[PATCH linux dev-4.10 3/5] leds: pca955x: Don't invert requested value in pca955x_gpio_set_value()

[Andrew Jeffery]

The PCA9552 lines can be used either for driving LEDs or as GPIOs. The
manual states that for LEDs, the operation is open-drain:

         The LSn LED select registers determine the source of the LED data.

           00 = output is set LOW (LED on)
           01 = output is set high-impedance (LED off; default)
           10 = output blinks at PWM0 rate
           11 = output blinks at PWM1 rate

For GPIOs it suggests a pull-up so that the open-case drives the line
high:

         For use as output, connect external pull-up resistor to the pin
         and size it according to the DC recommended operating
         characteristics.  LED output pin is HIGH when the output is
         programmed as high-impedance, and LOW when the output is
         programmed LOW through the ‘LED selector’ register.  The output
         can be pulse-width controlled when PWM0 or PWM1 are used.

Now, I have a hardware design that uses the LED controller to control
LEDs. However, for $reasons, we're using the leds-gpio driver to drive
the LEDs which means wending our way through the gpiochip abstractions.
So with that in mind we need to describe an active-low GPIO
configuration to drive the LEDs as though they were GPIOs.

The set() gpiochip callback in leds-pca955x does the following:

         ...
         if (val)
                pca955x_led_set(&led->led_cdev, LED_FULL);
         else
                pca955x_led_set(&led->led_cdev, LED_OFF);
         ...

Where LED_FULL = 255. pca955x_led_set() in turn does:

         ...
         switch (value) {
         case LED_FULL:
                ls = pca955x_ledsel(ls, ls_led, PCA955X_LS_LED_ON);
                break;
         ...

Where PCA955X_LS_LED_ON is defined as:

         #define PCA955X_LS_LED_ON	0x0	/* Output LOW */

So here we have some type confusion: We've crossed domains from GPIO
behaviour to LED behaviour without accounting for possible inversions
in the process.

Stepping back to leds-gpio for a moment, during probe() we call
create_gpio_led(), which eventually executes:

         if (template->default_state == LEDS_GPIO_DEFSTATE_KEEP) {
                state = gpiod_get_value_cansleep(led_dat->gpiod);
                if (state < 0)
                        return state;
         } else {
                state = (template->default_state == LEDS_GPIO_DEFSTATE_ON);
         }
         ...
         ret = gpiod_direction_output(led_dat->gpiod, state);

In the devicetree the GPIO is annotated as active-low, and
gpiod_get_value_cansleep() handles this for us:

         int gpiod_get_value_cansleep(const struct gpio_desc *desc)
         {
                 int value;

                 might_sleep_if(extra_checks);
                 VALIDATE_DESC(desc);
                 value = _gpiod_get_raw_value(desc);
                 if (value < 0)
                         return value;

                 if (test_bit(FLAG_ACTIVE_LOW, &desc->flags))
                         value = !value;

                 return value;
         }

_gpiod_get_raw_value() in turn calls through the get() callback for the
gpiochip implementation, so returning to our get() implementation in
leds-pca955x we find we extract the raw value from hardware:

         static int pca955x_gpio_get_value(struct gpio_chip *gc, unsigned int offset)
         {
                 struct pca955x *pca955x = gpiochip_get_data(gc);
                 struct pca955x_led *led = &pca955x->leds[offset];
                 u8 reg = pca955x_read_input(pca955x->client, led->led_num / 8);

                 return !!(reg & (1 << (led->led_num % 8)));
         }

This behaviour is not symmetric with that of set(), where the val is
inverted by the driver.

Closing the loop on the GPIO_ACTIVE_LOW inversions,
gpiod_direction_output(), like gpiod_get_value_cansleep, handles it for
us:

         int gpiod_direction_output(struct gpio_desc *desc, int value)
         {
                  VALIDATE_DESC(desc);
                  if (test_bit(FLAG_ACTIVE_LOW, &desc->flags))
                           value = !value;
                  else
                           value = !!value;
                  return _gpiod_direction_output_raw(desc, value);
         }

All-in-all, with a value of 'keep' for default-state property in the
gpio-leds child node, the current state of the hardware will in-fact be
inverted; precisely the opposite of what was intended.

Rework leds-pca955x so that we avoid the incorrect inversion and clarify
the semantics with respect to GPIO.

Signed-off-by: Andrew Jeffery <andrew at aj.id.au>
---
 drivers/leds/leds-pca955x.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/leds/leds-pca955x.c b/drivers/leds/leds-pca955x.c
index 0217bac2f47b..a02c1fd5dee9 100644
--- a/drivers/leds/leds-pca955x.c
+++ b/drivers/leds/leds-pca955x.c
@@ -61,6 +61,9 @@
 #define PCA955X_LS_BLINK0	0x2	/* Blink at PWM0 rate */
 #define PCA955X_LS_BLINK1	0x3	/* Blink at PWM1 rate */
 
+#define PCA955X_GPIO_HIGH	LED_OFF
+#define PCA955X_GPIO_LOW	LED_FULL
+
 enum pca955x_type {
 	pca9550,
 	pca9551,
@@ -292,9 +295,9 @@ static void pca955x_gpio_set_value(struct gpio_chip *gc, unsigned int offset,
 	struct pca955x_led *led = &pca955x->leds[offset];
 
 	if (val)
-		pca955x_led_set(&led->led_cdev, LED_FULL);
+		pca955x_led_set(&led->led_cdev, PCA955X_GPIO_HIGH);
 	else
-		pca955x_led_set(&led->led_cdev, LED_OFF);
+		pca955x_led_set(&led->led_cdev, PCA955X_GPIO_LOW);
 }
 
 static int pca955x_gpio_get_value(struct gpio_chip *gc, unsigned int offset)
-- 
2.11.0