pwm: Make it explicit that pwm_apply_state() might sleep

At least some implementations sleep. So mark pwm_apply_state() with a
might_sleep() to make callers aware. In the worst case this uncovers a
valid atomic user, then we revert this patch and at least gained some more
knowledge and then can work on a concept similar to
gpio_get_value/gpio_get_value_cansleep.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/core.c

@@ -532,6 +532,15 @@ int pwm_apply_state(struct pwm_device *pwm, const struct pwm_state *state)
 	struct pwm_chip *chip;
 	int err;
 
+	/*
+	 * Some lowlevel driver's implementations of .apply() make use of
+	 * mutexes, also with some drivers only returning when the new
+	 * configuration is active calling pwm_apply_state() from atomic context
+	 * is a bad idea. So make it explicit that calling this function might
+	 * sleep.
+	 */
+	might_sleep();
+
 	if (!pwm || !state || !state->period ||
 	    state->duty_cycle > state->period)
 		return -EINVAL;

include/linux/pwm.h

@@ -441,6 +441,7 @@ static inline void pwm_free(struct pwm_device *pwm)
 static inline int pwm_apply_state(struct pwm_device *pwm,
 				  const struct pwm_state *state)
 {
+	might_sleep();
 	return -ENOTSUPP;
 }
 
@@ -452,6 +453,7 @@ static inline int pwm_adjust_config(struct pwm_device *pwm)
 static inline int pwm_config(struct pwm_device *pwm, int duty_ns,
 			     int period_ns)
 {
+	might_sleep();
 	return -EINVAL;
 }
 
@@ -464,11 +466,13 @@ static inline int pwm_capture(struct pwm_device *pwm,
 
 static inline int pwm_enable(struct pwm_device *pwm)
 {
+	might_sleep();
 	return -EINVAL;
 }
 
 static inline void pwm_disable(struct pwm_device *pwm)
 {
+	might_sleep();
 }
 
 static inline int pwm_set_chip_data(struct pwm_device *pwm, void *data)