linux-kernel - clocksource with changing frequency?

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Date:	Thu, 20 Jun 2013 17:01:22 -0700
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	Sören Brinkmann <soren.brinkmann@...inx.com>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Michal Simek <michal.simek@...inx.com>
Subject: clocksource with changing frequency?

Hi all,

I have a question/problem with a clocksource whose input frequency can
change.
On Zynq we use the cadece_ttc timer as clocksource. Its input clock is
tightly coupled to the CPU clock. I.e. it changes when cpufreq scales
the CPU clocks.
To compensate this, I added a clock notifier doing this on a frequency
change:
	clocksource_unregister(&ttccs->cs);                              
	clocksource_register_hz(&ttccs->cs, ndata->new_rate / PRESCALE);

This just seems not to be the most elegant way, a heavy weight operation
and results in a couple of locking errors in combination with
suspend/hotplug.

Is there a sane solution for such a case? E.g. for clockevent devices
there is a 'clockevents_update_freq()' to adjust to changing
frequencies.

I assume the sanest solution is an external oscillator as clock input,
but I think our platforms don't provide that :(

	Thanks,
	Sören


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