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From: Geert Uytterhoeven <geert@linux-m68k.org>
Newsgroups: linux.kernel
Subject: Re: [PATCH/RFC 0/6] PSCI: Fix non-PMIC wake-up if SYSTEM_SUSPEND cuts power
Date: Tue, 21 Feb 2017 17:30:02 +0100
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Hi Sudeep,

On Tue, Feb 21, 2017 at 11:38 AM, Sudeep Holla <sudeep.holla@arm.com> wrote:
> On 20/02/17 20:33, Geert Uytterhoeven wrote:
>> This patch series adds support for using non-PMIC wake-up sources on the
>> Renesas R-Car Gen3 (H3 or M3-W) Salvator-X development boards.
>>
>> Nothing in the PSCI specification requires the SoC to remain powered and
>> to support wake-up sources when suspended using SYSTEM_SUSPEND.
>> If the firmware implements the PSCI SYSTEM_SUSPEND operation by cutting
>> power to the SoC, the only possibly wake-up sources are thus the ones
>> connected to the PMIC.
>
> OK, but I don't see any issue with that. That's exactly how it works on

How do you use other wake-up sources, like wake on LAN, UART or GPIO?

> ARM Juno platform. The SoC is powered down.

Good to hear this is not limited to Renesas platforms, so there's a common
problem to solve.

>> To allow other wake-up sources, this patch series documents and adds
>> support for an "arm,psci-system-suspend-is-power-down" DT property, so
>
> NACK, you don't need any such properties.

If this is true for all PSCI platforms, there's indeed no need for such a
property, and drivers/firmware/psci.c should default to this case.

>> Linux uses a different suspend method when other wake-up sources (e.g.
>> wake on LAN, UART or GPIO) are enabled.  Hence the user no longer has to
>> manually restrict "mem" suspend to "s2idle" or "shallow" states using:
>
> Have you explored suspend-to-idle instead ? It looks like thats exactly
> what you are doing in this patch set. You also get low latency for free
> as it just enters the deepest idle state on all CPUs instead of
> hotplugging out all the secondaries.

Yes, cfr. "s2idle" above.
The user can specify to use "s2idle" manually:

    $ echo s2idle > /sys/power/mem_sleep # or "shallow"

However, how to handle this automatically, e.g. by a distro?
On most other platforms, userspace can just do e.g.

    ethtool -s eth0 wol g

to enable wake-on-LAN, and suspend to the deepest supported state using:

    echo mem > /sys/power/state

On systems where PSCI SYSTEM_SUSPEND powers down the SoC, userspace must
make sure to configure to use "s2idle" (or "shallow) instead, else the
configured wake-up sources won't work.
I want Linux to handle this automatically.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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