Re: [PATCH] clk: divider: Use DIV_ROUND_CLOSEST

[SÃren Brinkmann]

On Thu, Mar 21, 2013 at 10:15:31AM +0100, Uwe Kleine-KÃnig wrote:
> Hello,
> 
> On Wed, Mar 20, 2013 at 07:50:51PM +0100, Sascha Hauer wrote:
> > On Wed, Mar 20, 2013 at 09:32:51AM -0700, SÃren Brinkmann wrote:
> > > If the caller
> > > doesn't like the returned frequency he can request a different one.
> > > And he's eventually happy with the return value he calls
> > > clk_set_rate() requesting the frequency clk_round_rate() returned.
> > > Always rounding down seems a bit odd to me.
> > > 
> > > Another issue with the current implmentation:
> > > clk_divider_round_rate() calls clk_divider_bestdiv(), which uses the ROUND_UP macro, returning a rather low frequency.
> > 
> > And that is correct. clk_divider_bestdiv is used to calculate the
> > maximum parent frequency for which a given divider value does not
> > exceed the desired rate.
> The reason for that is that the (more?) usual constraint is like: This
> mmc card can handle up to 100 MHz. Or this i2c device can handle up to
> this and that frequency. Of course there are different constraints, e.g.
> for a UART if the target baud speed is 38400 you better run at 38402
> than at 19201.
> 
> I wonder if it depends on the clock if you want "best approximation <=
> requested value" or "best approximation" or on the caller. In the former
> case a flag for the clock would be the right thing (as suggested in this
> thread). If however it's the caller of round_rate who knows better which
> rounding is preferred than better extend the clk API.
> 
> Extending the API could just be a convenience function that doesn't
> affect the implementations of the clk API. E.g.:
> 
> 	long clk_round_rate_nearest(struct clk *clk, unsigned long rate)
> 	{
> 		long lower_limit = clk_round_rate(clk, rate);
> 		long upper_limit = clk_round_rate(clk, rate + (rate - lower_limit));
> 
> 		if (rate - lower_limit < upper_limit - rate)
> 			return lower_limit;
> 		else
> 			return upper_limit;
> 	}
> 
I guess both approaches may work. Anybody has a preference?

	SÃren


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