linux-kernel - Re: [PATCH v2] drivers: cpuidle: assign enter_freeze to same as enter callback function

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Date:   Thu, 10 Nov 2016 12:18:43 -0600
From:   Andy Gross <andy.gross@...aro.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     linux-pm@...r.kernel.org, "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Linux Kernel list <linux-kernel@...r.kernel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v2] drivers: cpuidle: assign enter_freeze to same as enter
 callback function

On 10 November 2016 at 08:24, Sudeep Holla <sudeep.holla@....com> wrote:
> enter_freeze() callback is expected atleast to do the same as enter()
> but it has to guarantee that interrupts aren't enabled at any point
> in its execution, as the tick is frozen.
>
> CPUs execute ->enter_freeze with the local tick or entire timekeeping
> suspended, so it must not re-enable interrupts at any point (even
> temporarily) or attempt to change states of clock event devices.
>
> It will be called when the system goes to suspend-to-idle and will
> reduce power usage because CPUs won't be awaken for unnecessary IRQs
> (i.e. woken up only on IRQs from "wakeup sources")
>
> We can reuse the same code for both the enter() and enter_freeze()
> callbacks as along as they don't re-enable interrupts. Only "coupled"
> cpuidle mechanism enables interrupts and doing that with timekeeping
> suspended is generally not safe.
>
> Since this generic DT based idle driver doesn't support "coupled"
> states, it is safe to assume that the interrupts are not re-enabled.
>
> This patch assign enter_freeze to same as enter callback function which
> helps to save power without any intermittent spurious wakeups from
> suspend-to-idle.
>
> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> Signed-off-by: Sudeep Holla <sudeep.holla@....com>

Tested this on a Qualcomm 410c dragonboard.  Worked great.  Thanks for
the patch!


Tested-by: Andy Gross <andy.gross@...aro.org>