gpio: regmap: Bypass cache for aliased inputs

gpio: regmap: Bypass cache for aliased inputs

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>
Link: https://lore.kernel.org/r/20251021142407.307753-3-sander@svanheule.net
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>