linux-kernel - Re: [PATCH] clk: divider: Use DIV_ROUND

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Message-ID: <20130321091531.GN20530@pengutronix.de>
Date:	Thu, 21 Mar 2013 10:15:31 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Sören Brinkmann <soren.brinkmann@...inx.com>
Cc:	Sascha Hauer <s.hauer@...gutronix.de>,
	Mike Turquette <mturquette@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] clk: divider: Use DIV_ROUND_CLOSEST

Hello,

On Wed, Mar 20, 2013 at 07:50:51PM +0100, Sascha Hauer wrote:
> On Wed, Mar 20, 2013 at 09:32:51AM -0700, Sören 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;
	}

Best regards
Uwe

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