linux-kernel - Re: [PATCH] cpuidle: fix fallback mechanism for suspend to idle in absence of enter

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Date:	Fri, 22 Jan 2016 14:45:43 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Sudeep Holla <sudeep.holla@....com>
Cc:	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] cpuidle: fix fallback mechanism for suspend to idle in absence of enter_freeze

On Friday, January 22, 2016 09:20:15 AM Sudeep Holla wrote:
> 
> On 22/01/16 01:15, Rafael J. Wysocki wrote:
> > On Thursday, January 21, 2016 11:19:29 AM Sudeep Holla wrote:
> >> Commit 51164251f5c3 ("sched / idle: Drop default_idle_call() fallback
> >> from call_cpuidle()") made find_deepest_state() return non-negative
> >> value and check all the states with index > 0. Also a result,
> >> find_deepest_state() returns 0 even when enter_freeze callbacks are not
> >> implemented and enter_freeze_proper is called which ends up crashing
> >> the kernel.
> >>
> >> This patch updates the check for index > 0 in cpuidle_enter_freeze and
> >> cpuidle_idle_call(when idle_should_freeze is true) to restore the
> >> suspend-to-idle functionality in absence of enter_freeze callback.
> >>
> >> Fixes: 51164251f5c3 ("sched / idle: Drop default_idle_call() fallback from call_cpuidle()")
> >> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> >> Cc: Ingo Molnar <mingo@...hat.com>
> >> Cc: Peter Zijlstra <peterz@...radead.org>
> >> Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> >> ---
> >>   drivers/cpuidle/cpuidle.c | 2 +-
> >>   kernel/sched/idle.c       | 2 +-
> >>   2 files changed, 2 insertions(+), 2 deletions(-)
> >>
> >> Hi Rafael,
> >
> > Hi,
> >
> > Sorry for the breakage.
> >
> 
> Sorry that I missed to test suspend-to-idle before it got merged.
> 
> >> I assume you prefer to retain find_deepest_state return non-negative
> >> values, so I took this approach for fixing the bug. Do you think we
> >> need to support enter_freeze_proper for index 0 ?
> >
> > Zero is a special case on x86, so supporting enter_freeze_proper() for it
> > is not necessary.
> >
> 
> Even on ARM, 0 is used for WFI only, which will not be used for freeze.
> 
> > If you think we can also make 0 a special case on ARM, the others should
> > not object to that either.
> >
> 
> Makes sense and since it's already reserved for WFI on ARM, it should be
> fine. If there are no objections, can you pick up this fix ?

Yes, I've already done that.

Thanks,
Rafael