Hello,
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
- change the return values to unsigned long
Do we also need a round_up implementation?
Mike? Russell? Any thoughts from your side?
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | http://www.pengutronix.de/ |
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