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

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Message-ID: <e79767a7-03be-2073-b78e-3ec8d29d5f8a@arm.com>
Date:   Tue, 15 Nov 2016 14:20:21 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     linux-pm@...r.kernel.org, "Rafael J . Wysocki" <rjw@...ysocki.net>
Cc:     Sudeep Holla <sudeep.holla@....com>, linux-kernel@...r.kernel.org,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Andy Gross <andy.gross@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v2] drivers: cpuidle: assign enter_freeze to same as enter
 callback function

Hi Rafael,

On 10/11/16 14:24, Sudeep Holla 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>
> ---
>  drivers/cpuidle/dt_idle_states.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> v1->v2:
> 	- Dropped checking and using only states with CPUIDLE_FLAG_TIMER_STOP
> 	  enabled.
>

Can you pick up this patch directly if there are no concerns ?

-- 
Regards,
Sudeep