[RFC PATCH 1/2] gpio: regmap: Force writes for aliased data regs

[Sander Vanheule <sander@svanheule.net>]

GPIO chips often have data input and output fields aliased to the same
offset. Since gpio-regmap performs a value update before the direction
update (to prevent glitches), a pin currently configured as input may
cause regmap_update_bits() to not perform a write.

This may cause unexpected line states when the current input state
equals the requested output state:

        OUT   IN      OUT
    DIR ''''''\...|.../''''''

    pin ....../'''|'''\......
             (1) (2) (3)

    1. Line was configurad as out-low, but is reconfigured to input.
       External logic results in high value.
    2. Set output value high. regmap_update_bits() sees the value is
       already high and discards the register write.
    3. Line is switched to output, maintaining the stale output config
       (low) instead of the requested config (high).

By switching to regmap_write_bits(), a write of the requested output
value can be forced, irrespective of the read state. Do this only for
aliased registers, so the more efficient regmap_update_bits() can still
be used for distinct registers.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
---
 drivers/gpio/gpio-regmap.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/gpio/gpio-regmap.c b/drivers/gpio/gpio-regmap.c
index ab9e4077fa60..ba3c19206ccf 100644
--- a/drivers/gpio/gpio-regmap.c
+++ b/drivers/gpio/gpio-regmap.c
@@ -93,7 +93,7 @@ static int gpio_regmap_set(struct gpio_chip *chip, unsigned int offset,
 {
 	struct gpio_regmap *gpio = gpiochip_get_data(chip);
 	unsigned int base = gpio_regmap_addr(gpio->reg_set_base);
-	unsigned int reg, mask;
+	unsigned int reg, mask, mask_val;
 	int ret;
 
 	ret = gpio->reg_mask_xlate(gpio, base, offset, &reg, &mask);
@@ -101,9 +101,15 @@ static int gpio_regmap_set(struct gpio_chip *chip, unsigned int offset,
 		return ret;
 
 	if (val)
-		ret = regmap_update_bits(gpio->regmap, reg, mask, mask);
+		mask_val = mask;
 	else
-		ret = regmap_update_bits(gpio->regmap, reg, mask, 0);
+		mask_val = 0;
+
+	/* ignore input values which shadow the old output value */
+	if (gpio->reg_dat_base == gpio->reg_set_base)
+		ret = regmap_write_bits(gpio->regmap, reg, mask, mask_val);
+	else
+		ret = regmap_update_bits(gpio->regmap, reg, mask, mask_val);
 
 	return ret;
 }
-- 
2.51.0