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

From: Dmitry Osipenko <digetx@gmail.com>
To: Ulf Hansson <ulf.hansson@linaro.org>
Cc: "Rafael J . Wysocki" <rafael@kernel.org>,
	linux-pm@vger.kernel.org, Kevin Hilman <khilman@kernel.org>,
	Alexandre Torgue <alexandre.torgue@foss.st.com>,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	Rajendra Nayak <rnayak@codeaurora.org>,
	Dong Aisheng <aisheng.dong@nxp.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] PM: domains: Prevent power off for parent unless child is in deepest state
Date: Thu, 17 Feb 2022 13:02:04 +0300	[thread overview]
Message-ID: <25e6658c-3241-198b-4240-1055c8a6c630@gmail.com> (raw)
In-Reply-To: <CAPDyKFoDP-LT9nKwa0oKF0jW9DD3tZh4+v9zE0uS_KpJeFC-MQ@mail.gmail.com>

16.02.2022 12:45, Ulf Hansson пишет:
> On Tue, 15 Feb 2022 at 22:49, Dmitry Osipenko <digetx@gmail.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