From patchwork Fri Jul 16 13:34:46 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andy Shevchenko X-Patchwork-Id: 12382113 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=-17.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=unavailable 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 12EE6C12002 for ; Fri, 16 Jul 2021 13:36:43 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id DA552613F5 for ; Fri, 16 Jul 2021 13:36:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DA552613F5 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:Message-Id:Date:Subject:Cc :To:From:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:In-Reply-To:References: List-Owner; bh=QDjoVEURMBRVzliBZu7d822LeV7XuxBH995OP2CGS9U=; b=z9UWLkyB3tHcUd 9yZABNA57pxeM38N69AJgE+nIo+LgtlFP/sQHC5TuprgKMD+QYmYt6DBMOxrNI5fmIVSNNGpoWGZ8 QEcK+9qZHaqcEFtOx/POMwxy57yvZqK+X7i28ivLEnX1zfrDHdo7dthcpqbbPGkmpLSBf6098IlXo ycZMMbzfLKybwffNWuJsrTxN0e1XSu3sTUR5Jc8GwQ7GtpA0GN1dvXYdBuQAeG2L8W54MlQrtp4rn gSuCNzoX2ryqLEOv6X7k284+tPtC4sC2GPXD269RcLTYlBTEgam04n0BsDCR8p/G7lDPoj1VP4yKr MiG9xvyL9phJPXrbqfEg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m4NzS-004Tem-60; Fri, 16 Jul 2021 13:34:54 +0000 Received: from mga12.intel.com ([192.55.52.136]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1m4Nz5-004TY4-Gr; Fri, 16 Jul 2021 13:34:33 +0000 X-IronPort-AV: E=McAfee;i="6200,9189,10046"; a="190410236" X-IronPort-AV: E=Sophos;i="5.84,245,1620716400"; d="scan'208";a="190410236" Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 16 Jul 2021 06:34:26 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.84,245,1620716400"; d="scan'208";a="431208956" Received: from black.fi.intel.com ([10.237.72.28]) by orsmga002.jf.intel.com with ESMTP; 16 Jul 2021 06:34:23 -0700 Received: by black.fi.intel.com (Postfix, from userid 1003) id 3467F262; Fri, 16 Jul 2021 16:34:50 +0300 (EEST) From: Andy Shevchenko To: Andy Shevchenko , Heiko Stuebner , Elaine Zhang , Stephen Boyd , linux-kernel@vger.kernel.org, linux-clk@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-rockchip@lists.infradead.org Cc: Michael Turquette Subject: [PATCH v2 1/3] clk: fractional-divider: Export approximation algo to the CCF users Date: Fri, 16 Jul 2021 16:34:46 +0300 Message-Id: <20210716133448.24890-1-andriy.shevchenko@linux.intel.com> X-Mailer: git-send-email 2.30.2 MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210716_063431_623888_8F0702DD X-CRM114-Status: GOOD ( 17.84 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org At least one user currently duplicates some functions that are provided by fractional divider module. Let's export approximation algo and replace the open-coded variant. As a bonus the exported function will get better documentation in place. Signed-off-by: Andy Shevchenko Tested-by: Heiko Stuebner Acked-by: Heiko Stuebner --- v2: fixed compilation error (LKP), successfully compile-tested on x86 drivers/clk/clk-fractional-divider.c | 11 +++++++---- drivers/clk/clk-fractional-divider.h | 9 +++++++++ drivers/clk/rockchip/clk.c | 17 +++-------------- 3 files changed, 19 insertions(+), 18 deletions(-) create mode 100644 drivers/clk/clk-fractional-divider.h diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c index b1e556f20911..535d299af646 100644 --- a/drivers/clk/clk-fractional-divider.c +++ b/drivers/clk/clk-fractional-divider.c @@ -14,6 +14,8 @@ #include #include +#include "clk-fractional-divider.h" + static inline u32 clk_fd_readl(struct clk_fractional_divider *fd) { if (fd->flags & CLK_FRAC_DIVIDER_BIG_ENDIAN) @@ -68,9 +70,10 @@ static unsigned long clk_fd_recalc_rate(struct