Messages in this thread | | | Date | Thu, 21 Mar 2013 10:15:31 +0100 | From | Uwe Kleine-König <> | Subject | Re: [PATCH] clk: divider: Use DIV_ROUND_CLOSEST |
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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/ | -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.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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