Re: Common/typical fractional divider HW API

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Fri, 2016-02-05 at 17:29 +0100, Mason wrote:
> On 05/02/2016 17:12, Andy Shevchenko wrote:
> 
> > On Fri, 2016-02-05 at 17:01 +0100, Mason wrote:
> > 
> > > So, do you agree that
> > > 
> > >   DIV = 2^I * (1 + F/256)
> > > 
> > > gives a more useful DIV distribution than
> > > 
> > >   DIV = I + F/16
> > 
> > If you can change hardware why not to use any existing approach
> > which
> > suits better to your device?
> 
> Sorry, I don't understand the question.
> 

There are plenty of implementations of the divider. You might consider
to use one than inventing new one:

https://xkcd.com/927/

> In this part of my message, I was trying to argue that one HW API
> "2^I * (1 + F/256)" seemed better than another one "I + F/16" on
> any hardware.

I disagree in a part 2^I.

> 
> > I don't remember any existing, though I didn't check much, divider
> > register which takes something like that.
> 
> IIUC, you are saying that you've never seen hardware use the
> "2^I * (1 + F/256)" formula, is that correct?

Yep, though it doesn't mean there is no such.

> I'm not sure how to parse that. I'm using the divider driver
> > > for a CPU clock, to do D(V)FS in cpufreq.
> > 
> > If you are using custom stuff for custom hardware, I hope it's
> > okay.
> > But if we are talking about generic solutions (like clk-fractional-
> > divider), I would suggest to consider existing users / hardware.
> 
> Are you saying that I could use the clk-fractional-divider with
> hardware that computes "I + F/16" ?

No.

> Maybe the clk-fractional-divider could be made more generic by having
> the register update part done in a call-back function?

Why do you need to touch that module at all if your hardware doesn't
suit it?

-- 
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Intel Finland Oy