Re: [PATCH/RFC] clockevents: Ignore C3STOP when CPUIdle is disabled

From: Daniel Lezcano
Date: Tue Jun 18 2013 - 04:24:38 EST

On 06/18/2013 09:39 AM, Magnus Damm wrote:
> On Tue, Jun 18, 2013 at 4:32 PM, Daniel Lezcano
> <daniel.lezcano@xxxxxxxxxx> wrote:
>> On 06/18/2013 09:17 AM, Magnus Damm wrote:
>>> From: Magnus Damm <damm@xxxxxxxxxxxxx>
>>> Introduce the function tick_device_may_c3stop() that
>>> ignores the C3STOP flag in case CPUIdle is disabled.
>>> The C3STOP flag tells the system that a clock event
>>> device may be stopped during deep sleep, but if this
>>> will happen or not depends on things like if CPUIdle
>>> is enabled and if a CPUIdle driver is available.
>>> This patch assumes that if CPUIdle is disabled then
>>> the sleep mode triggering C3STOP will never be entered.
>>> So by ignoring C3STOP when CPUIdle is disabled then it
>>> becomes possible to use high resolution timers with only
>>> per-cpu local timers - regardless if they have the
>>> C3STOP flag set or not.
>>> Observed on the r8a73a4 SoC that at this point only uses
>>> ARM architected timers for clock event and clock sources.
>>> Without this patch high resolution timers are run time
>>> disabled on the r8a73a4 SoC - this regardless of CPUIdle
>>> is disabled or not.
>>> The less short term fix is to add support for more timers
>>> on the r8a73a4 SoC, but until CPUIdle support is enabled
>>> it must be possible to use high resoultion timers without
>>> additional timers.
>>> I'd like to hear some feedback and also test this on more
>>> systems before merging the code, see the non-SOB below.
>> Do we need a broadcast timer when cpuidle is not compiled in the kernel ?
> Yes, if there is no per-cpu timer available. It depends on what the
> SMP support code for a particular SoC or architecture happen to
> enable.

Ok thanks for the information.

There is here a multiple occurrence of the information "the timer will
stop when power is saved": CLOCK_EVT_FEAT_C3STOP and
CPUIDLE_FLAG_TIMER_STOP, so I am wondering if some code simplification
couldn't be done before your patch.

The function:

tick_broadcast_oneshot_control is called from clockevents_notify. This
one is called from the cpuidle framework or the back-end cpuidle driver.
The caller knows the timer will be stop and this is why it is switching
to the broadcast mode. But we have a sanity check in
tick_broadcast_oneshot_control function:

if (!(dev->features & CLOCK_EVT_FEAT_C3STOP))

In other words, CPUIDLE_FLAG_TIMER_STOP will tell the framework to call
clockevents_notify and the tick broadcast code will re-check the device
will effectively go down. IMHO, we can get rid of this check.

The same happens for the tick_do_broadcast_on_off function.

That reduces the number of C3STOP usage.

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