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

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Date:   Wed, 22 Feb 2017 14:47:39 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Pavel Machek <pavel@....cz>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Mark Rutland <mark.rutland@....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 Sudeep,

On Tue, Feb 21, 2017 at 6:22 PM, Sudeep Holla <sudeep.holla@....com> wrote:
> On 21/02/17 16:32, 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?
>
> I am not sure if we have such selective configuration of wakeup source
> implemented in Linux.
>
> ACPI specification has some provisions where each device can state if it
> can specify device state in each system sleeping state that can wake the
> system.
>
> DT has no mechanism today to express this relations. I had brought up
> this discussion in plumbers(2015). Refer slide 7 in [0]
>
> And the way you are trying to do that is not correct IMO especially
> making it just PSCI specific.
>
>> Note that "it will not, ever" is an accepted answer.
>
> IIUC, it's not implemented today. I can't talk about future ;), but your

Good, so there's no need for the DT property, and drivers/firmware/psci.c
should aways call do_cpu_idle() instead of PSCI SYSTEM_SUSPEND if any
other wake-up sources are configured?

That follows the principle of least surprise: it doesn't leave the user with
a system that won't wake up the way he configured it to wake up.

> proposal is horrible hack.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds