From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4B46BC07E9C for ; Wed, 14 Jul 2021 10:46:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 29EA06128B for ; Wed, 14 Jul 2021 10:46:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239199AbhGNKtM (ORCPT ); Wed, 14 Jul 2021 06:49:12 -0400 Received: from mga03.intel.com ([134.134.136.65]:24002 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237788AbhGNKtK (ORCPT ); Wed, 14 Jul 2021 06:49:10 -0400 X-IronPort-AV: E=McAfee;i="6200,9189,10044"; a="210367740" X-IronPort-AV: E=Sophos;i="5.84,238,1620716400"; d="scan'208";a="210367740" Received: from orsmga003.jf.intel.com ([10.7.209.27]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 14 Jul 2021 03:46:18 -0700 X-IronPort-AV: E=Sophos;i="5.84,238,1620716400"; d="scan'208";a="413247577" Received: from smile.fi.intel.com (HELO smile) ([10.237.68.40]) by orsmga003-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 14 Jul 2021 03:46:15 -0700 Received: from andy by smile with local (Exim 4.94.2) (envelope-from ) id 1m3cP3-00DHKT-Rc; Wed, 14 Jul 2021 13:46:09 +0300 Date: Wed, 14 Jul 2021 13:46:09 +0300 From: Andy Shevchenko To: Liu Ying Cc: linux-clk@vger.kernel.org, linux-kernel@vger.kernel.org, Heikki Krogerus , Michael Turquette , Stephen Boyd , Dong Aisheng , NXP Linux Team , Jacky Bai Subject: Re: [RFC PATCH] clk: fractional-divider: Correct max_{m,n} handed over to rational_best_approximation() Message-ID: References: <20210714064129.1321277-1-victor.liu@nxp.com> <1f1b38f9c42093bea5e6a8ccb458bdf799069157.camel@nxp.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 14, 2021 at 01:38:22PM +0300, Andy Shevchenko wrote: > On Wed, Jul 14, 2021 at 06:10:46PM +0800, Liu Ying wrote: > > On Wed, 2021-07-14 at 12:12 +0300, Andy Shevchenko wrote: > > > On Wed, Jul 14, 2021 at 02:41:29PM +0800, Liu Ying wrote: > > ... > > > > > /* > > > > * Get rate closer to *parent_rate to guarantee there is no overflow > > > > * for m and n. In the result it will be the nearest rate left shifted > > > > * by (scale - fd->nwidth) bits. > > > > */ > > > > > > I don't know how to rephrase above comment better. > > > > > > > scale = fls_long(*parent_rate / rate - 1); > > > > if (scale > fd->nwidth) > > > > rate <<= scale - fd->nwidth; > > > > > > This takes an advantage of the numbers be in a form of > > > > > > n = k * 2^m, (1) > > > > > > where m will be scale in the snippet above. Thus, if n can be represented by > > > (1), we opportunistically reduce amount of bits needed for it by shifting right > > > by m bits. > > > Does it make sense? > > > > Thanks for your explaination. > > But, sorry, Jacky and I still don't understand this. It seems I poorly chose the letters for (1). Let's rewrite above as This takes an advantage of the numbers be in a form of a = k * 2^b, (1) where b will be scale in the snippet above. Thus, if a can be represented by (1), we opportunistically reduce amount of bits needed for it by shifting right by b bits. Also note, "shifting right" here is about the result of n (see below). > Okay, We have two values in question: > r_o (original rate of the parent clock) > r_u (the rate user asked for) > > We have a pre-scaler block which asks for > m (denominator) > n (nominator) > values to be provided to satisfy the (2) > > r_u ~= r_o * m / n, (2) > > where we try our best to make it "=" instead of "~=". > > Now, m and n have the limitation by a range, e.g. > > n >= 1, n < N_lim, where N_lim = 2^nlim. (3) > > Hence, from (2) and (3), assuming the worst case m = 1, > > ln2(r_o / r_u) <= nlim. (4) > > The above code tries to satisfy (4). > > Have you got it now? > > > > The code looks good to me, btw, although I dunno if you need to call the newly > > > introduced function before or after the above mentioned snippet. > > > > Assuming that snippet is fully orthogonal to this patch, then it > > doesn't matter if it's before or after. > > Please, double check this. Because you play with limits, while we expect them > to satisfy (3). > > -- > With Best Regards, > Andy Shevchenko > > -- With Best Regards, Andy Shevchenko