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

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Message-ID: <3ff23a63-f05f-4e46-835c-564e05e01048@BN1BFFO11FD016.protection.gbl>
Date:	Wed, 21 May 2014 13:19:40 -0700
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
CC:	Mike Turquette <mturquette@...aro.org>,
	Russell King <linux@....linux.org.uk>,
	Stephen Boyd <sboyd@...eaurora.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>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 2/5] clk: Introduce 'clk_round_rate_nearest()'

On Wed, 2014-05-21 at 08:23PM +0200, Uwe Kleine-König wrote:
> Hello Sören,
> 
> On Wed, May 21, 2014 at 08:58:10AM -0700, Sören Brinkmann wrote:
> > On Wed, 2014-05-21 at 09:34AM +0200, Uwe Kleine-König wrote:
> > > On Tue, May 20, 2014 at 02:48:20PM -0700, Sören Brinkmann wrote:
> > > > 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.
> > > I don't really care which semantic (i.e. round up, round down or round
> > > closest) is picked, but I'd vote that all should pick up the same. I
> > > think the least surprising definition is to choose rounding down and add
> > > the function that is under discussion here to get a nearest match.
> > > 
> > > So I suggest:
> > > 
> > > 	- if round_rate is given a rate that is smaller than the
> > > 	  smallest available rate, return 0
> > > 	- add WARN_ONCE to round_rate and set_rate if they return with a
> > > 	  rate bigger than requested
> > 
> > Why do you think 0 is always valid? I think for a clock that can
> > generate 40, 70, 120, clk_round_rate(20) should return 40.
> I didn't say it's a valid value. It just makes the it possible to check
> for clk_round_rate(clk, rate) <= rate.

But if the set_rate() doesn't set the rate to 0 in that case, the
implementation was buggy.

> 
> I grepped a bit around and found da850_round_armrate which implements a
> round_rate callback returning the best match.
> omap1_clk_round_rate_ckctl_arm can return a value < 0.
> s3c2412_roundrate_usbsrc can return values that are bigger than
> requested. (I wonder if that is a bug though.)
> 
> > > 	- change the return values to unsigned long
> > 
> > Yep, I agree, this should happen.
> And we're using 0 as error value? e.g. for the case where
> omap1_clk_round_rate_ckctl_arm returns -EIO now?

IMHO, there should always be a valid frequency be returned from these
calls. The API says: return the frequency the clock provides when
set_rate() is called with the same argument. This should never result in
an error.

	Sören

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