pwm: lpss: Make pwm_lpss_apply() not rely on existing hardware state

Before this commit pwm_lpss_apply() was assuming 2 pre-conditions
were met by the existing hardware state:

1. That the base-unit and on-time-div read back from the
control register are those actually in use, so that it
can skip setting the update bit if the read-back value
matches the desired values.

2. That the controller is enabled when the cached
pwm_state.enabled says that the controller is enabled.

As the long history of fixes for subtle (often suspend/resume)
lpss-pwm issues shows, these assumptions are not necessary
always true.

1. Specifically is not true on some (*) Cherry Trail devices
with a nasty GFX0._PS3 method which: a. saves the ctrl reg value.
b. sets the base-unit to 0 and writes the update bit to apply/commit
c. restores the original ctrl value without setting the update bit,
so that the 0 base-unit value is still in use.

2. Assumption 2. currently is true, but only because of the code which
saves/restores the state on suspend/resume. By convention restoring the
PWM state should be done by the PWM consumer and the presence of this
code in the pmw-lpss driver is a bug. Therefor the save/restore code will
be dropped in the next patch in this series, after which this assumption
also is no longer true.

This commit changes the pwm_lpss_apply() to not make any assumptions about
the state the hardware is in. Instead it makes pwm_lpss_apply() always
fully program the PWM controller, making it much less fragile.

*) Seen on the Acer One 10 S1003, Lenovo Ideapad Miix 310 and 320 models
and various Medion models.
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200903112337.4113-7-hdegoede@redhat.com

drivers/pwm/pwm-lpss.c

@@ -85,7 +85,7 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
 	unsigned long long on_time_div;
 	unsigned long c = lpwm->info->clk_rate, base_unit_range;
 	unsigned long long base_unit, freq = NSEC_PER_SEC;
-	u32 orig_ctrl, ctrl;
+	u32 ctrl;
 
 	do_div(freq, period_ns);
 
@@ -104,16 +104,14 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
 	do_div(on_time_div, period_ns);
 	on_time_div = 255ULL - on_time_div;
 
-	orig_ctrl = ctrl = pwm_lpss_read(pwm);
+	ctrl = pwm_lpss_read(pwm);
 	ctrl &= ~PWM_ON_TIME_DIV_MASK;
 	ctrl &= ~((base_unit_range - 1) << PWM_BASE_UNIT_SHIFT);
 	ctrl |= (u32) base_unit << PWM_BASE_UNIT_SHIFT;
 	ctrl |= on_time_div;
 
-	if (orig_ctrl != ctrl) {
-		pwm_lpss_write(pwm, ctrl);
-		pwm_lpss_write(pwm, ctrl | PWM_SW_UPDATE);
-	}
+	pwm_lpss_write(pwm, ctrl);
+	pwm_lpss_write(pwm, ctrl | PWM_SW_UPDATE);
 }
 
 static inline void pwm_lpss_cond_enable(struct pwm_device *pwm, bool cond)
@@ -124,8 +122,7 @@ static inline void pwm_lpss_cond_enable(struct pwm_device *pwm, bool cond)
 
 static int pwm_lpss_prepare_enable(struct pwm_lpss_chip *lpwm,
 				   struct pwm_device *pwm,
-				   const struct pwm_state *state,
-				   bool enable)
+				   const struct pwm_state *state)
 {
 	int ret;
 
@@ -134,12 +131,12 @@ static int pwm_lpss_prepare_enable(struct pwm_lpss_chip *lpwm,
 		return ret;
 
 	pwm_lpss_prepare(lpwm, pwm, state->duty_cycle, state->period);
-	pwm_lpss_cond_enable(pwm, enable && lpwm->info->bypass == false);
+	pwm_lpss_cond_enable(pwm, lpwm->info->bypass == false);
 	ret = pwm_lpss_wait_for_update(pwm);
 	if (ret)
 		return ret;
 
-	pwm_lpss_cond_enable(pwm, enable && lpwm->info->bypass == true);
+	pwm_lpss_cond_enable(pwm, lpwm->info->bypass == true);
 	return 0;
 }
 
@@ -152,11 +149,11 @@ static int pwm_lpss_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	if (state->enabled) {
 		if (!pwm_is_enabled(pwm)) {
 			pm_runtime_get_sync(chip->dev);
-			ret = pwm_lpss_prepare_enable(lpwm, pwm, state, true);
+			ret = pwm_lpss_prepare_enable(lpwm, pwm, state);
 			if (ret)
 				pm_runtime_put(chip->dev);
 		} else {
-			ret = pwm_lpss_prepare_enable(lpwm, pwm, state, false);
+			ret = pwm_lpss_prepare_enable(lpwm, pwm, state);
 		}
 	} else if (pwm_is_enabled(pwm)) {
 		pwm_lpss_write(pwm, pwm_lpss_read(pwm) & ~PWM_ENABLE);