linux-kernel - Re: One of these things (CONFIG

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Message-ID: <20130121210341.GW23505@n2100.arm.linux.org.uk>
Date:	Mon, 21 Jan 2013 21:03:41 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Matt Sealey <matt@...esi-usa.com>,
	Linux ARM Kernel ML <linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	Ben Dooks <ben@...tec.co.uk>
Subject: Re: One of these things (CONFIG_HZ) is not like the others..

On Mon, Jan 21, 2013 at 08:41:17PM +0000, Arnd Bergmann wrote:
> On Monday 21 January 2013, Matt Sealey wrote:
> > 
> > ARM seems to be the only "major" platform not using the
> > kernel/Kconfig.hz definitions, instead rolling it's own and setting
> > what could be described as both reasonable and unreasonable defaults
> > for platforms.

No, you've got this totally wrong.

They're not defaults.  And I object to your use of "unreasonable" too.
I've no idea where you get that from.

There's a reason why we have different HZ rates - some platforms just
can't do the standard 100Hz tick rate.  No way - their timers can't
divide down to that interrupt rate.  Sorry to spoil your ivory tower
with a few facts, but your statement is just rediculous.

The reason we don't use kernel/Kconfig.hz is precisely because of that;
we _HAVE_ to have different HZ definitions on different platforms, and
you'll notice that kernel/Kconfig.hz makes _no_ prevision for this.

Now, while things have moved forwards and we have clocksource/clockevent
support, not every platform can support this timekeeping structure;
ebsa110 certainly can't.  There's one timer and one timer only which
is usable, which even needs to be manually reloaded by the CPU.  No
other independent counter to act as a clock source.

As for Samsung and the rest I can't comment.  The original reason OMAP
used this though was because the 32768Hz counter can't produce 100Hz
without a .1% error - too much error under pre-clocksource
implementations for timekeeping.  Whether that's changed with the
clocksource/clockevent support needs to be checked.

It's entirely possible with the modern clocksource/clockevent support
that many of these platforms can have their alternative HZ tick rates
removed - but there will continue to be a subset which can't, and all
the time that we have such a subset, kernel/Kconfig.hz can't be used
without modification.
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