From mboxrd@z Thu Jan 1 00:00:00 1970 From: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= Subject: Re: [RFC PATCH 2/5] clk: Introduce 'clk_round_rate_nearest()' Date: Wed, 21 May 2014 20:23:08 +0200 Message-ID: <20140521182308.GN31687@pengutronix.de> References: <20140515073816.GI16662@pengutronix.de> <91822600-39d0-4e71-b0f5-9eda35b76ec0@BN1AFFO11FD016.protection.gbl> <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> Mime-Version: 1.0 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-pm-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="iso-8859-1" To: =?iso-8859-1?Q?S=F6ren?= Brinkmann Cc: Mike Turquette , 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 Hello S=F6ren, On Wed, May 21, 2014 at 08:58:10AM -0700, S=F6ren Brinkmann wrote: > On Wed, 2014-05-21 at 09:34AM +0200, Uwe Kleine-K=F6nig wrote: > > On Tue, May 20, 2014 at 02:48:20PM -0700, S=F6ren Brinkmann wrote: > > > On Tue, 2014-05-20 at 10:48AM -0700, Stephen Boyd wrote: > > > > On 05/20/14 09:01, S=F6ren Brinkmann wrote: > > > > > > > > > >>>>> +{ > > > > >>>>> + unsigned long lower, upper, cur, lower_last, upper_last= ; > > > > >>>>> + > > > > >>>>> + lower =3D clk_round_rate(clk, rate); > > > > >>>>> + if (lower >=3D rate) > > > > >>>>> + return lower; > > > > >>>> Is the >-case worth a warning? > > > > >>> No, it's correct behavior. If you request a rate that is wa= y 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 i= mplementation > > > > >>> choice that had been made for clk-divider. > > > > >> I'm sure it's more than an implementation choice for clk-div= ider. But I > > > > >> don't find any respective documentation (but I didn't try ha= rd). > > > > > A similar discussion - without final conclusion: > > > > > https://lkml.org/lkml/2010/7/14/260 > > > > > > > > > > > > > >=20 > > > > 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. > > >=20 > > > 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 ro= und > > 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 an= d add > > the function that is under discussion here to get a nearest match. > >=20 > > So I suggest: > >=20 > > - 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 >=20 > 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) <=3D 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 >=20 > 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? Best regards Uwe --=20 Pengutronix e.K. | Uwe Kleine-K=F6nig = | Industrial Linux Solutions | http://www.pengutronix.de/= |