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>

diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
index cf6d91f1a9e629d802243f4bce365194e03257a0..8acbcf5b66739f13be83026aea37b1d4d573bc89 100644 (file)
--- a/drivers/pwm/core.c
+++ b/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
@@ -147,10 +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 &&