[RFC PATCH 2/2] gpio: regmap: Bypass cache for aliased outputs

[Sander Vanheule <sander@svanheule.net>]

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 gpiochip implementation stores a shadow value of the pin
output data, which is updated and written to hardware on output data
changes. Pin input values are always obtained by reading the aliased
data register from hardware.

For gpio-regmap the output data could be in multiple registers, but we
can use the regmap cache support to shadow the output values by 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>
---
 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