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

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Message-ID: <CAPDyKFofrEj2LdqXh-L256b2Tcz=qYQgzTUBVuvx0rOR58SrVg@mail.gmail.com>
Date:   Fri, 3 Sep 2021 10:22:59 +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 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.

Kind regards
Uffe