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pwm: Allow pwm state transitions from an invalid state

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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>
This commit is contained in:
Uwe Kleine-König 2024-06-28 12:35:19 +02:00 committed by Uwe Kleine-König
parent 14b9dc66e9
commit 9dd42d019e
1 changed files with 37 additions and 2 deletions
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39
drivers/pwm/core.c
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@ -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
@ -147,10 +166,26 @@ 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;
if (state->period == pwm->state.period &&