Re: [PATCH 2/2] pwm: lpss: Check PWM powerstate after resume on Cherry Trail devices

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

Hi,

On 24-09-18 11:18, Andy Shevchenko wrote:
> On Mon, Sep 24, 2018 at 11:10:28AM +0200, Hans de Goede wrote:
> 
>>>> +	/* The PWM may be turned on by AML code, update our state to match */
>>>> +	if (pm_runtime_suspended(dev) && lpwm->info->check_power_on_resume) {
>>>
>>>> +		status = acpi_evaluate_integer(ACPI_HANDLE(dev), "_PSC",
>>>> +					       NULL, &psc);
>>>
>>> AFAIU this is a standard power source method for ACPI, shouldn't ACPI core take
>>> care of being in sync?
>>
>> This is not about ACPI power-resources, this is about the power state (D0 or D3)
>> of the device itself. The ACPI core does not expect the state of devices to
>> magically change underneath it when using s2idle, since then everything is
>> under the kernel's control. But the _PS0 method of the GPU messing with the PWM
>> controller (hurray for firmware) messes things up.
> 
> What I mean is shouldn't we care about this on a ACPI core level to be sure
> that states are kept in sync on OS level?

The ACPI / pm core does care about this when doing a firmware supported suspend/resume
(so going to regular S3) because the firmware can then make all sort of changes
to the device state.

But with s2idle the kernel is fully in control and we never hand control over to the
firmware, so checking the device state then should not be necessary and is a somewhat
expensive operation (esp. to do on all devices), while one of the advantages of s2idle
is supposed to be shorter suspend/resume times.

IOW for s2idle it is undesirable for the core to check the power-state of all
devices after a resume, since it simply should not have changed, with the PWM
device being a nasty exception because of the _PS0 method for *another* device
mucking with it.

Anyways lets see what Rafael has to say about this.

Regards,

Hans