Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756226Ab3C0BhL (ORCPT ); Tue, 26 Mar 2013 21:37:11 -0400 Received: from mail-da0-f54.google.com ([209.85.210.54]:61519 "EHLO mail-da0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751984Ab3C0BhJ convert rfc822-to-8bit (ORCPT ); Tue, 26 Mar 2013 21:37:09 -0400 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8BIT To: =?utf-8?q?S=C3=B6ren_Brinkmann?= , =?utf-8?q?Uwe_Kleine-K=C3=B6nig?= From: Mike Turquette In-Reply-To: Cc: Sascha Hauer , , References: <20130320001609.8663.21043@quantum> <20130320185051.GA28349@pengutronix.de> <20130321091531.GN20530@pengutronix.de> Message-ID: <20130327013703.4014.45615@quantum> User-Agent: alot/0.3.3+ Subject: Re: [PATCH] clk: divider: Use DIV_ROUND_CLOSEST Date: Tue, 26 Mar 2013 18:37:03 -0700 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2675 Lines: 59 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@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/