From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932169AbbCFT2Y (ORCPT ); Fri, 6 Mar 2015 14:28:24 -0500 Received: from metis.ext.pengutronix.de ([92.198.50.35]:50436 "EHLO metis.ext.pengutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755469AbbCFT2V (ORCPT ); Fri, 6 Mar 2015 14:28:21 -0500 Date: Fri, 6 Mar 2015 20:28:13 +0100 From: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= To: Mike Turquette Cc: Stephen Boyd , linux-arm-kernel@lists.infradead.org, =?iso-8859-1?Q?S=F6ren?= Brinkmann , linux-kernel@vger.kernel.org, kernel@pengutronix.de Subject: Re: [PATCH 0/3] clk: divider: three exactness fixes (and a rant) Message-ID: <20150306192813.GG10717@pengutronix.de> References: <20150221085620.GV19388@pengutronix.de> <1424515225-6929-1-git-send-email-u.kleine-koenig@pengutronix.de> <20150306185730.11109.45998@quantum> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20150306185730.11109.45998@quantum> User-Agent: Mutt/1.5.21 (2010-09-15) X-SA-Exim-Connect-IP: 2001:67c:670:100:1d::c0 X-SA-Exim-Mail-From: ukl@pengutronix.de X-SA-Exim-Scanned: No (on metis.ext.pengutronix.de); SAEximRunCond expanded to false X-PTX-Original-Recipient: linux-kernel@vger.kernel.org Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello Mike, On Fri, Mar 06, 2015 at 10:57:30AM -0800, Mike Turquette wrote: > Quoting Uwe Kleine-König (2015-02-21 02:40:22) > > Hello, > > > > TLDR: only apply patch 1 and rip of CLK_DIVIDER_ROUND_CLOSEST. > > > > I stared at clk-divider.c for some time now given Sascha's failing test > > case. I found a fix for the failure (which happens to be what Sascha > > suspected). > > > > The other two patches fix problems only present when handling dividers > > that have CLK_DIVIDER_ROUND_CLOSEST set. Note that these are still > > heavily broken however. So having a 4bit-divider and a parent clk of > > 10000 (as in Sascha's test case) requesting > > > > clk_set_rate(clk, 666) > > > > sets the rate to 625 (div=15) instead of 667 (div=16). The reason is the > > choice of parent_rate in clk_divider_bestdiv's loop is wrong for > > CLK_DIVIDER_ROUND_CLOSEST (with and without patch 1). A fix here is > > non-trivial and for sure more than one rate must be tested here. This is > > complicated by the fact that clk_round_rate might return a value bigger > > than the requested rate which convinces me (once more) that it's a bad > > idea to allow that. Even if this was fixed for .round_rate, > > clk_divider_set_rate is still broken because it also uses > > > > div = DIV_ROUND_UP(parent_rate, rate); > > > > to calculate the (pretended) best divider to get near rate. > > > > Note this makes at least two reasons to remove support for > > CLK_DIVIDER_ROUND_CLOSEST! > > > > Instead I'd favour creating a function > > > > clk_round_rate_nearest > > > > as was suggested some time ago by Soren Brinkmann and me[1] that doesn't > > Uwe, > > Thanks for the fixes. I'm thinking of taking all three for 4.0. I also > agree on clk_round_rate_nearest (along with a _ceil and _floor version > as well). That's something we can do for 4.1 probably. I'd say that we make round_rate the _floor version. I guess in most cases that already does the right thing. Also I think 4.1 is very ambitious, so my suggestion for 4.1 is: - add a WARN_ON_ONCE to clk_round_rate catching calls that return a value bigger than requested. - implement clk_round_rate_nearest using clk_round_rate and the assumption that it returns a value that is <= the requested rate. I think without that there are too many special cases to handle and probably not even a reliable way to determine the nearest rate. - while we're at it tightening the requirements for clk_round_rate let's also specify the expected rounding. I'd vote for the mathematical rounding, that is clk_round_rate(someclk, 333) explicitly is allowed to return a rate bigger than 333 as long as it is less than 333.5. At one point while developing patch 1 I had the dividers fixed for the rounding issue. I think I still have that patch somewhere so can post it as RFC. > There are currently 3 users of CLK_DIVIDER_ROUND_CLOSEST: > > Loongson > QCOM > ST > > So now is probably the right time to remove the flag if we're going to > do it. "now" is before we have clk_round_rate_nearest, right? So what do you want to do with these three users? Move them to the default implementation? Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ |