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

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Date:	Thu, 15 May 2014 07:05:28 -0700
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	Mike Turquette <mturquette@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Russell King <linux@....linux.org.uk>,
	Michal Simek <michal.simek@...inx.com>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 0/5] Frequency resolution in CCF vs. cpufreq

Hi Viresh,

On Thu, 2014-05-15 at 11:42AM +0530, Viresh Kumar wrote:
> On 15 May 2014 04:00, Soren Brinkmann <soren.brinkmann@...inx.com> wrote:
> > 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
> > 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
> > causes issue 1: cpufreq stats are broken.
> 
> I know it might a big exercise, but wouldn't it be worth to make cpufreq start
> using frequencies in Hz ?

I haven't looked into this. As you say yourself, that might be a rather
big project. I will take a look at it, but I can't promise that I have
time to dedicate to this. Also, as said above, even though our CPU is
supposed to run at 666666666 Hz, rounding lets the last 6 Hz disappear.
I think we have to handle deviances either way.

	Sören

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