linux-kernel - Re: [PATCH] Add sched_clock to AT91 TCB clocksource driver

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Date:	Mon, 1 Aug 2011 10:28:06 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Remy Bohmer <linux@...mer.net>
Cc:	Remy Bohmer <l.pinguin@...il.com>, Arnd Bergmann <arnd@...db.de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	Andrew Victor <linux@...im.org.za>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] Add sched_clock to AT91 TCB clocksource driver

On Mon, Aug 01, 2011 at 10:42:35AM +0200, Remy Bohmer wrote:
> OK, I know that too, but the jiffies based fallback does it even
> worse, since it does not run at all...  (It only updates in steps of a
> couple of 100msecs and stops after a while, and even does not display
> a real time...) This sched_clock implementation at least works before
> the drivers are being initialised...

You're forgetting that jiffies doesn't jump about.  A late initializing
sched_clock could jump.

> Anyway, do you have a better suggestion how to fix this? The tcb
> clocksource is loaded during a arch_initcall(), perhaps we need
> something before that point.
> I do not see an easy way to integrate it in MACHINE_START(.timer).
> Would 'late_time_init' be a better solution?

late_time_init() is not that much better as that still happens after
sched_init() has been called.  sched_init() initializes various
structures which involves reading sched_clock().

Why can't it initialize itself at the standard point during the boot
sequence, which is time_init() - which in turn is as you say the
.timer callback?
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