linux-kernel - Re: [PATCH/RFC 3/6] drivers: firmware: psci: Implement shallow suspend mode

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Date:   Tue, 21 Feb 2017 17:20:13 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Sudeep Holla <sudeep.holla@....com>, Pavel Machek <pavel@....cz>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Lina Iyer <lina.iyer@...aro.org>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>,
        Rob Herring <robh+dt@...nel.org>,
        Magnus Damm <magnus.damm@...il.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC 3/6] drivers: firmware: psci: Implement shallow
 suspend mode

Hi,

On Tue, Feb 21, 2017 at 05:32:50PM +0100, Geert Uytterhoeven wrote:
> On Tue, Feb 21, 2017 at 12:14 PM, Sudeep Holla <sudeep.holla@....com> wrote:
> > On 21/02/17 11:07, Pavel Machek wrote:
> >>> Enable support for "shallow" suspend mode, also known as "Standby" or
> >>> "Power-On Suspend".
> >>>
> >>> As secondary CPU cores are taken offline, "shallow" suspend mode saves
> >>> slightly more power than "s2idle", but less than "deep" suspend mode.
> >>> However, unlike "deep" suspend mode, "shallow" suspend mode can be used
> >>> regardless of the presence of support for PSCI_SYSTEM_SUSPEND, which is
> >>> an optional API in PSCI v1.0.
> >>
> >> If system supports "shallow" suspend, why does not PSCI implement it?
> >
> > Yes it can, and IIUC it already does on this platform with CPU_SUSPEND.
> > All it now needs is just to use existing "freeze" suspend mode in Linux.
> 
> How can Linux know if using "deep" suspend will allow to wake-up the system
> according to configured wake-up sources, or not?

My understanding is that if a device can wake the system from
PSCI_SYSTEM_SUSPEND, it should be described in the DT as a wakeup source
[1]. So we should be able to determine the set of devices which can wake
the system from a suspend. We shouldn't assume that other devices can
(though I don't precisely what we do currently).

Otherwise, where PSCI_CPU_SUSPEND, we'd expect that most devices
(barring cpu-local timers) can wake up CPUs, and hence the system, by
raising an interrupt.

Thanks,
Mark.

[1] Documentation/devicetree/bindings/power/wakeup-source.txt