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

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Date:	Tue, 18 Jun 2013 16:39:32 +0900
From:	Magnus Damm <magnus.damm@...il.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Mark Rutland <mark.rutland@....com>,
	SH-Linux <linux-sh@...r.kernel.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	"Simon Horman [Horms]" <horms@...ge.net.au>,
	John Stultz <john.stultz@...aro.org>,
	Shinya Kuribayashi <shinya.kuribayashi.px@...esas.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH/RFC] clockevents: Ignore C3STOP when CPUIdle is disabled

On Tue, Jun 18, 2013 at 4:32 PM, Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
> On 06/18/2013 09:17 AM, Magnus Damm wrote:
>> From: Magnus Damm <damm@...nsource.se>
>>
>> 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.

/ magnus
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