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

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Date:   Thu, 17 Feb 2022 13:02:04 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
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

16.02.2022 12:45, Ulf Hansson пишет:
> 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?

Yes, I see now what this patch does.

So "deepest state" in the code's comment means powered off state. Maybe
the comment could clarify it better a tad.

- * The children must be in their deepest states to allow the parent to
+ * The children must be in their deepest (powered-off) states to allow
the parent to