clk_hw *hw, return ret; } -static void clk_fd_general_approximation(struct clk_hw *hw, unsigned long rate, - unsigned long *parent_rate, - unsigned long *m, unsigned long *n) +void clk_fractional_divider_general_approximation(struct clk_hw *hw, + unsigned long rate, + unsigned long *parent_rate, + unsigned long *m, unsigned long *n) { struct clk_fractional_divider *fd = to_clk_fd(hw); unsigned long scale; @@ -102,7 +105,7 @@ static long clk_fd_round_rate(struct clk_hw *hw, unsigned long rate, if (fd->approximation) fd->approximation(hw, rate, parent_rate, &m, &n); else - clk_fd_general_approximation(hw, rate, parent_rate, &m, &n); + clk_fractional_divider_general_approximation(hw, rate, parent_rate, &m, &n); ret = (u64)*parent_rate * m; do_div(ret, n); diff --git a/drivers/clk/clk-fractional-divider.h b/drivers/clk/clk-fractional-divider.h new file mode 100644 index 000000000000..4fa359a12ef4 --- /dev/null +++ b/drivers/clk/clk-fractional-divider.h @@ -0,0 +1,9 @@ +/* SPDX-License-Identifier: GPL-2.0 */ + +struct clk_hw; + +void clk_fractional_divider_general_approximation(struct clk_hw *hw, + unsigned long rate, + unsigned long *parent_rate, + unsigned long *m, + unsigned long *n); diff --git a/drivers/clk/rockchip/clk.c b/drivers/clk/rockchip/clk.c index 049e5e0b64f6..b7be7e11b0df 100644 --- a/drivers/clk/rockchip/clk.c +++ b/drivers/clk/rockchip/clk.c @@ -22,6 +22,8 @@ #include #include #include + +#include "../clk-fractional-divider.h" #include "clk.h" /* @@ -178,10 +180,8 @@ static void rockchip_fractional_approximation(struct clk_hw *hw, unsigned long rate, unsigned long *parent_rate, unsigned long *m, unsigned long *n) { - struct clk_fractional_divider *fd = to_clk_fd(hw); unsigned long p_rate, p_parent_rate; struct clk_hw *p_parent; - unsigned long scale; p_rate = clk_hw_get_rate(clk_hw_get_parent(hw)); if ((rate * 20 > p_rate) && (p_rate % rate != 0)) { @@ -190,18 +190,7 @@ static void rockchip_fractional_approximation(struct clk_hw *hw, *parent_rate = p_parent_rate; } - /* - * 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. - */ - scale = fls_long(*parent_rate / rate - 1); - if (scale > fd->nwidth) - rate <<= scale - fd->nwidth; - - rational_best_approximation(rate, *parent_rate, - GENMASK(fd->mwidth - 1, 0), GENMASK(fd->nwidth - 1, 0), - m, n); + clk_fractional_divider_general_approximation(hw, rate, parent_rate, m, n); } static struct clk *rockchip_clk_register_frac_branch( From patchwork Fri Jul 16 13:34:47 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andy Shevchenko X-Patchwork-Id: 12382109 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=-17.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=unavailable 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 9CFE8C07E95 for ; Fri, 16 Jul 2021 13:36:30 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 6066D613F7 for ; Fri, 16 Jul 2021 13:36:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6066D613F7 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To: Message-Id:Date:Subject:Cc:To:From:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=ugkgBYVsq9P21k41+6cN+joo/1pCEaiWivrM+iNrbcI=; b=wyGuQXNbUvciYX F8PBiIKQS9jPXIV6dQ6jBz3fJRhr6QU2J1u66NkTbOheeEOp6eorE6CrYWkhyPIFLtk5TMmtrBP4r 8cfGkl3SeUcK3io/+Rf+SghMHIcpZEC3k/xY/eoO3c7oS7voFloLIzq2OfrDPiD1+y8LtuuIe52xB zd1RZx74L5MBlcBhGo+Dsh8QtwRnG6jkkSKbGX1m2DajNsCOLnvLKFKa9OpZDAJRt/MH9/JJP0Lkj PnfYBPe4ix42R1Tbdsup/osPiDOc/JLXwmDaiVRuP3H6l/lra+E4dqkt6lP1fIlCkHL+CD2RGxRKq SKjLALt+Yhfs5p8ZpXAw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m4NzH-004Tbk-NU; Fri, 16 Jul 2021 13:34:43 +0000 Received: from mga09.intel.com ([134.134.136.24]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1m4Nz2-004TWx-Er; Fri, 16 Jul 2021 13:34:33 +0000 X-IronPort-AV: E=McAfee;i="6200,9189,10046"; a="210719645" X-IronPort-AV: E=Sophos;i="5.84,245,1620716400"; d="scan'208";a="210719645" Received: from fmsmga001.fm.intel.com ([10.253.24.23]) by orsmga102.