From patchwork Thu Jul 22 19:50:10 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andy Shevchenko X-Patchwork-Id: 483963 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=-16.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS, URIBL_BLOCKED, USER_AGENT_GIT autolearn=ham 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 C3891C43216 for ; Thu, 22 Jul 2021 19:50:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A4F4C60EB1 for ; Thu, 22 Jul 2021 19:50:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229897AbhGVTJw (ORCPT ); Thu, 22 Jul 2021 15:09:52 -0400 Received: from mga01.intel.com ([192.55.52.88]:51144 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229756AbhGVTJd (ORCPT ); Thu, 22 Jul 2021 15:09:33 -0400 X-IronPort-AV: E=McAfee;i="6200,9189,10053"; a="233553644" X-IronPort-AV: E=Sophos;i="5.84,261,1620716400"; d="scan'208";a="233553644" Received: from fmsmga006.fm.intel.com ([10.253.24.20]) by fmsmga101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Jul 2021 12:50:08 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.84,261,1620716400"; d="scan'208";a="659226766" Received: from black.fi.intel.com ([10.237.72.28]) by fmsmga006.fm.intel.com with ESMTP; 22 Jul 2021 12:50:03 -0700 Received: by black.fi.intel.com (Postfix, from userid 1003) id 9D99835D; Thu, 22 Jul 2021 22:50:31 +0300 (EEST) From: Andy Shevchenko To: "Rafael J. Wysocki" , Andy Shevchenko , Heiko Stuebner , Elaine Zhang , linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org, linux-clk@vger.kernel.org, linux-imx@nxp.com, linux-arm-kernel@lists.infradead.org, linux-rockchip@lists.infradead.org Cc: "Rafael J. Wysocki" , Len Brown , Michael Turquette , Stephen Boyd , Abel Vesa , Shawn Guo , Sascha Hauer , Pengutronix Kernel Team , Fabio Estevam , Lee Jones , Liu Ying Subject: [PATCH v3 4/4] clk: fractional-divider: Document the arithmetics used behind the code Date: Thu, 22 Jul 2021 22:50:10 +0300 Message-Id: <20210722195010.45940-4-andriy.shevchenko@linux.intel.com> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20210722195010.45940-1-andriy.shevchenko@linux.intel.com> References: <20210722195010.45940-1-andriy.shevchenko@linux.intel.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org It appears that some code lines raise the question why they are needed and how they are participated in the calculus of the resulting values. Document this in a form of the top comment in the module file. Reported-by: Liu Ying Signed-off-by: Andy Shevchenko --- v3: mentioned the flag in the comment drivers/clk/clk-fractional-divider.c | 35 +++++++++++++++++++++++++++- 1 file changed, 34 insertions(+), 1 deletion(-) diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c index ae1927f9c08b..86cead75e02c 100644 --- a/drivers/clk/clk-fractional-divider.c +++ b/drivers/clk/clk-fractional-divider.c @@ -3,8 +3,39 @@ * Copyright (C) 2014 Intel Corporation * * Adjustable fractional divider clock implementation. - * Output rate = (m / n) * parent_rate. * Uses rational best approximation algorithm. + * + * Output is calculated as + * + * rate = (m / n) * parent_rate (1) + * + * This is useful when on die we have a prescaler block which asks for + * m (numerator) and n (denominator) values to be provided to satisfy + * the (1) as much as possible. + * + * Since m and n have the limitation by a range, e.g. + * + * n >= 1, n < N_width, where N_width = 2^nwidth (2) + * + * for some cases the output may be saturated. Hence, from (1) and (2), + * assuming the worst case when m = 1, the inequality + * + * floor(log2(parent_rate / rate)) <= nwidth (3) + * + * may be derived. Thus, in cases when + * + * (parent_rate / rate) >> N_width (4) + * + * we might scale up the rate by 2^scale (see the description of + * CLK_FRAC_DIVIDER_POWER_OF_TWO_PS for additional information), where + * + * scale = floor(log2(parent_rate / rate)) - nwidth (5) + * + * and assume that the IP, that needs m and n, has also its own + * prescaler, which is capable to divide by 2^scale. In this way + * we get the denominator to satisfy the desired range (2) and + * at the same time much much better result of m and n than simple + * saturated values. */ #include @@ -81,6 +112,8 @@ void clk_fractional_divider_general_approximation(struct clk_hw *hw, * 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. + * + * For the detailed explanation see the top comment in this file. */ if (fd->flags & CLK_FRAC_DIVIDER_POWER_OF_TWO_PS) { unsigned long scale = fls_long(*parent_rate / rate - 1);