linux-kernel - RE: [RFT PATCH 5/7] gpio: exar: unduplicate address and offset computation

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Date:   Mon, 2 Nov 2020 11:27:57 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andy Shevchenko' <andy.shevchenko@...il.com>,
        Bartosz Golaszewski <brgl@...ev.pl>
CC:     Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: RE: [RFT PATCH 5/7] gpio: exar: unduplicate address and offset
 computation

From: Andy Shevchenko
> Sent: 02 November 2020 10:59
> 
> On Mon, Oct 26, 2020 at 4:23 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:
> 
> ...
> 
> > +static unsigned int
> > +exar_offset_to_sel_addr(struct exar_gpio_chip *exar_gpio, unsigned int offset)
> > +{
> > +       return (offset + exar_gpio->first_pin) / 8 ? EXAR_OFFSET_MPIOSEL_HI
> > +                                                  : EXAR_OFFSET_MPIOSEL_LO;
> > +}
> > +
> > +static unsigned int
> > +exar_offset_to_lvl_addr(struct exar_gpio_chip *exar_gpio, unsigned int offset)
> > +{
> > +       return (offset + exar_gpio->first_pin) / 8 ? EXAR_OFFSET_MPIOLVL_HI
> > +                                                  : EXAR_OFFSET_MPIOLVL_LO;
> > +}
> > +
> > +static unsigned int
> > +exar_offset_to_bit(struct exar_gpio_chip *exar_gpio, unsigned int offset)
> > +{
> > +       return (offset + exar_gpio->first_pin) % 8;
> > +}
> 
> Answering to your question...
> 
> It can be done line this:
> 
> static unsigned int exar_offset_to_bank_and_bit(..., *bit)
> {
>        *bit = (offset + exar_gpio->first_pin) % 8;
>        return (offset + exar_gpio->first_pin) / 8;
> }

That is likely to require the compiler reload exar_gpio->first_pin
after the write to *bit.

> static unsigned int exar_offset_to_lvl_addr_and_bit(, *bit)
> {
>     return exar_offset_to_bank_and_bit(..., bit) ?
>         EXAR_OFFSET_MPIOLVL_HI : EXAR_OFFSET_MPIOLVL_LO;
> }

Gah why is it using divide then ?: ?
AFAICT (from the above) there are at most 16 pins.

Much better would be using:
	tmp =	offset + exar_gpio->first_pin;
	*bit = tmp & 7;
	return tmp & 8;

Inlined the compiler may well compute:
	exar_offset_to_bank_and_bit() ? HI : LO;
as:
	LO + (HI - LO) * exar_offset_to_bank_and_bit().
The latter term is likely to be just (tmp & 8) >> n.

I also bet the code actually wants (1 << bit).

	David

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