linux-kernel - Re: [PATCH v2 0/2] pinctrl: Allow indicating loss of state across suspend/resume

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Message-ID: <952312f9-2c26-58bc-e34f-76ab1fb5f05d@gmail.com>
Date:   Tue, 7 Nov 2017 16:28:24 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Tony Lindgren <tony@...mide.com>
Cc:     Charles Keepax <ckeepax@...nsource.cirrus.com>,
        linux-gpio@...r.kernel.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        ckeepax@...nsource.wolfsonmicro.com, swarren@...dia.com,
        andy.shevchenko@...il.com, alcooperx@...il.com,
        bcm-kernel-feedback-list@...adcom.com, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 0/2] pinctrl: Allow indicating loss of state across
 suspend/resume

On 11/07/2017 04:23 PM, Rafael J. Wysocki wrote:
> On Tuesday, November 7, 2017 5:00:06 PM CET Tony Lindgren wrote:
>> * Florian Fainelli <f.fainelli@...il.com> [171104 17:21]:
>>>
>>>
>>> On 11/04/2017 05:25 AM, Rafael J. Wysocki wrote:
>>>> On Friday, November 3, 2017 6:33:53 PM CET Tony Lindgren wrote:
>>>>> * Florian Fainelli <f.fainelli@...il.com> [171103 17:04]:
>>>>>> On 11/03/2017 09:11 AM, Tony Lindgren wrote:
>>>>>> The pinctrl provider is losing its state, hence these two patches.
>>>>>
>>>>> OK
>>>>>
>>>>>>> Anyways, the context lost flag should be managed in the PM core for
>>>>>>> the device, so adding linux-pm and Rafael to Cc.
>>>>>>
>>>>>> I don't think it's that simple but sure, why not.
>>>>>
>>>>> Just having bool context_lost in struct dev_pm_info would probably
>>>>> be enough to allow drivers to deal with it. This flag could then
>>>>> be set for a device by power domain related code that knows if
>>>>> context got lost.
>>>>
>>>> Something like: if the driver sees "context_lost" set, it should restore
>>>> the context to the device from memory?
>>>
>>> That is what is being proposed here, except that the actual mechanism
>>> where this matters needs to be in the core pinctrl code, otherwise the
>>> state (context) is not restored due to a check that attempts not to
>>> (re)apply a previous state.
>>>
>>>>
>>>> But the it would also need to save the context beforehand, so why not to
>>>> restore it unconditionally on resume?
>>>
>>> That's what my original attempts did here:
>>>
>>> https://patchwork.kernel.org/patch/9598969/
>>>
>>> but Linus rightfully requested this to be done differently, hence this
>>> attempt now to solve it in a slightly more flexible way based on DT
>>> properties.
>>
>> For runtime PM, restoring the state constantly is unnecessary and not
>> good for battery life. The logic can be just:
>>
>> 1. Device driver runtime PM suspend saves the state when needed
>>
>> 2. Device driver runtime PM resume checks if context_lost was set by
>>    the bus or power domain code
>>
>> 3. If context was lost, device driver restores the state, or in some
>>    cases may need re-run the driver register init related parts
>>    to bring the driver back up, then clears the context_lost flag
>>
>> How about something like the following patch? So far only compile
>> tested with CONFIG_PM enabled. If that looks like the way to go,
>> I'll test it properly and add some comments for the functions and
>> post a proper patch :)
> 
> Honestly, I'm not sure.
> 
> I'd rather have a context_lost flag to start with and see how/if
> drivers will use that before adding any common infra for handling
> this.

I am afraid we are being slightly side tracked here on this context_loss
flag, because the crux of the problem is not whether a driver knows or
not when it loses state, it's more than the pinctrl core refuses to
re-apply the same state upon resumption even when the consumer driver
tells it to, because it has not seen a transition (consider it as a
stale software cache of the state), and there is only one state defined.

I don't particularly care how its gets solved, at the generic device
driver model or at the pinctrl level, but I think the pinctrl code needs
to change in that regard no matter what we do, because right now, if you
call pinctrl_select_state() in your driver's resume function, and there
is only one state defined, nothing happens, that's a problem.
-- 
Florian