linux-kernel - [PATCH 3/3] pwm: pca9685: Support staggered output ON times

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Message-Id: <20201118174417.278011-3-clemens.gruber@pqgruber.com>
Date:   Wed, 18 Nov 2020 18:44:17 +0100
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     linux-pwm@...r.kernel.org
Cc:     Thierry Reding <thierry.reding@...il.com>,
        u.kleine-koenig@...gutronix.de, Lee Jones <lee.jones@...aro.org>,
        linux-kernel@...r.kernel.org,
        Sven Van Asbroeck <TheSven73@...il.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        David Jander <david@...tonic.nl>,
        Clemens Gruber <clemens.gruber@...ruber.com>
Subject: [PATCH 3/3] pwm: pca9685: Support staggered output ON times

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@...ruber.com>
---
 drivers/pwm/pwm-pca9685.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
index 88028222335f..95958b5cce0d 100644
--- a/drivers/pwm/pwm-pca9685.c
+++ b/drivers/pwm/pwm-pca9685.c
@@ -74,6 +74,7 @@ struct pca9685 {
 	struct pwm_chip chip;
 	struct regmap *regmap;
 	int prescale;
+	bool staggered_outputs;
 #if IS_ENABLED(CONFIG_GPIOLIB)
 	struct mutex lock;
 	struct gpio_chip gpio;
@@ -348,7 +349,7 @@ static int pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 {
 	struct pca9685 *pca = to_pca(chip);
 	unsigned long long duty, prescale;
-	unsigned int reg;
+	unsigned int on, off, reg;
 
 	if (state->polarity != PWM_POLARITY_NORMAL)
 		return -EOPNOTSUPP;
@@ -395,6 +396,32 @@ static int pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	duty = (PCA9685_COUNTER_RANGE - 1) * state->duty_cycle;
 	duty = DIV_ROUND_UP_ULL(duty, state->period);
 
+	if (pca->staggered_outputs) {
+		if (pwm->hwpwm < 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 = pwm->hwpwm * PCA9685_COUNTER_RANGE /
+				PCA9685_MAXCHAN;
+			off = (on + duty) % PCA9685_COUNTER_RANGE;
+			regmap_write(pca->regmap, LED_N_ON_L(pwm->hwpwm),
+				     on & 0xff);
+			regmap_write(pca->regmap, LED_N_ON_H(pwm->hwpwm),
+				     (on >> 8) & 0xf);
+			regmap_write(pca->regmap, LED_N_OFF_L(pwm->hwpwm),
+				     off & 0xff);
+			regmap_write(pca->regmap, LED_N_OFF_H(pwm->hwpwm),
+				     (off >> 8) & 0xf);
+			return 0;
+		}
+
+		/* No staggering possible if "all LEDs" channel is used */
+		regmap_write(pca->regmap, PCA9685_ALL_LED_ON_L, 0);
+		regmap_write(pca->regmap, PCA9685_ALL_LED_ON_H, 0);
+	}
+
 	if (pwm->hwpwm >= PCA9685_MAXCHAN)
 		reg = PCA9685_ALL_LED_OFF_L;
 	else
@@ -494,6 +521,9 @@ 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, "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);
@@ -502,6 +532,9 @@ static int pca9685_pwm_probe(struct i2c_client *client,
 	/* Reset OFF registers to HW default (only full OFF bit is set) */
 	regmap_write(pca->regmap, PCA9685_ALL_LED_OFF_L, 0);
 	regmap_write(pca->regmap, PCA9685_ALL_LED_OFF_H, LED_FULL);
+	/* Reset ON registers to HW default */
+	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 */
-- 
2.29.2