linux-kernel - Re: [PATCH 1/3] PM: domains: Drop the performance state vote for a device at detach

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Date:   Mon, 6 Sep 2021 12:24:51 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Stephan Gerhold <stephan@...hold.net>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] PM: domains: Drop the performance state vote for a
 device at detach

On Sun, 5 Sept 2021 at 10:26, Dmitry Osipenko <digetx@...il.com> wrote:
>
> 03.09.2021 17:03, Ulf Hansson пишет:
> > On Fri, 3 Sept 2021 at 11:58, Dmitry Osipenko <digetx@...il.com> wrote:
> >>
> >> 03.09.2021 11:22, Ulf Hansson пишет:
> >>> On Fri, 3 Sept 2021 at 08:01, Dmitry Osipenko <digetx@...il.com> wrote:
> >>>>
> >>>> 02.09.2021 13:16, Ulf Hansson пишет:
> >>>>> When a device is detached from its genpd, genpd loses track of the device,
> >>>>> including its performance state vote that may have been requested for it.
> >>>>>
> >>>>> Rather than relying on the consumer driver to drop the performance state
> >>>>> vote for its device, let's do it internally in genpd when the device is
> >>>>> getting detached. In this way, we makes sure that the aggregation of the
> >>>>> votes in genpd becomes correct.
> >>>>
> >>>> This is a dangerous behaviour in a case where performance state
> >>>> represents voltage. If hardware is kept active on detachment, say it's
> >>>> always-on, then it may be a disaster to drop the voltage for the active
> >>>> hardware.
> >>>>
> >>>> It's safe to drop performance state only if you assume that there is a
> >>>> firmware behind kernel which has its own layer of performance management
> >>>> and it will prevent the disaster by saying 'nope, I'm not doing this'.
> >>>>
> >>>> The performance state should be persistent for a device and it should be
> >>>> controlled in a conjunction with runtime PM. If platform wants to drop
> >>>> performance state to zero on detachment, then this behaviour should be
> >>>> specific to that platform.
> >>>
> >>> I understand your concern, but at this point, genpd can't help to fix this.
> >>>
> >>> Genpd has no information about the device, unless it's attached to it.
> >>> For now and for these always on HWs, we simply need to make sure the
> >>> device stays attached, in one way or the other.
> >>
> >> This indeed requires to redesign GENPD to make it more coupled with a
> >> device, but this is not a real problem for any of the current API users
> >> AFAIK. Ideally the state should be persistent to make API more universal.
> >
> > Right. In fact this has been discussed in the past. In principle, the
> > idea was to attach to genpd at device registration, rather than at
> > driver probe.
> >
> > Although, this is not very easy to implement - and it seems like the
> > churns to do, have not been really worth it. At least so far.
> >
> >>
> >> Since for today we assume that device should be suspended at the time of
> >> the detachment (if the default OPP state isn't used), it may be better
> >> to add a noisy warning message if pstate!=0, keeping the state untouched
> >> if it's not zero.
> >
> > That would just be very silly in my opinion.
> >
> > When the device is detached (suspended or not), it may cause it's PM
> > domain to be powered off - and there is really nothing we can do about
> > that from the genpd point of view.
> >
> > As stated, the only current short term solution is to avoid detaching
> > the device. Anything else, would just be papering of the issue.
>
> What about to re-evaluate the performance state of the domain after
> detachment instead of setting the state to zero?

I am not suggesting to set the performance state of the genpd to zero,
but to drop a potential vote for a performance state for the *device*
that is about to be detached.

Calling genpd_set_performance_state(dev, 0), during detach will have
the same effect as triggering a re-evaluation of the performance state
for the genpd, but after the detach.

> This way PD driver may
> take an action on detachment if performance isn't zero, before hardware
> is crashed, for example it may emit a warning.

Not sure I got that. Exactly when do you want to emit a warning and
for what reason?

Do you want to add a check somewhere to see if
'gpd_data->performance_state' is non zero - and then print a warning?

Kind regards
Uffe