Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755577Ab3CZWpa (ORCPT ); Tue, 26 Mar 2013 18:45:30 -0400 Received: from va3ehsobe004.messaging.microsoft.com ([216.32.180.14]:41027 "EHLO va3outboundpool.messaging.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753281Ab3CZWp1 convert rfc822-to-8bit (ORCPT ); Tue, 26 Mar 2013 18:45:27 -0400 X-Forefront-Antispam-Report: CIP:149.199.60.83;KIP:(null);UIP:(null);IPV:NLI;H:xsj-gw1;RD:unknown-60-83.xilinx.com;EFVD:NLI X-SpamScore: -2 X-BigFish: VPS-2(zz98dIc89bh1432I1521Izz1f42h1ee6h1de0h1202h1e76h1d1ah1d2ahzzz2fh95h668h839h93fhd24hf0ah119dh1288h12a5h12a9h12bdh137ah13b6h1441h14ddh1504h1537h153bh162dh1631h1758h18e1h1946h19b5h1b0ah906i1155h) Date: Tue, 26 Mar 2013 15:45:22 -0700 From: =?utf-8?B?U8O2cmVu?= Brinkmann To: Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= CC: Sascha Hauer , Mike Turquette , , Subject: Re: [PATCH] clk: divider: Use DIV_ROUND_CLOSEST References: <20130320001609.8663.21043@quantum> <20130320185051.GA28349@pengutronix.de> <20130321091531.GN20530@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Disposition: inline In-Reply-To: <20130321091531.GN20530@pengutronix.de> User-Agent: Mutt/1.5.21 (2010-09-15) X-RCIS-Action: ALLOW Message-ID: Content-Transfer-Encoding: 8BIT X-OriginatorOrg: xilinx.com Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2275 Lines: 53 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? Sören -- 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/