6524

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Message-ID: <20180606073525.GA18743@amd>
Date:   Wed, 6 Jun 2018 09:35:25 +0200
From:   Pavel Machek <pavel@....cz>
To:     "H. Nikolaus Schaller" <hns@...puter.org>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Kumar Gala <galak@...eaurora.org>,
        Rob Herring <robh+dt@...nel.org>,
        Pawel Moll <pawel.moll@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ian Campbell <ijc+devicetree@...lion.org.uk>,
        Linus Walleij <linus.walleij@...aro.org>,
        Alexandre Courbot <gnurou@...il.com>,
        devicetree <devicetree@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>, kernel@...a-handheld.com
Subject: Re: [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@....cz>:
> > 
> > On Tue 2018-06-05 18:37:21, Andy Shevchenko wrote:
> >> On Wed, May 23, 2018 at 5:06 PM, Pavel Machek <pavel@....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@...il.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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