linux-kernel - Re: [PATCH] clockevents: Add (missing) default case for switch blocks

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Date:	Fri, 20 Feb 2015 15:04:32 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Kevin Hilman <khilman@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linaro Networking <linaro-networking@...aro.org>,
	Steven Miao <realmz6@...il.com>,
	Mark Salter <msalter@...hat.com>,
	Michal Simek <monstr@...str.eu>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Ley Foon Tan <lftan@...era.com>,
	Jonas Bonn <jonas@...thpole.se>,
	"David S. Miller" <davem@...emloft.net>,
	Jeff Dike <jdike@...toit.com>,
	Guan Xuetao <gxt@...c.pku.edu.cn>
Subject: Re: [PATCH] clockevents: Add (missing) default case for switch blocks


* Viresh Kumar <viresh.kumar@...aro.org> wrote:

> On 20 February 2015 at 18:52, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > * Viresh Kumar <viresh.kumar@...aro.org> wrote:
> >> +       CLOCK_EVT_DEV_MODE_UNUSED = 0,
> >
> > What is 'unused' - not initialized yet?
> 
> Unused. Initially all clockevent devices are supposed to 
> be in this mode but later if another device replaces an 
> existing one, the existing one is put into this mode.

I'd suggest to rename it to MODE_INIT - at first glance it 
gave me the impression that it's some sort of API 
placeholder - i.e. an unused flag or so.

Also, I'd suggest to rename all 'modes' to true state 
machine naming: STATE_INITIALIZED, STATE_SHUT_DOWN, 
STATE_PERIODIC, STATE_RESUMED, etc.: if these are enums for 
states and not state transition names, see my later 
questions:

> >> +       CLOCK_EVT_DEV_MODE_SHUTDOWN,
> >> +       CLOCK_EVT_DEV_MODE_PERIODIC,
> >> +       CLOCK_EVT_DEV_MODE_ONESHOT,
> >> +       CLOCK_EVT_DEV_MODE_RESUME,
> >
> > What is 'resume' mode?
> 
> Introduced with: 18de5bc4c1f1 ("clockevents: fix resume 
> logic") and is only called during system resume to resume 
> the clockevent devices before resuming the tick. Only few 
> implementations do meaningful stuff here.

So is it a state that a clockevents device reaches, or a 
state transition? The two purposes seem to be mixed up in 
the nomenclature.

> >> +       CLOCK_EVT_DEV_MODE_ONESHOT_STOPPED,  /* This would be the new
> >> mode which I will add later */
> >
> > What does this mode express?
> 
> I have added it here to show how things would look like 
> eventually, but it wouldn't be present in the patch which 
> splits the enum into two parts..

Yeah.

> Its only important for NOHZ_FULL (IDLE ? Maybe). When we 
> decide that the tick (LOWRES) or hrtimer interrupt 
> (HIGHRES) isn't required for indefinite period of time 
> (i.e. no timers/hrtimers are present to serve), we skip 
> reprogramming the clockevent device. But its already 
> reprogrammed from the tick-handler and so will fire 
> atleast once again.

So this new 'mode' appears to be a true state of the 
device?

Thanks,

	Ingo
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