Re: [PATCH] pwm: lpss: Fix get_state runtime-pm reference handling

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Hi,

On 5/12/20 9:01 PM, Andy Shevchenko wrote:
On Tue, May 12, 2020 at 01:00:44PM +0200, Hans de Goede wrote:
Before commit cfc4c189bc70 ("pwm: Read initial hardware state at request
time"), a driver's get_state callback would get called once per PWM from
pwmchip_add().

pwm-lpss' runtime-pm code was relying on this, getting a runtime-pm ref for
PWMs which are enabled at probe time from within its get_state callback,
before enabling runtime-pm.

The change to calling get_state at request time causes a number of
problems:

1. PWMs enabled at probe time may get runtime suspended before they are
requested, causing e.g. a LCD backlight controlled by the PWM to turn off.

2. When the request happens when the PWM has been runtime suspended, the
ctrl register will read all 1 / 0xffffffff, causing get_state to store
bogus values in the pwm_state.

3. get_state was using an async pm_runtime_get() call, because it assumed
that runtime-pm has not been enabled yet. If shortly after the request an
apply call is made, then the pwm_lpss_is_updating() check may trigger
because the resume triggered by the pm_runtime_get() call is not complete
yet, so the ctrl register still reads all 1 / 0xffffffff.

This commit fixes these issues by moving the initial pm_runtime_get() call
for PWMs which are enabled at probe time to the pwm_lpss_probe() function;
and by making get_state take a runtime-pm ref before reading the ctrl reg.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>

Thank you.

One thing to consider below.

BugLink: https://bugzilla.redhat.com/show_bug.cgi?id=1828927
Fixes: cfc4c189bc70 ("pwm: Read initial hardware state at request time")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>
---
  drivers/pwm/pwm-lpss.c | 15 +++++++++++----
  1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
index 75bbfe5f3bc2..9d965ffe66d1 100644
--- a/drivers/pwm/pwm-lpss.c
+++ b/drivers/pwm/pwm-lpss.c
@@ -158,7 +158,6 @@ static int pwm_lpss_apply(struct pwm_chip *chip, struct pwm_device *pwm,
  	return 0;
  }
-/* This function gets called once from pwmchip_add to get the initial state */
  static void pwm_lpss_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
  			       struct pwm_state *state)
  {
@@ -167,6 +166,8 @@ static void pwm_lpss_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
  	unsigned long long base_unit, freq, on_time_div;
  	u32 ctrl;
+ pm_runtime_get_sync(chip->dev);
+
  	base_unit_range = BIT(lpwm->info->base_unit_bits);
ctrl = pwm_lpss_read(pwm);
@@ -187,8 +188,7 @@ static void pwm_lpss_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
  	state->polarity = PWM_POLARITY_NORMAL;
  	state->enabled = !!(ctrl & PWM_ENABLE);
- if (state->enabled)
-		pm_runtime_get(chip->dev);
+	pm_runtime_put(chip->dev);
  }
static const struct pwm_ops pwm_lpss_ops = {
@@ -202,7 +202,8 @@ struct pwm_lpss_chip *pwm_lpss_probe(struct device *dev, struct resource *r,
  {
  	struct pwm_lpss_chip *lpwm;
  	unsigned long c;
-	int ret;
+	int i, ret;
+	u32 ctrl;
if (WARN_ON(info->npwm > MAX_PWMS))
  		return ERR_PTR(-ENODEV);
@@ -232,6 +233,12 @@ struct pwm_lpss_chip *pwm_lpss_probe(struct device *dev, struct resource *r,
  		return ERR_PTR(ret);
  	}
+ for (i = 0; i < lpwm->info->npwm; i++) {

+		ctrl = pwm_lpss_read(&lpwm->chip.pwms[i]);
+		if (ctrl & PWM_ENABLE)

I would create a helper for this as opposite to pwm_lpss_cond_enable(),
something like pwm_lpss_is_enabled().

pwm_lpss_cond_enable() is used in multiple places, this check
we only need once, so I would prefer to just keep it as is.

Regards,

Hans





+			pm_runtime_get(dev);
+	}
+
  	return lpwm;
  }
  EXPORT_SYMBOL_GPL(pwm_lpss_probe);
--
2.26.0






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