linux-kernel - Re: [RFC PATCH 0/5] Frequency resolution in CCF vs. cpufreq

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Message-ID: <6fe97032-ba6b-4dac-9064-34a9c788c57c@BN1BFFO11FD001.protection.gbl>
Date:	Thu, 15 May 2014 07:00:26 -0700
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
CC:	Mike Turquette <mturquette@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Russell King <linux@....linux.org.uk>,
	Michal Simek <michal.simek@...inx.com>,
	<linux-pm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<cpufreq@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 0/5] Frequency resolution in CCF vs. cpufreq

Hi Uwe,

On Thu, 2014-05-15 at 09:47AM +0200, Uwe Kleine-König wrote:
> On Wed, May 14, 2014 at 03:30:50PM -0700, Soren Brinkmann wrote:
> > Hi,
> > 
> > I have one or two problems with cpufreq and the CCF, which are caused by
> > rounding/different frequency resolutions.
> > 
> > cpufreq works with kHz, while the CCF uses Hz. On Zynq our default frequency is
> > 666666666 Hz which the CCF, due to rounding, reports as 666666660. And for
> Why does this happen? Isn't that a bug? What is the actual freqency?
> 666666666 Hz or 2000000000/3 Hz?

I don't know for sure. I think it's rounding that takes place in the
CCF. That output clock goes from a x-tal through an PLL and various
muxes and dividers. I guess some inaccuracies in the magnitude of a few
Hz are probably not avoidable in that processing chain.

> 
> > cpufreq, which simply divides values it obtains through clk_round_rate() by
> > 1000, 666666.
> > Since passing 666666 to clk_round_rate() does not result in 666666660
> > (clk_round_rate() always rounds down!), we chose to put 666667 in the OPP. This
> What is OPP?

As Nishanth already said, Operating Performance Points. Sorry for being
lazy and not having spelled that out.

	Sören

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