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))