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 16 Jul 2021 06:34:26 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.84,245,1620716400"; d="scan'208";a="574019903" Received: from black.fi.intel.com ([10.237.72.28]) by fmsmga001.fm.intel.com with ESMTP; 16 Jul 2021 06:34:23 -0700 Received: by black.fi.intel.com (Postfix, from userid 1003) id 459A7B9; Fri, 16 Jul 2021 16:34:51 +0300 (EEST) From: Andy Shevchenko To: Andy Shevchenko , Heiko Stuebner , Elaine Zhang , Stephen Boyd , linux-kernel@vger.kernel.org, linux-clk@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-rockchip@lists.infradead.org Cc: Michael Turquette , Liu Ying Subject: [PATCH v2 2/3] clk: fractional-divider: Introduce NO_PRESCALER flag Date: Fri, 16 Jul 2021 16:34:47 +0300 Message-Id: <20210716133448.24890-2-andriy.shevchenko@linux.intel.com> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20210716133448.24890-1-andriy.shevchenko@linux.intel.com> References: <20210716133448.24890-1-andriy.shevchenko@linux.intel.com> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210716_063428_581312_4CE79320 X-CRM114-Status: GOOD ( 18.39 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org The newly introduced flag, when set, makes the flow to skip the assumption that the caller will use an additional 2^scale prescaler to get the desired clock rate. Reported-by: Liu Ying Signed-off-by: Andy Shevchenko --- v2: moved entire snipped under new flag check (Liu) drivers/clk/clk-fractional-divider.c | 10 ++++++---- include/linux/clk-provider.h | 5 +++++ 2 files changed, 11 insertions(+), 4 deletions(-) diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c index 535d299af646..5f4b6a8aef67 100644 --- a/drivers/clk/clk-fractional-divider.c +++ b/drivers/clk/clk-fractional-divider.c @@ -76,16 +76,18 @@ void clk_fractional_divider_general_approximation(struct clk_hw *hw, unsigned long *m, unsigned long *n) { struct clk_fractional_divider *fd = to_clk_fd(hw); - unsigned long scale; /* * 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. */ - scale = fls_long(*parent_rate / rate - 1); - if (scale > fd->nwidth) - rate <<= scale - fd->nwidth; + if (!(fd->flags & CLK_FRAC_DIVIDER_NO_PRESCALER)) { + unsigned long scale = fls_long(*parent_rate / rate - 1); + + if (scale > fd->nwidth) + rate <<= scale - fd->nwidth; + } rational_best_approximation(rate, *parent_rate, GENMASK(fd->mwidth - 1, 0), GENMASK(fd->nwidth - 1, 0), diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h index d83b829305c0..f74d0afe275f 100644 --- a/include/linux/clk-provider.h +++ b/include/linux/clk-provider.h @@ -1001,6 +1001,10 @@ struct clk_hw *devm_clk_hw_register_fixed_factor(struct device *dev, * CLK_FRAC_DIVIDER_BIG_ENDIAN - By default little endian register accesses are * used for the divider register. Setting this flag makes the register * accesses big endian. + * CLK_FRAC_DIVIDER_NO_PRESCALER - By default the resulting rate may be shifted + * left by a few bits in case when the asked one is quite small to satisfy + * the desired range of denominator. If the caller wants to get the best + * rate without using an additional prescaler, this flag may be set. */ struct clk_fractional_divider { struct clk_hw hw; @@ -1022,6 +1026,7 @@ struct clk_fractional_divider { #define CLK_FRAC_DIVIDER_ZERO_BASED BIT(0) #define CLK_FRAC_DIVIDER_BIG_ENDIAN BIT(1) +#define CLK_FRAC_DIVIDER_NO_PRESCALER BIT(2) extern const struct clk_ops clk_fractional_divider_ops; struct clk *clk_register_fractional_divider(struct device *dev, From patchwork Fri Jul 16 13:34:48 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andy Shevchenko X-Patchwork-Id: 12382111 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=-17.