linux-kernel - Re: [PATCH v3 1/6] PM / core: Add LEAVE

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Date:   Fri, 17 Nov 2017 16:57:05 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux Documentation <linux-doc@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kevin Hilman <khilman@...nel.org>
Subject: Re: [PATCH v3 1/6] PM / core: Add LEAVE_SUSPENDED driver flag

On 17 November 2017 at 15:31, Rafael J. Wysocki <rafael@...nel.org> wrote:
> On Fri, Nov 17, 2017 at 2:49 PM, Ulf Hansson <ulf.hansson@...aro.org> 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