Subject: [PATCH 2/2] clockevents: Prefer clockevents that don't suffer from FEAT_C3_STOP

From: [email-masked] (Stephen Boyd)
Date: Thu, 22 Aug 2013 10:40:05 -0700

On 08/22/13 10:33, Santosh Shilimkar wrote:
> On Thursday 22 August 2013 01:06 PM, Stephen Boyd wrote:
>> On some ARM systems there are two sets of per-cpu timers: the TWD
>> timers and the global timers. The TWD timers are rated at 350 and
>> the global timers are rated at 300 but the TWD timers suffer from
>> FEAT_C3_STOP while the arm global timers don't. The tick device
>> selection logic doesn't consider the FEAT_C3_STOP flag so we'll
>> always end up with the TWD as the tick device although we don't
>> want that.
>>
>> Extend the preference logic to take the FEAT_C3_STOP flag into
>> account and always prefer tick devices that don't suffer from
>> FEAT_C3_STOP even if their rating is lower than tick devices that
>> do suffer from FEAT_C3_STOP. This will remove the need to do any
>> broadcasting on such ARM systems.
>>
>> Signed-off-by: Stephen Boyd <[email-masked]>
>> ---
>>  kernel/time/tick-common.c | 14 ++++++++++++--
>>  1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
>> index 64522ec..3ae437d 100644
>> --- a/kernel/time/tick-common.c
>> +++ b/kernel/time/tick-common.c
>> @@ -244,12 +244,22 @@ static bool tick_check_preferred(struct clock_event_device *curdev,
>>  			return false;
>>  	}
>>  
>> +	if (!curdev)
>> +		return true;
>> +
>> +	/* Always prefer a tick device that doesn't suffer from FEAT_C3STOP */
>> +	if (!(newdev->features & CLOCK_EVT_FEAT_C3STOP) &&
>> +			(curdev->features & CLOCK_EVT_FEAT_C3STOP))
>> +		return true;
>> +	if ((newdev->features & CLOCK_EVT_FEAT_C3STOP) &&
>> +			!(curdev->features & CLOCK_EVT_FEAT_C3STOP))
>> +		return false;
>> +
> I don't think preferring the clock-event which doesn't suffer
> from FEAT_C3STOP is a good idea if the quality of the time source
> is not same. Generally the global timers are slow and far away from
> CPU(cycle cost). So as long as we don't get impacted because of low power
> states, the tick should run out of local timers which are faster access
> as well as high resolution.
>

Fair enough. I have no data either way to convince anyone that this is a
good or bad idea so this patch should have probably been an RFC. Are you
hinting at something like switching to a per-cpu timer that doesn't
suffer from FEAT_C3_STOP when a CPU goes into a deep idle state?
Interesting idea but I think I'll leave that to someone else if they
really care to do that.

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