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pwm: Prevent a glitch for legacy drivers

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If a running PWM is reconfigured to disabled calling the ->config()
callback before disabling the hardware might result in a glitch where
the (maybe) new period and duty_cycle are visible on the output before
disabling the hardware.

So handle disabling before calling ->config(). Also exit early in this case
which is possible because period and duty_cycle don't matter for disabled PWMs.
In return however ->config has to be called even if state->period ==
pwm->state.period && state->duty_cycle != pwm->state.duty_cycle because setting
these might have been skipped in the previous call.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
This commit is contained in:
Uwe Kleine-König 2021-07-01 09:29:26 +02:00 committed by Thierry Reding
parent 77965c98cf
commit 92f69e582e
1 changed files with 24 additions and 17 deletions
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41
drivers/pwm/core.c
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@ -555,26 +555,33 @@ static int pwm_apply_legacy(struct pwm_chip *chip, struct pwm_device *pwm,
pwm->state.polarity = state->polarity;
}
if (state->period != pwm->state.period ||
state->duty_cycle != pwm->state.duty_cycle) {
err = chip->ops->config(pwm->chip, pwm,
state->duty_cycle,
state->period);
if (err)
return err;
if (!state->enabled) {
if (pwm->state.enabled)
chip->ops->disable(chip, pwm);
pwm->state.period = state->period;
pwm->state.duty_cycle = state->duty_cycle;
return 0;
}
if (state->enabled != pwm->state.enabled) {
if (!pwm->state.enabled) {
err = chip->ops->enable(chip, pwm);
if (err)
return err;
} else {
chip->ops->disable(chip, pwm);
}
/*
* We cannot skip calling ->config even if state->period ==
* pwm->state.period && state->duty_cycle == pwm->state.duty_cycle
* because we might have exited early in the last call to
* pwm_apply_state because of !state->enabled and so the two values in
* pwm->state might not be configured in hardware.
*/
err = chip->ops->config(pwm->chip, pwm,
state->duty_cycle,
state->period);
if (err)
return err;
pwm->state.period = state->period;
pwm->state.duty_cycle = state->duty_cycle;
if (!pwm->state.enabled) {
err = chip->ops->enable(chip, pwm);
if (err)
return err;
}
return 0;