linux-kernel - Re: [RFC PATCH 2/5] clk: Introduce 'clk_round_rate

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Message-ID: <668683e3-856e-4f30-9b11-8f3e91e12d1d@BL2FFO11FD038.protection.gbl>
Date:	Tue, 20 May 2014 14:48:20 -0700
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Stephen Boyd <sboyd@...eaurora.org>
CC:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Mike Turquette <mturquette@...aro.org>,
	<linux-pm@...r.kernel.org>, Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Michal Simek <michal.simek@...inx.com>,
	<cpufreq@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 2/5] clk: Introduce 'clk_round_rate_nearest()'

On Tue, 2014-05-20 at 10:48AM -0700, Stephen Boyd wrote:
> On 05/20/14 09:01, Sören Brinkmann wrote:
> >
> >>>>> +{
> >>>>> +	unsigned long lower, upper, cur, lower_last, upper_last;
> >>>>> +
> >>>>> +	lower = clk_round_rate(clk, rate);
> >>>>> +	if (lower >= rate)
> >>>>> +		return lower;
> >>>> Is the >-case worth a warning?
> >>> No, it's correct behavior. If you request a rate that is way lower than what the
> >>> clock can generate, returning something larger is perfectly valid, IMHO.
> >>> Which reveals one problem in this whole discussion. The API does not
> >>> require clk_round_rate() to round down. It is actually an implementation
> >>> choice that had been made for clk-divider.
> >> I'm sure it's more than an implementation choice for clk-divider. But I
> >> don't find any respective documentation (but I didn't try hard).
> > A similar discussion - without final conclusion:
> > https://lkml.org/lkml/2010/7/14/260
> >
> >
> 
> Please call this new API something like clk_find_nearest_rate() or
> something. clk_round_rate() is supposed to return the rate that will be
> set if you call clk_set_rate() with the same arguments. It's up to the
> implementation to decide if that means rounding the rate up or down or
> to the nearest value.

Sounds good to me. Are there any cases of clocks that round up? I think
that case would not be handled correctly. But I also don't see a use
case for such an implementation.

	Sören

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