On Tue, Oct 21, 2025 at 4:24 PM Sander Vanheule <sander@svanheule.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@svanheule.net>
> Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
> Reviewed-by: Michael Walle <mwalle@kernel.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
>