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

["Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>]

On Wednesday, November 8, 2017 1:28:24 AM CET Florian Fainelli wrote:
> 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@xxxxxxxxx> [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@xxxxxxxxx> [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.

I see, thanks for explaining that clearly.

Then it looks like that there needs to be a way for the "cached" state to be
invalidated and the question is what that way should be and how it is going to
be triggered.

Thanks,
Rafael

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