linux-kernel - Re: [PATCH] arm: kernel: utilize hrtimer based broadcast

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Date:	Tue, 5 Jan 2016 10:42:57 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Arnd Bergmann <arnd@...db.de>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Huan Wang <alison.wang@....com>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Huan Wang <alison.wang@...escale.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH] arm: kernel: utilize hrtimer based broadcast

On Sat, 2 Jan 2016, Russell King - ARM Linux wrote:
> On Tue, Dec 29, 2015 at 02:54:10PM +0100, Thomas Gleixner wrote:
> > I have no real opinion about that patch. It does no harm to unconditionally
> > setup the hrtimer based broadcast even if it's never used.
> > 
> > Up to the arch maintainer to decide. 
> 
> That's really not fair to keep shovelling these kinds of decisions onto
> architecture maintainers without any kind of explanation about how an
> architecture maintainer should make such a decision.
> 
> Do I roll a 6-face dice, and if it gives an odd number, I apply this
> patch, otherwise I reject it?
> 
> Is there a technical basis for making the decision?  If so, please
> explain what the technical arguments are against having or not having
> this change.

The hrtimer based broadcast device is used when you have per cpu timers which
stop in deeper power states, but you have no other timer hardware on the chip
which can backup the per cpu timer in deep power states. The trick is that it
emulates a timer hardware via a hrtimer and then tells the cpu idle code not
to go into deep power states on the cpu which owns that hrtimer. All other
cpus can go as deep as they want and still get woken up.

The only downside of adding this unconditionally is extra code in case that it
is not needed on a particular platform.

Hope that helps.

     tglx
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