pwm: Allow pwm state transitions from an invalid state

While driving a PWM via the sysfs API it's hard to determine the right
order of writes to the pseudo files "period" and "duty_cycle":

If you want to go from duty_cycle/period = 50/100 to 150/300 you have to
write period first (because 150/100 is invalid). If however you start at
400/500 the duty_cycle must be configured first. The rule that works is:
If you increase period write period first, otherwise write duty_cycle
first. A complication however is that it's usually sensible to configure
the polarity before both period and duty_cycle. This can only be done if
the current state's duty_cycle and period configuration isn't bogus
though. It is still worse (but I think only theoretic) if you have a PWM
that only supports inverted polarity and you start with period = 0 and
polarity = normal. Then you can change neither period (because polarity
= normal is refused) nor polarity (because there is still period = 0).

To simplify the corner cases for userspace, let invalid target states
pass if the current state is invalid already.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20240628103519.105020-2-u.kleine-koenig@baylibre.comSigned-off-by: Uwe Kleine-König <ukleinek@kernel.org>

drivers/pwm/core.c

@@ -137,6 +137,25 @@ static void pwm_apply_debug(struct pwm_device *pwm,
 	}
 }
 
+static bool pwm_state_valid(const struct pwm_state *state)
+{
+	/*
+	 * For a disabled state all other state description is irrelevant and
+	 * and supposed to be ignored. So also ignore any strange values and
+	 * consider the state ok.
+	 */
+	if (state->enabled)
+		return true;
+
+	if (!state->period)
+		return false;
+
+	if (state->duty_cycle > state->period)
+		return false;
+
+	return true;
+}
+
 /**
  * __pwm_apply() - atomically apply a new state to a PWM device
  * @pwm: PWM device
@@ -147,9 +166,25 @@ static int __pwm_apply(struct pwm_device *pwm, const struct pwm_state *state)
 	struct pwm_chip *chip;
 	int err;
 
-	if (!pwm || !state || !state->period ||
-	    state->duty_cycle > state->period)
+	if (!pwm || !state)
+		return -EINVAL;
+
+	if (!pwm_state_valid(state)) {
+		/*
+		 * Allow to transition from one invalid state to another.
+		 * This ensures that you can e.g. change the polarity while
+		 * the period is zero. (This happens on stm32 when the hardware
+		 * is in its poweron default state.) This greatly simplifies
+		 * working with the sysfs API where you can only change one
+		 * parameter at a time.
+		 */
+		if (!pwm_state_valid(&pwm->state)) {
+			pwm->state = *state;
+			return 0;
+		}
+
 		return -EINVAL;
+	}
 
 	chip = pwm->chip;