[PATCH v6 4/7] pwm: pca9685: Support staggered output ON times

[Clemens Gruber]

The PCA9685 supports staggered LED output ON times to minimize current
surges and reduce EMI.
When this new option is enabled, the ON times of each channel are
delayed by channel number x counter range / 16, which avoids asserting
all enabled outputs at the same counter value while still maintaining
the configured duty cycle of each output.

Signed-off-by: Clemens Gruber <clemens.gruber@xxxxxxxxxxxx>
---
Changes since v5:
- Simplified staggered outputs special case in set/get_duty

drivers/pwm/pwm-pca9685.c | 58 +++++++++++++++++++++++++++++----------
 1 file changed, 44 insertions(+), 14 deletions(-)

diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
index 4d6684b90819..a61eafdd2335 100644
--- a/drivers/pwm/pwm-pca9685.c
+++ b/drivers/pwm/pwm-pca9685.c
@@ -78,6 +78,7 @@
 struct pca9685 {
 	struct pwm_chip chip;
 	struct regmap *regmap;
+	bool staggered_outputs;
 #if IS_ENABLED(CONFIG_GPIOLIB)
 	struct mutex lock;
 	struct gpio_chip gpio;
@@ -93,46 +94,70 @@ static inline struct pca9685 *to_pca(struct pwm_chip *chip)
 /* Helper function to set the duty cycle ratio to duty/4096 (e.g. duty=2048 -> 50%) */
 static void pca9685_pwm_set_duty(struct pca9685 *pca, int channel, unsigned int duty)
 {
+	unsigned int on, off;
+
 	if (duty == 0) {
 		/* Set the full OFF bit, which has the highest precedence */
 		regmap_write(pca->regmap, REG_OFF_H(channel), LED_FULL);
+		return;
 	} else if (duty >= PCA9685_COUNTER_RANGE) {
 		/* Set the full ON bit and clear the full OFF bit */
 		regmap_write(pca->regmap, REG_ON_H(channel), LED_FULL);
 		regmap_write(pca->regmap, REG_OFF_H(channel), 0);
-	} else {
-		/* Set OFF time (clears the full OFF bit) */
-		regmap_write(pca->regmap, REG_OFF_L(channel), duty & 0xff);
-		regmap_write(pca->regmap, REG_OFF_H(channel), (duty >> 8) & 0xf);
-		/* Clear the full ON bit */
-		regmap_write(pca->regmap, REG_ON_H(channel), 0);
+		return;
 	}
+
+	if (pca->staggered_outputs && channel < PCA9685_MAXCHAN) {
+		/*
+		 * To reduce EMI, the ON times of each channel are
+		 * spread out evenly within the counter range, while
+		 * still maintaining the configured duty cycle
+		 */
+		on = channel * PCA9685_COUNTER_RANGE / PCA9685_MAXCHAN;
+	} else
+		on = 0;
+
+	off = (on + duty) % PCA9685_COUNTER_RANGE;
+
+	/* Set ON time (clears full ON bit) */
+	regmap_write(pca->regmap, REG_ON_L(channel), on & 0xff);
+	regmap_write(pca->regmap, REG_ON_H(channel), (on >> 8) & 0xf);
+	/* Set OFF time (clears full OFF bit) */
+	regmap_write(pca->regmap, REG_OFF_L(channel), off & 0xff);
+	regmap_write(pca->regmap, REG_OFF_H(channel), (off >> 8) & 0xf);
 }
 
 static unsigned int pca9685_pwm_get_duty(struct pca9685 *pca, int channel)
 {
-	unsigned int off_h = 0, val = 0;
+	unsigned int off = 0, on = 0, val = 0;
 
 	if (WARN_ON(channel >= PCA9685_MAXCHAN)) {
 		/* HW does not support reading state of "all LEDs" channel */
 		return 0;
 	}
 
-	regmap_read(pca->regmap, LED_N_OFF_H(channel), &off_h);
-	if (off_h & LED_FULL) {
+	regmap_read(pca->regmap, LED_N_OFF_H(channel), &off);
+	if (off & LED_FULL) {
 		/* Full OFF bit is set */
 		return 0;
 	}
 
-	regmap_read(pca->regmap, LED_N_ON_H(channel), &val);
-	if (val & LED_FULL) {
+	regmap_read(pca->regmap, LED_N_ON_H(channel), &on);
+	if (on & LED_FULL) {
 		/* Full ON bit is set */
 		return PCA9685_COUNTER_RANGE;
 	}
 
-	val = 0;
 	regmap_read(pca->regmap, LED_N_OFF_L(channel), &val);
-	return ((off_h & 0xf) << 8) | (val & 0xff);
+	off = ((off & 0xf) << 8) | (val & 0xff);
+	if (!pca->staggered_outputs)
+		return off;
+
+	/* Read ON register to calculate duty cycle of staggered output */
+	val = 0;
+	regmap_read(pca->regmap, LED_N_ON_L(channel), &val);
+	on = ((on & 0xf) << 8) | (val & 0xff);
+	return (off - on) & (PCA9685_COUNTER_RANGE - 1);
 }
 
 #if IS_ENABLED(CONFIG_GPIOLIB)
@@ -443,14 +468,19 @@ static int pca9685_pwm_probe(struct i2c_client *client,
 
 	regmap_write(pca->regmap, PCA9685_MODE2, reg);
 
+	pca->staggered_outputs = device_property_read_bool(
+		&client->dev, "nxp,staggered-outputs");
+
 	/* Disable all LED ALLCALL and SUBx addresses to avoid bus collisions */
 	regmap_read(pca->regmap, PCA9685_MODE1, &reg);
 	reg &= ~(MODE1_ALLCALL | MODE1_SUB1 | MODE1_SUB2 | MODE1_SUB3);
 	regmap_write(pca->regmap, PCA9685_MODE1, reg);
 
-	/* Reset OFF registers to POR default */
+	/* Reset OFF/ON registers to POR default */
 	regmap_write(pca->regmap, PCA9685_ALL_LED_OFF_L, LED_FULL);
 	regmap_write(pca->regmap, PCA9685_ALL_LED_OFF_H, LED_FULL);
+	regmap_write(pca->regmap, PCA9685_ALL_LED_ON_L, 0);
+	regmap_write(pca->regmap, PCA9685_ALL_LED_ON_H, 0);
 
 	pca->chip.ops = &pca9685_pwm_ops;
 	/* Add an extra channel for ALL_LED */
-- 
2.31.1