From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753107AbaEUUdl (ORCPT ); Wed, 21 May 2014 16:33:41 -0400 Received: from mail-pb0-f51.google.com ([209.85.160.51]:59493 "EHLO mail-pb0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751801AbaEUUdj convert rfc822-to-8bit (ORCPT ); Wed, 21 May 2014 16:33:39 -0400 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8BIT To: =?utf-8?q?Uwe_Kleine-K=C3=B6nig?= , =?utf-8?q?S=C3=B6ren_Brinkmann?= From: Mike Turquette In-Reply-To: <20140521182308.GN31687@pengutronix.de> Cc: "Russell King" , "Stephen Boyd" , linux-pm@vger.kernel.org, "Viresh Kumar" , "Rafael J. Wysocki" , "Michal Simek" , cpufreq@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org References: <20140515073816.GI16662@pengutronix.de> <20140519161949.GG16662@pengutronix.de> <20140520073358.GJ16662@pengutronix.de> <4bb5f44a-60bb-4e34-8f88-f91b8419be8d@BL2FFO11FD050.protection.gbl> <537B957B.5010001@codeaurora.org> <668683e3-856e-4f30-9b11-8f3e91e12d1d@BL2FFO11FD038.protection.gbl> <20140521073457.GD31687@pengutronix.de> <20140521182308.GN31687@pengutronix.de> Message-ID: <20140521203300.9521.67546@quantum> User-Agent: alot/0.3.5 Subject: Re: [RFC PATCH 2/5] clk: Introduce 'clk_round_rate_nearest()' Date: Wed, 21 May 2014 13:33:00 -0700 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Quoting Uwe Kleine-König (2014-05-21 11:23:08) > Hello Sören, > > On Wed, May 21, 2014 at 08:58:10AM -0700, Sören Brinkmann wrote: > > On Wed, 2014-05-21 at 09:34AM +0200, Uwe Kleine-König wrote: > > > On Tue, May 20, 2014 at 02:48:20PM -0700, Sören Brinkmann wrote: > > > > On Tue, 2014-05-20 at 10:48AM -0700, Stephen Boyd wrote: > > > > > On 05/20/14 09:01, Sören Brinkmann wrote: > > > > > > > > > > > >>>>> +{ > > > > > >>>>> + unsigned long lower, upper, cur, lower_last, upper_last; > > > > > >>>>> + > > > > > >>>>> + lower = clk_round_rate(clk, rate); > > > > > >>>>> + if (lower >= rate) > > > > > >>>>> + return lower; > > > > > >>>> Is the >-case worth a warning? > > > > > >>> No, it's correct behavior. If you request a rate that is way lower than what the > > > > > >>> clock can generate, returning something larger is perfectly valid, IMHO. > > > > > >>> Which reveals one problem in this whole discussion. The API does not > > > > > >>> require clk_round_rate() to round down. It is actually an implementation > > > > > >>> choice that had been made for clk-divider. > > > > > >> I'm sure it's more than an implementation choice for clk-divider. But I > > > > > >> don't find any respective documentation (but I didn't try hard). > > > > > > A similar discussion - without final conclusion: > > > > > > https://lkml.org/lkml/2010/7/14/260 > > > > > > > > > > > > > > > > > > > > > > Please call this new API something like clk_find_nearest_rate() or > > > > > something. clk_round_rate() is supposed to return the rate that will be > > > > > set if you call clk_set_rate() with the same arguments. It's up to the > > > > > implementation to decide if that means rounding the rate up or down or > > > > > to the nearest value. > > > > > > > > Sounds good to me. Are there any cases of clocks that round up? I think > > > > that case would not be handled correctly. But I also don't see a use > > > > case for such an implementation. > > > I don't really care which semantic (i.e. round up, round down or round > > > closest) is picked, but I'd vote that all should pick up the same. I > > > think the least surprising definition is to choose rounding down and add > > > the function that is under discussion here to get a nearest match. > > > > > > So I suggest: > > > > > > - if round_rate is given a rate that is smaller than the > > > smallest available rate, return 0 > > > - add WARN_ONCE to round_rate and set_rate if they return with a > > > rate bigger than requested > > > > Why do you think 0 is always valid? I think for a clock that can > > generate 40, 70, 120, clk_round_rate(20) should return 40. > I didn't say it's a valid value. It just makes the it possible to check > for clk_round_rate(clk, rate) <= rate. > > I grepped a bit around and found da850_round_armrate which implements a > round_rate callback returning the best match. > omap1_clk_round_rate_ckctl_arm can return a value < 0. > s3c2412_roundrate_usbsrc can return values that are bigger than > requested. (I wonder if that is a bug though.) > > > > - change the return values to unsigned long > > > > Yep, I agree, this should happen. > And we're using 0 as error value? e.g. for the case where > omap1_clk_round_rate_ckctl_arm returns -EIO now? No. clk_round_rate returns long for a reason, which is that we can provide an error code to the caller. From include/linux/clk.h: /** * clk_round_rate - adjust a rate to the exact rate a clock can provide * @clk: clock source * @rate: desired clock rate in Hz * * Returns rounded clock rate in Hz, or negative errno. */ This has the unfortunate side effect that the max value we can return safely is 2147483647 (~2GHz). So another issue here is converting clock rates to 64-bit values. Regards, Mike > > Best regards > Uwe > > -- > Pengutronix e.K. | Uwe Kleine-König | > Industrial Linux Solutions | http://www.pengutronix.de/ |