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Date: Tue, 26 Mar 2013 18:37:03 -0700 From: Mike Turquette <mturquette@...aro.org> To: Sören Brinkmann <soren.brinkmann@...inx.com>, Uwe Kleine-König <u.kleine-koenig@...gutronix.de> Cc: Sascha Hauer <s.hauer@...gutronix.de>, <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org> Subject: Re: [PATCH] clk: divider: Use DIV_ROUND_CLOSEST Quoting Sören Brinkmann (2013-03-26 15:45:22) > On Thu, Mar 21, 2013 at 10:15:31AM +0100, Uwe Kleine-König 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, 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; > > } > > > I guess both approaches may work. Anybody has a preference? > A dedicated function like the one Uwe defined is better than adding subtlety to the existing clk_round_rate via a flag in a clock driver. Regards, Mike > Sören -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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