linux-kernel - Re: [PATCH v1] gpio: mmio: restroe get multiple gpio mask

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Message-ID: <CACRpkdZ8CVWb6=-sgK3BdW-ssivY1oz4cc_mYoXmyrBMLxsX=A@mail.gmail.com>
Date:   Mon, 24 Apr 2023 08:56:52 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Yan Wang <rk.code@...look.com>
Cc:     brgl@...ev.pl, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] gpio: mmio: restroe get multiple gpio mask

Hi Yan,

thanks for your patch!

On Sun, Apr 23, 2023 at 11:07 AM Yan Wang <rk.code@...look.com> wrote:

> Simplify the code,should not modify its logic.

I don't see how it simplifies the code, it seems to me that it is
making it more complex?

> Fixes: 761b5c30c206 ("gpio: mmio: replace open-coded for_each_set_bit()")
> Signed-off-by: Yan Wang <rk.code@...look.com>
> ---
>  drivers/gpio/gpio-mmio.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpio/gpio-mmio.c b/drivers/gpio/gpio-mmio.c
> index d9dff3dc92ae..c2347ef3a4df 100644
> --- a/drivers/gpio/gpio-mmio.c
> +++ b/drivers/gpio/gpio-mmio.c
> @@ -271,10 +271,14 @@ static void bgpio_multiple_get_masks(struct gpio_chip *gc,
>         *clear_mask = 0;
>
>         for_each_set_bit(i, mask, gc->bgpio_bits) {
> -               if (test_bit(i, bits))
> -                       *set_mask |= bgpio_line2mask(gc, i);
> -               else
> -                       *clear_mask |= bgpio_line2mask(gc, i);
> +               if (*mask == 0)
> +                       break;
> +               if (__test_and_clear_bit(i, mask)) {
> +                       if (test_bit(i, bits))
> +                               *set_mask |= bgpio_line2mask(gc, i);
> +                       else
> +                               *clear_mask |= bgpio_line2mask(gc, i);
> +               }

The intention of the change seems to be to break out of the loop
when all set bits are handled, by successively masking off one
bit at a time from mask. So this is intended as an optimization,
not a simplification.

I think for_each_set_bit() is already skipping over every bit
that is zero, see include/linux/find.h.

So this optimization gains us nothing.

Yours,
Linus Walleij