linux-kernel - Re: [PATCH/RFC] arm: arch_timer: Do not set C3STOP in case CPU

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Message-ID: <CANqRtoTu7U6x6QmiV8JsCxeF8WU3n_bndFgR0Tif5TwEpFXbLA@mail.gmail.com>
Date:	Tue, 18 Jun 2013 16:32:07 +0900
From:	Magnus Damm <magnus.damm@...il.com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"horms@...ge.net.au" <horms@...ge.net.au>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"shinya.kuribayashi.px@...esas.com" 
	<shinya.kuribayashi.px@...esas.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH/RFC] arm: arch_timer: Do not set C3STOP in case CPU_IDLE=n

Hi Mark,

On Mon, Jun 17, 2013 at 11:53 PM, Mark Rutland <mark.rutland@....com> wrote:
> On Mon, Jun 17, 2013 at 01:20:56AM +0100, Magnus Damm wrote:
>> From: Magnus Damm <damm@...nsource.se>
>>
>> Modify the ARM architected timer driver to not set C3STOP
>> in case CPU_IDLE is disabled. This is a short term fix that
>> allows use of high resolution timers even though no additional
>> clock event is registered.
>>
>> Not-really-Signed-off-by: Magnus Damm <damm@...nsource.se>
>> ---
>>
>>  If someone cares about this case then perhaps it should be
>>  moved up to the clock event main code. The same issue should
>>  in theory trigger on all architectures, perhaps x86 people
>>  hunting for low latency may try to disable CPU_IDLE?
>
> I think that changing tick_is_oneshot_capable and friends to only worry about
> C3STOP when CPU_IDLE is enabled would be a nicer solution. That way you enable
> all clock_event_devices with C3STOP to function as high resolution timers when
> CPU_IDLE's selected. Presenting the hardware differently depending on CPU_IDLE
> feels wrong.

I agree that doing this in the clock event driver is a bit odd. So
because of that I just posted this patch:

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

> Having some other clock_event_device would be a nicer solution still.

No doubt that another clock event device helps, but mainly together
with CPU Idle IMO.

Thanks for your comments!

/ magnus
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