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 09:20:15 +0000
From:	Sudeep Holla <sudeep.holla@....com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Sudeep Holla <sudeep.holla@....com>, 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 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 ?

-- 
Regards,
Sudeep