Re: [PATCH] gpio: exar set value handling for hw with gpio pull-up or pull-down

[Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>]

On Tue, Jul 30, 2024 at 07:16:10PM +0530, Sai Kumar Cholleti wrote:

Please, refer to the functions in the text as func(), e.g. exar_set_value().
Use proper acronym, i.e. GPIO (capitalised).

> Setting gpio direction = high, sometimes results in gpio value = 0.
> 
> If a gpio is pulled high, the following construction results in the
> value being 0 when the desired value is 1:
> 
> $ echo "high" > /sys/class/gpio/gpio336/direction
> $ cat /sys/class/gpio/gpio336/value
> 0
> 
> Before the gpio direction is changed from input to output,
> exar_set_value is set to 1, but since direction is input when
> exar_set_value is called, _regmap_update_bits reads a 1 due to an
> external pull-up.  When force_write is not set (regmap_set_bits has
> force_write = false), the value is not written.  When the direction is
> then changed, the gpio becomes an output with the value of 0 (the
> hardware default).
> 
> regmap_write_bits sets force_write = true, so the value is always
> written by exar_set_value and an external pull-up doesn't affect the
> outcome of setting direction = high.
> 
> 
> The same can happen when a gpio is pulled low, but the scenario is a
> little more complicated.
> 
> $ echo high > /sys/class/gpio/gpio351/direction
> $ cat /sys/class/gpio/gpio351/value
> 1
> 
> $ echo in > /sys/class/gpio/gpio351/direction
> $ cat /sys/class/gpio/gpio351/value
> 0
> 
> $ echo low > /sys/class/gpio/gpio351/direction
> $ cat /sys/class/gpio/gpio351/value
> 1

Okay, shouldn't you instead mark the respective registers as volatile or so?
I believe regmap has some settings for this case. But I haven't checked myself.

-- 
With Best Regards,
Andy Shevchenko