linux-kernel - Re: [PATCH v2] PM: domains: Prevent power off for parent unless child is in deepest state

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Date:   Wed, 16 Feb 2022 10:45:56 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     "Rafael J . Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org,
        Kevin Hilman <khilman@...nel.org>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Dong Aisheng <aisheng.dong@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] PM: domains: Prevent power off for parent unless child
 is in deepest state

On Tue, 15 Feb 2022 at 22:49, Dmitry Osipenko <digetx@...il.com> wrote:
>
> 04.02.2022 13:16, Ulf Hansson пишет:
> > A PM domain managed by genpd may support multiple idlestates. During
> > genpd_power_off() a genpd governor may be asked to select one of the
> > idlestates based upon the dev PM QoS constraints, for example.
> >
> > However, there is a problem with the behaviour around this in genpd. More
> > precisely, a parent-domain is allowed to be powered off, no matter of what
> > idlestate that has been selected for the child-domain.
> >
> > So far, we have not received any reports about errors from the current
> > behaviour. However, there is an STMicro platform that is being worked on,
> > which can't cope with this.
>
> Could you please provide some technical info about why STMicro platform
> can't cope with that?

There is a parent domain with one power-off state. The parent domain
has a few devices attached to it, which means they need to be managed
to together. The parent domain is controlling a shared power rail.

The child domain, which has multiple idle states to choose from, also
has some devices attached to it. The child domain is controlling a
system clock, but also relies on the shared power rail that is managed
by the parent domain.

Obviously, these idle states are managed by firmware.

I hope that made it a bit more clear?

Kind regards
Uffe