Re: [PATCH v3 1/6] PM / core: Add LEAVE_SUSPENDED driver flag

[Ulf Hansson]

On 17 November 2017 at 15:31, Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> On Fri, Nov 17, 2017 at 2:49 PM, Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
>> [...]
>>
>>>>
>>>>>> Second, have you considered setting the default value of
>>>>>> dev->power.may_skip_resume to true?
>>>>>
>>>>> Yes.
>>>>>
>>>>>> That would means the subsystem
>>>>>> instead need to implement an opt-out method. I am thinking that it may
>>>>>> not be an issue, since we anyway at this point, don't have drivers
>>>>>> using the LEAVE_SUSPENDED flag.
>>>>>
>>>>> Opt-out doesn't work because of the need to invoke the "noirq" callbacks.
>>>>
>>>> I am not sure I follow that.
>>>>
>>>> Whatever needs to be fixed on the subsystem level, that could be done
>>>> before the driver starts using the LEAVE_SUSPENDED flag. No?
>>>
>>> That requires a bit of explanation, sorry for being overly concise.
>>>
>>> The core calls ->resume_noirq from the middle layer regardless of
>>> whether or not the device will be left suspended, so the
>>> ->resume_noirq cannot do arbitrary things to it.  Setting
>>> may_skip_resume by the middle layer tells the core that the middle
>>> layer is ready for that and is going to cooperate.  If may_skip_resume
>>> had been set by default, that piece of information would have been
>>> missing.
>>
>> Huh, I still don't get that. Sorry.
>>
>> If the "may_skip_resume" is default set to true by the PM core,
>> wouldn't that just mean that the middle-layer needs to implement an
>> opt-out method, rather than opt-in. In principle to opt-out the
>> middle-layer needs to set may_skip_resume to false in suspend_noirq
>> phase, no?
>
> Yes, but if the middle-layer doesn't clear it, that may mean two
> things.  First, the middle layer is ready and so on.  Good.  Second,
> the middle layer is not aware of the whole thing.  Not good.  The core
> cannot tell.
>
> In the opt-in case, however, all is clear. :-)

Okay.

>
>> Then we only need to make sure drivers don't starts use
>> LEAVE_SUSPENDED, before we make sure the middle layers is adopted. But
>> that should not be a problem.
>>
>> The benefit would be that those middle layers that can cope with
>> LEAVE_SUSPENDED as of today don't need to change.
>
> I'm not sure if that's the case.
>
> The middle layer has to evaluate dev_pm_may_skip_resume() in
> ->resume_noirq() to check if the device can be left in suspend, as it
> cannot determine that in ->suspend_noirq() yet.

Right. Okay, let's stick with the chosen method.

Kind regards
Uffe