Re: [PATCH 1/1] PM: s2idle: Introduce syscore callbacks in s2idle flow

From: Claude.Yen <Claude.Yen@mediatek.com>
To: Stephen Boyd <swboyd@chromium.org>
Cc: wsd_upstream@mediatek.com, linux-pm@vger.kernel.org,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	"Rafael J.Wysocki" <rjw@rjwysocki.net>,
	Douglas Anderson <dianders@chromium.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	linux-mediatek@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] PM: s2idle: Introduce syscore callbacks in s2idle flow
Date: Thu, 3 Sep 2020 11:43:24 +0800	[thread overview]
Message-ID: <1599104604.4435.7.camel@mtksdccf07> (raw)
In-Reply-To: <159894602346.334488.4470501636825871587@swboyd.mtv.corp.google.com>

On Tue, 2020-09-01 at 00:40 -0700, Stephen Boyd wrote:
> Quoting Claude Yen (2020-09-01 00:04:19)
> > From: "claude.yen" <claude.yen@mediatek.com>
> > 
> > Adding syscore callbacks to s2idle makes the behavior of s2idle become
> > more similar to Suspend-to-Ram (S2R) and reduces potential porting
> > effort.
> > 
> > tick_freeze() in s2idle flow calls sched_clock_suspend() and
> > timekeeping_suspend(), which both functions are also registered as
> > syscore callback. sched_clock_suspend() introduced in
> > commit 3f2552f7e9c5 ("timers/sched_clock: Prevent generic sched_clock
> > wrap caused by tick_freeze()") is added due to clock wrap issue.
> > By adding syscore callbacks in s2idle, if there is any syscore callbacks
> > also required in s2idle, additional porting effort could be saved.
> > 
> > Besides, in both S2R and s2idle, before the system entering low power
> > state, there is a state when only one cpu alive with interrupt-disabled,
> > which is syscore callback designed for. Adding syscore callbacks in
> > s2idle is feasible option.
> > 
> > Scenarios to call syscore callback:
> > S2R: one cpu alive when nonboot cpus are hotplug-ed off
> > s2idle: one cpu alive when other cpus have enter idle state
> > 
> 
> Do you have syscore ops that can't be converted to something else like
> CPU_PM notifier? At this point most syscore code that is important has
> been converted so I don't see much benefit for this patch. If anything,
> it will prevent conversions to code that works for both cases.

In Mediatek SoC's implementation, battery voltage and remaining capacity
are tracked in syscore callback when system going to suspend. The reason
to hook callback in syscore is that all devices are suspended, and thus
battery output become steady and suitable for measurement. Such callback
is not suitable for CPU_PM notifier.

If s2idle is applied, the callback to track battery status is not
triggered due to syscore is not called by current design.

Regards,
Claude
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