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

[Hans de Goede <hdegoede@redhat.com>]

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@linux.intel.com>

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@vger.kernel.org
>> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
>> ---
>>   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
>>
>