linux-kernel - Re: [PATCH v6 2/8] gpio: regmap: Bypass cache for aliased inputs

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Message-ID: <CAMRc=Mevt+h-zjU2hmbTOjuTB=xQZN7O=FtdM0v=680VNiAKTg@mail.gmail.com>
Date: Thu, 23 Oct 2025 14:06:58 +0200
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Sander Vanheule <sander@...nheule.net>
Cc: Michael Walle <mwalle@...nel.org>, Linus Walleij <linus.walleij@...aro.org>, 
	linux-gpio@...r.kernel.org, Lee Jones <lee@...nel.org>, 
	Pavel Machek <pavel@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, linux-leds@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/8] gpio: regmap: Bypass cache for aliased inputs

On Tue, Oct 21, 2025 at 4:24 PM Sander Vanheule <sander@...nheule.net> wrote:
>
> GPIO chips often have data input and output registers aliased to the
> same offset. The output register is non-valitile and could in theory be
> cached. The input register however is volatile by nature and hence
> should not be cached, resulting in different requirements for reads and
> writes.
>
> The generic gpio chip implementation stores a shadow value of the pin
> output data, which is updated and written to hardware on output data
> changes in bgpio_set(), bgpio_set_set(). Pin input values are always
> obtained by reading the aliased data register from hardware.
>
> For gpio-regmap the situation is more complex as the output data could
> be in multiple registers, but we can use the regmap cache to shadow the
> output values when marking the data registers as non-volatile. By using
> regmap_read_bypassed() we can still treat the input values as volatile,
> irrespective of the regmap config. This ensures proper functioning of
> writing the output register with regmap_write_bits(), which will then
> use and update the cache only on data writes, gaining some performance
> from the cached output values.
>
> Signed-off-by: Sander Vanheule <sander@...nheule.net>
> Reviewed-by: Linus Walleij <linus.walleij@...aro.org>
> Reviewed-by: Michael Walle <mwalle@...nel.org>
> --
> Changes since RFC:
>   - Add review tags
>   - Slightly reworded the commit message

Nit: changelog should not be part of the commit message, I dropped it.

Bart

> ---
>  drivers/gpio/gpio-regmap.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpio/gpio-regmap.c b/drivers/gpio/gpio-regmap.c
> index ba3c19206ccf..afecacf7607f 100644
> --- a/drivers/gpio/gpio-regmap.c
> +++ b/drivers/gpio/gpio-regmap.c
> @@ -81,7 +81,11 @@ static int gpio_regmap_get(struct gpio_chip *chip, unsigned int offset)
>         if (ret)
>                 return ret;
>
> -       ret = regmap_read(gpio->regmap, reg, &val);
> +       /* ensure we don't spoil any register cache with pin input values */
> +       if (gpio->reg_dat_base == gpio->reg_set_base)
> +               ret = regmap_read_bypassed(gpio->regmap, reg, &val);
> +       else
> +               ret = regmap_read(gpio->regmap, reg, &val);
>         if (ret)
>                 return ret;
>
> --
> 2.51.0
>