Re: [PATCH v4 05/16] pwm: lpss: Use pwm_lpss_apply() when restoring state on resume

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

Hi,

On 7/9/20 3:36 PM, Andy Shevchenko wrote:
> On Wed, Jul 08, 2020 at 11:14:21PM +0200, Hans de Goede wrote:
>> Before this commit a suspend + resume of the LPSS PWM controller
>> would result in the controller being reset to its defaults of
>> output-freq = clock/256, duty-cycle=100%, until someone changes
>> to the output-freq and/or duty-cycle are made.
>>
>> This problem has been masked so far because the main consumer
>> (the i915 driver) was always making duty-cycle changes on resume.
>> With the conversion of the i915 driver to the atomic PWM API the
>> driver now only disables/enables the PWM on suspend/resume leaving
>> the output-freq and duty as is, triggering this problem.
>>
>> The LPSS PWM controller has a mechanism where the ctrl register value
>> and the actual base-unit and on-time-div values used are latched. When
>> software sets the SW_UPDATE bit then at the end of the current PWM cycle,
>> the new values from the ctrl-register will be latched into the actual
>> registers, and the SW_UPDATE bit will be cleared.
>>
>> The problem is that before this commit our suspend/resume handling
>> consisted of simply saving the PWM ctrl register on suspend and
>> restoring it on resume, without setting the PWM_SW_UPDATE bit.
>> When the controller has lost its state over a suspend/resume and thus
>> has been reset to the defaults, just restoring the register is not
>> enough. We must also set the SW_UPDATE bit to tell the controller to
>> latch the restored values into the actual registers.
>>
>> Fixing this problem is not as simple as just or-ing in the value which
>> is being restored with SW_UPDATE. If the PWM was enabled before we must
>> write the new settings + PWM_SW_UPDATE before setting PWM_ENABLE.
>> We must also wait for PWM_SW_UPDATE to become 0 again and depending on the
>> model we must do this either before or after the setting of PWM_ENABLE.
>>
>> All the necessary logic for doing this is already present inside
>> pwm_lpss_apply(), so instead of duplicating this inside the resume
>> handler, this commit makes the resume handler use pwm_lpss_apply() to
>> restore the settings when necessary. This fixes the output-freq and
>> duty-cycle being reset to their defaults on resume.
> 
> ...
> 
>> +static int __pwm_lpss_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>> +			    const struct pwm_state *state, bool from_resume)
>>   {
>>   	struct pwm_lpss_chip *lpwm = to_lpwm(chip);
>>   	int ret;
>>   
>>   	if (state->enabled) {
>>   		if (!pwm_is_enabled(pwm)) {
>> -			pm_runtime_get_sync(chip->dev);
>> +			if (!from_resume)
>> +				pm_runtime_get_sync(chip->dev);
>> +
>>   			ret = pwm_lpss_is_updating(pwm);
>>   			if (ret) {
>> -				pm_runtime_put(chip->dev);
>> +				if (!from_resume)
>> +					pm_runtime_put(chip->dev);
>> +
>>   				return ret;
>>   			}
>>   			pwm_lpss_prepare(lpwm, pwm, state->duty_cycle, state->period);
>>   			pwm_lpss_cond_enable(pwm, lpwm->info->bypass == false);
>>   			ret = pwm_lpss_wait_for_update(pwm);
>>   			if (ret) {
>> -				pm_runtime_put(chip->dev);
>> +				if (!from_resume)
>> +					pm_runtime_put(chip->dev);
>> +
>>   				return ret;
>>   			}
>>   			pwm_lpss_cond_enable(pwm, lpwm->info->bypass == true);
> 
>>   		}
>>   	} else if (pwm_is_enabled(pwm)) {
>>   		pwm_lpss_write(pwm, pwm_lpss_read(pwm) & ~PWM_ENABLE);
>> -		pm_runtime_put(chip->dev);
>> +
>> +		if (!from_resume)
>> +			pm_runtime_put(chip->dev);
>>   	}
> 
> I'm wondering if splitting more will make this look better, like:
> 
> 	...
> 	if (from_resume) {
> 		ret = pwm_lpss_prepare_enable(...); // whatever name you think suits better
> 	} else {
> 		pm_runtime_get_sync(...);
> 		ret = pwm_lpss_prepare_enable(...);
> 		if (ret)
> 			pm_runtime_put(...);
> 	}
> 	...
> 

That is a good idea, I like it. We already had multiple pm_runtime_put() calls
before for the error handlig and this patch did not make it any better.

So adding a pwm_lpss_prepare_enable() helper (the name works for)
will also cleanup the original code. I will add this helper as
a separate preparation patch for this one in v5 of the patch-set.

Regards,

Hans