From patchwork Thu Jul 15 12:07:50 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andy Shevchenko X-Patchwork-Id: 12379885 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,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 E683EC07E96 for ; Thu, 15 Jul 2021 12:08:12 +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 AD98B6136E for ; Thu, 15 Jul 2021 12:08:12 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AD98B6136E 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-rockchip-bounces+linux-rockchip=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=t3JgDdOZ9XdYjm3SGdglRM5GeOof7tzaeJT0foAZciU=; b=D45ZMwyyANfq/O gkrFuLCv+qNQpTZ8vl0iHKLp8N87Wm8xJTtE/q8TAF3qF16QoD8vmDZskeubR55G+cmMYcupuV+0h b6YxpT7TtK47osLpK2l+3aQqs73kR404jGupT0nRQaom7A+fmavJuIktTEh3z/n7Kh/r2d6v7hBqW 4SjLw/YnxSFnAvGxM1WiMqGBWUt6xAPotj7fupYPNdpoQSqa3XjDl/EMB6RmXDSimq/k/qedd/iU4 9K3cDPzO8jmKGg4oG9n2qy0Ev29r/924BlPd7WtWY5sT0ysIWtuO69L/SvtnJrPX5hp/fOHYAFgfK PPXqpKSAveq7neoJdQKQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m409v-000hDn-Op; Thu, 15 Jul 2021 12:08:07 +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 1m409Z-000h9H-Ox; Thu, 15 Jul 2021 12:07:47 +0000 X-IronPort-AV: E=McAfee;i="6200,9189,10045"; a="190212616" X-IronPort-AV: E=Sophos;i="5.84,240,1620716400"; d="scan'208";a="190212616" Received: from fmsmga003.fm.intel.com ([10.253.24.29]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Jul 2021 05:07:38 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.84,240,1620716400"; d="scan'208";a="495407872" Received: from black.fi.intel.com ([10.237.72.28]) by FMSMGA003.fm.intel.com with ESMTP; 15 Jul 2021 05:07:35 -0700 Received: by black.fi.intel.com (Postfix, from userid 1003) id 4413C262; Thu, 15 Jul 2021 15:08:03 +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 v1 1/3] clk: fractional-divider: Export approximation algo to the CCF users Date: Thu, 15 Jul 2021 15:07:50 +0300 Message-Id: <20210715120752.29174-1-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-20210715_050745_874723_D7C866DC X-CRM114-Status: GOOD ( 17.47 ) X-BeenThere: linux-rockchip@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Upstream kernel work for Rockchip platforms List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-rockchip" Errors-To: linux-rockchip-bounces+linux-rockchip=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 Reported-by: kernel test robot Reported-by: kernel test robot --- 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..ce7b71594827 100644 --- a/drivers/clk/rockchip/clk.c +++ b/drivers/clk/rockchip/clk.c @@ -22,7 +22,9 @@ #include #include #include + #include "clk.h" +#include "clk-fractional-divider.h" /* * Register a clock branch. @@ -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 Thu Jul 15 12:07:51 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andy Shevchenko X-Patchwork-Id: 12379887 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,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 E1816C1B08C for ; Thu, 15 Jul 2021 12:08:23 +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 AACF0611C0 for ; Thu, 15 Jul 2021 12:08:23 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AACF0611C0 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-rockchip-bounces+linux-rockchip=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=xTGvNSrXu5EpS9uIh56/JaHTuNqrsk4pDoc53jvdGJQ=; b=QeiUYG5xq4nlJh Z/W/roK7UqvDGndru3UrLdAETYl/ww8MM3qBEkSaZd8QRqCvuh+Fd8DIDhhKvyAx4LYVKXZfdMspz OnAlDk+6krfMjXQfS7MkCOcHOmgKFCQQHQaog1fYZSY5x3/5Lkd17FdWVjeICAmxx8RAkXzpE4/2I hcqAra9eIYKS/iHWq39lqffAJWJDx17tVwaG7HuJV5yYX+BeE/rhy/jTmUcnmhrUbfY2XsAstxcaK /60P5RuEmCBQggAEiJvKNwN77rTotU5emzbfVEQBdP03CyAiTPeLQUl/d4zkGo7qr8CuT9+t+fXFQ ayqtlpjR2G8sH7pwEXJw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m40A7-000hIY-GS; Thu, 15 Jul 2021 12:08:19 +0000 Received: from mga04.intel.com ([192.55.52.120]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1m409a-000h9c-4m; Thu, 15 Jul 2021 12:07:47 +0000 X-IronPort-AV: E=McAfee;i="6200,9189,10045"; a="208715427" X-IronPort-AV: E=Sophos;i="5.84,240,1620716400"; d="scan'208";a="208715427" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by fmsmga104.