Re: Discussion on device's runtime wake capability

[Aaron Lu <aaron.lu@intel.com>]

On 10/11/2012 06:59 AM, Rafael J. Wysocki wrote:
> On Wednesday 10 of October 2012 08:53:32 Aaron Lu wrote:
>> On 10/10/2012 04:48 AM, Rafael J. Wysocki wrote:
>>> On Tuesday 09 of October 2012 14:23:12 Aaron Lu wrote:
>>>> Hi all,
>>>>
>>>> We are using _PRW as a hint to see if a device supports wakeup, this is
>>>> fine for device which is able to wake the system in a sleep state, but
>>>> not to wake itself when system is at S0.
>>>
>>> Why not?
>>
>> Section 7.2.13 of ACPI spec has words like this:
>> _PRW is only required for devices that have the ability to wake the
>> system from a system sleeping state.
>>
>> This is why I don't think _PRW is necessary for run wake as we are still
>> at S0.
> 
> It may not be necessary, but if it is present, it gives us certain information
> about the device's wakeup capabilities.

Agree.

> 
>>>> Moreover, when we are to arm the device runtime wake, I think there is
>>>> no need to power on the power resources referenced in _PRW,
>>>
>>> Why do you think so?  How can you be sure that those resources are not needed
>>> to provide wakeup power to the device (or whatever generates the wakeup signal
>>> on its behalf))?
>>
>> The same reason as above, the power resources are needed to arm the
>> device to wake the system from a sleeping state, not S0.
> 
> The spec doesn't say pretty much anything about S0 in that context and we
> found out some things by experimentation.

OK.

> 
>> But it's really good to know if there are systems that has a requirement
>> for _PRW to be on for the device to wake itself up when system is at S0.
> 
> Yes, there are.  Please see the Matthew's response.

I was thinking in his example, the device may not have a _S0W object.
I was waiting for his reply.

I agree that for devices only have _PRW but not _S0W, if we want run
wake, we will need to power on _PRW.

> 
>>>> those power resources should be used to give the device ability to wake
>>>> the system from a sleep state, not to wake itself when system is at S0,
>>>> so powering thoses power resources on for run wake is a waste.
>>>
>>> Where did you get this idea from?
>>
>> Section 7.2.13 of ACPI spec.
> 
> I thought so. :-)
> 
> That part of the spec doesn't cover runtime wakeup at all, so I'm not sure it's
> a good reference.

OK.

> 
>>>> But I may miss something, so it would be very kind of you to point it
>>>> out if things are not like what I've thought, thanks.
>>>
>>> Yes, you do.  For example, _PRW gives us the number of the GPE to use for
>>> signalling wakeup, right?
>>
>> Yes, but I think this is a separate story.
>> The GPE will still be needed, but the power resources may not.
> 
> They may or may not be needed, that's the problem.  We basically cannot tell
> that just by reading the ACPI tables in many cases.

It doesn't hurt if we enable the GPE, since either we as OSPM will
enable it, or the ASL code will do that.

This is not like the power resources, which will waste some energy.

> 
>>>> BTW, _S0W seems to be a good hint whether the device supports run wake
>>>> from ACPI's perspective.
>>>
>>> Yes, it is a _hint_.
>>
>> And I think it is a better hint than _PRW.
>> I've seen a ACPI table that uses something like this to express the run
>> wake of the device:
>> - It has _S0W;
>> - It has _DSW;
>> - No _PRW;
>> - It has a _CRS method to describe an interrupt resource.
> 
> That's fine I suppose, because this seems to be a device whose interrupt
> wakes it up.  We didn't consider such devices before.
> 
> However, that doesn't mean that if _PRW _is_ defined and it lists some
> power resources, then those resources are only necessary for waking up
> the whole system from sleep states.  They may be necessary for remote
> wakeup to work in S0 too.
> 
>> And I've tested with a device which is able to both run wake and system
>> wake. If I do not power those _PRW power resources, there is no problem
>> for it to generate wake signal when system is at S0, and it even takes
>> care of the GPE thing in ASL code. But this may be special cases, so I
>> want to have a discussion on this.
>>
>> So I hope we can refresh the way we check if the device is able to run
>> wake.
> 
> Yes, we can.

Good :-)

> 
>> And if possible, we can change the way we do to arm the device for
>> run wake by not powering those _PRW power resources.
> 
> No, we can't.
> 
> Do you have examples of devices where it is harmful?

Yes, for the ZPODD device.
It has _PRW specifying it can wake the system up from S4. And I tried
not to powering on the power resources referenced in _PRW, no problem
for it to generate wake event in S0.


Let me try to conclude:
- The behaviour of devices which only have _PRW should not be changed;
- We can change the way we check if a device is run wake capable by
  checking if it has _PRW or _S0W;
- For devices which have both _PRW and _S0W, we need consider if
  powering on _PRW is needed for run wake.

Does this look correct to you?

Thanks,
Aaron