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=unavailable 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 74CD9C12002 for ; Fri, 16 Jul 2021 13:36:31 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 39004613F5 for ; Fri, 16 Jul 2021 13:36:31 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 39004613F5 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To: Message-Id:Date:Subject:Cc:To:From:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=/tAJ3S8n12k2QD1o4RNhpSLk+tU02YCv4OynGpCIKKA=; b=qRxRcXAHfH5bb5 RJIQl1mRnNGLt939Y3TqHuYVSnu3LyBxYdCJ/JVmRvlcW3VVjxYUOBLYGAv9ZbO0O5H60AO/2zXy8 nAHfHqf6i3DvycqQpi5T2fYxcrBJeGvXYfkYki4+8oyROYugRPALAflYov0L0UMupD7LbjUK5qx1k 5ehNOqMUZKj/YXmvcp97+W3mTI6IRC5y0Vr4fHcSdUK3+5muC4Key8nOEfJgeTCq3K7TnKHYW7V9H LUmhAwQHvTZjG14Wu9/Wqw2ViicNssvveVUBeU5udWEQ/9nfmFMhw6FYenwFkkbjwq+M5qbem182Q L/2uh/+dtmE050XLNbdQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m4Nz6-004TYZ-B7; Fri, 16 Jul 2021 13:34:32 +0000 Received: from mga03.intel.com ([134.134.136.65]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1m4Nz2-004TWw-6I; Fri, 16 Jul 2021 13:34:30 +0000 X-IronPort-AV: E=McAfee;i="6200,9189,10046"; a="210779373" X-IronPort-AV: E=Sophos;i="5.84,245,1620716400"; d="scan'208";a="210779373" Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 16 Jul 2021 06:34:26 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.84,245,1620716400"; d="scan'208";a="429191754" Received: from black.fi.intel.com ([10.237.72.28]) by fmsmga007.fm.intel.com with ESMTP; 16 Jul 2021 06:34:23 -0700 Received: by black.fi.intel.com (Postfix, from userid 1003) id 4C00A2CB; Fri, 16 Jul 2021 16:34:51 +0300 (EEST) From: Andy Shevchenko To: Andy Shevchenko , Heiko Stuebner , Elaine Zhang , Stephen Boyd , linux-kernel@vger.kernel.org, linux-clk@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-rockchip@lists.infradead.org Cc: Michael Turquette , Liu Ying Subject: [PATCH v2 3/3] clk: fractional-divider: Document the arithmetics used behind the code Date: Fri, 16 Jul 2021 16:34:48 +0300 Message-Id: <20210716133448.24890-3-andriy.shevchenko@linux.intel.com> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20210716133448.24890-1-andriy.shevchenko@linux.intel.com> References: <20210716133448.24890-1-andriy.shevchenko@linux.intel.com> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210716_063428_317465_CE9310CA X-CRM114-Status: GOOD ( 15.38 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.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 --- v2: renamed variables in formulas to follow the code, added floor() drivers/clk/clk-fractional-divider.c | 34 +++++++++++++++++++++++++++- 1 file changed, 33 insertions(+), 1 deletion(-) diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c index 5f4b6a8aef67..7f7f688f8de5 100644 --- a/drivers/clk/clk-fractional-divider.c +++ b/drivers/clk/clk-fractional-divider.c @@ -3,8 +3,38 @@ * 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 scale up the rate by 2^scale, 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 +111,8 @@ 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_NO_PRESCALER)) { unsigned long scale = fls_long(*parent_rate / rate - 1);