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Jul 2021 05:07:41 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.84,240,1620716400"; d="scan'208";a="460371672" Received: from black.fi.intel.com ([10.237.72.28]) by orsmga008.jf.intel.com with ESMTP; 15 Jul 2021 05:07:39 -0700 Received: by black.fi.intel.com (Postfix, from userid 1003) id 52CD32CB; Thu, 15 Jul 2021 15:08:06 +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 v1 2/3] clk: fractional-divider: Introduce NO_PRESCALER flag Date: Thu, 15 Jul 2021 15:07:51 +0300 Message-Id: <20210715120752.29174-2-andriy.shevchenko@linux.intel.com> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20210715120752.29174-1-andriy.shevchenko@linux.intel.com> References: <20210715120752.29174-1-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-20210715_050746_233020_E7CF93E7 X-CRM114-Status: GOOD ( 15.53 ) X-BeenThere: linux-rockchip@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Upstream kernel work for Rockchip platforms List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-rockchip" Errors-To: linux-rockchip-bounces+linux-rockchip=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 --- drivers/clk/clk-fractional-divider.c | 2 +- include/linux/clk-provider.h | 5 +++++ 2 files changed, 6 insertions(+), 1 deletion(-) diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c index 535d299af646..b2f9aae9f172 100644 --- a/drivers/clk/clk-fractional-divider.c +++ b/drivers/clk/clk-fractional-divider.c @@ -84,7 +84,7 @@ void clk_fractional_divider_general_approximation(struct clk_hw *hw, * by (scale - fd->nwidth) bits. */ scale = fls_long(*parent_rate / rate - 1); - if (scale > fd->nwidth) + if (scale > fd->nwidth && !(fd->flags & CLK_FRAC_DIVIDER_NO_PRESCALER)) rate <<= scale - fd->nwidth; rational_best_approximation(rate, *parent_rate, 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 Thu Jul 15 12:07:52 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andy Shevchenko X-Patchwork-Id: 12379883 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,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 0DD1DC07E96 for ; Thu, 15 Jul 2021 12:08:05 +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 A7CEE6136E for ; Thu, 15 Jul 2021 12:08:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A7CEE6136E 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-rockchip-bounces+linux-rockchip=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=XOsS2e7Z7PaCGz43wXFUgBOumQ/Yi1nALzZJsdKeMI8=; b=zC1i2NvSCoHL2a nx/fE2NBk1MidSYDai/FvsK0K/W5Xfts5qgwzr31mNNVgsdCNsgCCRBevAhSB5zg6IExap/h6j9MM z8B/qmEByZf3H/Wwqj2BJZW4E4rX4uFmi3Cs4S3OFnbd0z5k+gAGmWC+X+t9APDFTyewuXg6CZl1N DVxumcWzFk3nTOfN/hWX6gaLtAcA2aoRuSW7CBoLTdoagyz1aVKkq7/s7nic32B8joQzB0o5YSKJE QeIVLfWBfHbt+CB1NUkXpLmMUyBz0ZDN44fjd8HhHKD2ueStN5KEArBoorTY8zFOaM9jbU6YQHGm5 e9gakxAr6ieFmfjbZEBw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m409k-000hC4-L9; Thu, 15 Jul 2021 12:07:56 +0000 Received: from mga07.intel.com ([134.134.136.100]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1m409X-000h9f-1g; Thu, 15 Jul 2021 12:07:44 +0000 X-IronPort-AV: E=McAfee;i="6200,9189,10045"; a="274358683" X-IronPort-AV: E=Sophos;i="5.84,240,1620716400"; d="scan'208";a="274358683" Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Jul 2021 05:07:42 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.84,240,1620716400"; d="scan'208";a="430812515" Received: from black.fi.intel.com ([10.237.72.28]) by orsmga002.jf.intel.com with ESMTP; 15 Jul 2021 05:07:39 -0700 Received: by black.fi.intel.com (Postfix, from userid 1003) id 03182262; Thu, 15 Jul 2021 15:08:06 +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 v1 3/3] clk: fractional-divider: Document the arithmetics used behind the code Date: Thu, 15 Jul 2021 15:07:52 +0300 Message-Id: <20210715120752.29174-3-andriy.shevchenko@linux.intel.com> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20210715120752.29174-1-andriy.shevchenko@linux.intel.com> References: <20210715120752.29174-1-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-20210715_050743_224649_3F06DA07 X-CRM114-Status: GOOD ( 16.48 ) X-BeenThere: linux-rockchip@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Upstream kernel work for Rockchip platforms List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-rockchip" Errors-To: linux-rockchip-bounces+linux-rockchip=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 --- 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 b2f9aae9f172..29b32b51210d 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_lim, where N_lim = 2^nlim (2) + * + * for some cases the output may be saturated. Hence, from (1) and (2), + * assuming the worst case when m = 1, the inequality + * + * ln2(parent_rate / rate) <= nlim (3) + * + * may be derived. Thus, in cases when + * + * (parent_rate / rate) >> N_lim (4) + * + * we scale up the rate by 2^scale, where + * + * scale = ln2(parent_rate / rate) - nlim (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 @@ -82,6 +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. */ scale = fls_long(*parent_rate / rate - 1); if (scale > fd->nwidth && !(fd->flags & CLK_FRAC_DIVIDER_NO_PRESCALER))