pwm: pca9685: Restrict period change for enabled PWMs

Previously, the last used PWM channel could change the global prescale
setting, even if other channels are already in use.

Fix it by only allowing the first enabled PWM to change the global
chip-wide prescale setting. If there is more than one channel in use,
the prescale settings resulting from the chosen periods must match.

GPIOs do not count as enabled PWMs as they are not using the prescaler
and can't change it.

Signed-off-by: Clemens Gruber <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>

drivers/pwm/pwm-pca9685.c

···
 #include <linux/bitmap.h>
 
 /*
- * Because the PCA9685 has only one prescaler per chip, changing the period of
- * one channel affects the period of all 16 PWM outputs!
- * However, the ratio between each configured duty cycle and the chip-wide
- * period remains constant, because the OFF time is set in proportion to the
- * counter range.
+ * Because the PCA9685 has only one prescaler per chip, only the first channel
+ * that is enabled is allowed to change the prescale register.
+ * PWM channels requested afterwards must use a period that results in the same
+ * prescale setting as the one set by the first requested channel.
+ * GPIOs do not count as enabled PWMs as they are not using the prescaler.
  */
 
 #define PCA9685_MODE1		0x00
···
 struct pca9685 {
 	struct pwm_chip chip;
 	struct regmap *regmap;
-#if IS_ENABLED(CONFIG_GPIOLIB)
 	struct mutex lock;
+	DECLARE_BITMAP(pwms_enabled, PCA9685_MAXCHAN + 1);
+#if IS_ENABLED(CONFIG_GPIOLIB)
 	struct gpio_chip gpio;
 	DECLARE_BITMAP(pwms_inuse, PCA9685_MAXCHAN + 1);
 #endif
···
 static inline struct pca9685 *to_pca(struct pwm_chip *chip)
 {
 	return container_of(chip, struct pca9685, chip);
+}
+
+/* This function is supposed to be called with the lock mutex held */
+static bool pca9685_prescaler_can_change(struct pca9685 *pca, int channel)
+{
+	/* No PWM enabled: Change allowed */
+	if (bitmap_empty(pca->pwms_enabled, PCA9685_MAXCHAN + 1))
+		return true;
+	/* More than one PWM enabled: Change not allowed */
+	if (bitmap_weight(pca->pwms_enabled, PCA9685_MAXCHAN + 1) > 1)
+		return false;
+	/*
+	 * Only one PWM enabled: Change allowed if the PWM about to
+	 * be changed is the one that is already enabled
+	 */
+	return test_bit(channel, pca->pwms_enabled);
 }
 
 /* Helper function to set the duty cycle ratio to duty/4096 (e.g. duty=2048 -> 50%) */
···
 {
 	struct device *dev = pca->chip.dev;
 
-	mutex_init(&pca->lock);
-
 	pca->gpio.label = dev_name(dev);
 	pca->gpio.parent = dev;
 	pca->gpio.request = pca9685_pwm_gpio_request;
···
 	}
 }
 
-static int pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
-			     const struct pwm_state *state)
+static int __pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
+			       const struct pwm_state *state)
 {
 	struct pca9685 *pca = to_pca(chip);
 	unsigned long long duty, prescale;
···
 
 	regmap_read(pca->regmap, PCA9685_PRESCALE, &val);
 	if (prescale != val) {
+		if (!pca9685_prescaler_can_change(pca, pwm->hwpwm)) {
+			dev_err(chip->dev,
+				"pwm not changed: periods of enabled pwms must match!\n");
+			return -EBUSY;
+		}
+
 		/*
 		 * Putting the chip briefly into SLEEP mode
 		 * at this point won't interfere with the
···
 	duty = DIV_ROUND_UP_ULL(duty, state->period);
 	pca9685_pwm_set_duty(pca, pwm->hwpwm, duty);
 	return 0;
+}
+
+static int pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
+			     const struct pwm_state *state)
+{
+	struct pca9685 *pca = to_pca(chip);
+	int ret;
+
+	mutex_lock(&pca->lock);
+	ret = __pca9685_pwm_apply(chip, pwm, state);
+	if (ret == 0) {
+		if (state->enabled)
+			set_bit(pwm->hwpwm, pca->pwms_enabled);
+		else
+			clear_bit(pwm->hwpwm, pca->pwms_enabled);
+	}
+	mutex_unlock(&pca->lock);
+
+	return ret;
 }
 
 static void pca9685_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
···
 
 	if (pca9685_pwm_test_and_set_inuse(pca, pwm->hwpwm))
 		return -EBUSY;
+
+	if (pwm->hwpwm < PCA9685_MAXCHAN) {
+		/* PWMs - except the "all LEDs" channel - default to enabled */
+		mutex_lock(&pca->lock);
+		set_bit(pwm->hwpwm, pca->pwms_enabled);
+		mutex_unlock(&pca->lock);
+	}
+
 	pm_runtime_get_sync(chip->dev);
 
 	return 0;
···
 {
 	struct pca9685 *pca = to_pca(chip);
 
+	mutex_lock(&pca->lock);
 	pca9685_pwm_set_duty(pca, pwm->hwpwm, 0);
+	clear_bit(pwm->hwpwm, pca->pwms_enabled);
+	mutex_unlock(&pca->lock);
+
 	pm_runtime_put(chip->dev);
 	pca9685_pwm_clear_inuse(pca, pwm->hwpwm);
 }
···
 	}
 
 	i2c_set_clientdata(client, pca);
+
+	mutex_init(&pca->lock);
 
 	regmap_read(pca->regmap, PCA9685_MODE2, &reg);