Commit 9dd42d01 authored Jun 28, 2024 by Uwe Kleine-König Committed by Uwe Kleine-König Jul 10, 2024

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.com


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

parent 14b9dc66

drivers/pwm/core.c

+37 −2

Original line number	Diff line number	Diff line
		@@ -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;