[RFC PATCH 2/5] clk: Introduce 'clk_round_rate_nearest()'

[Mike Turquette]

Quoting Uwe Kleine-König (2014-05-21 11:23:08)
> 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.
> 
> 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?

No. clk_round_rate returns long for a reason, which is that we can
provide an error code to the caller. From include/linux/clk.h:

/**
 * clk_round_rate - adjust a rate to the exact rate a clock can provide
 * @clk: clock source
 * @rate: desired clock rate in Hz
 *
 * Returns rounded clock rate in Hz, or negative errno.
 */

This has the unfortunate side effect that the max value we can return
safely is 2147483647 (~2GHz). So another issue here is converting clock
rates to 64-bit values.

Regards,
Mike

> 
> Best regards
> Uwe
> 
> -- 
> Pengutronix e.K.                           | Uwe Kleine-König            |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |