LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Heiko Stuebner <heiko@sntech.de>,
	Elaine Zhang <zhangqing@rock-chips.com>,
	Stephen Boyd <sboyd@kernel.org>,
	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" <rjw@rjwysocki.net>,
	Len Brown <lenb@kernel.org>,
	Michael Turquette <mturquette@baylibre.com>,
	Abel Vesa <abel.vesa@nxp.com>, Shawn Guo <shawnguo@kernel.org>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	Pengutronix Kernel Team <kernel@pengutronix.de>,
	Fabio Estevam <festevam@gmail.com>,
	Lee Jones <lee.jones@linaro.org>, Liu Ying <victor.liu@nxp.com>
Subject: [PATCH v4 4/4] clk: fractional-divider: Document the arithmetics used behind the code
Date: Thu, 12 Aug 2021 20:00:25 +0300	[thread overview]
Message-ID: <20210812170025.67074-4-andriy.shevchenko@linux.intel.com> (raw)
In-Reply-To: <20210812170025.67074-1-andriy.shevchenko@linux.intel.com>

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 <victor.liu@nxp.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
v4: rebased on top of latest CLK codebase

 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 6a3ed82fdae9..b4f365d97bd0 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 <linux/clk-provider.h>
@@ -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);
-- 
2.30.2


  parent reply	other threads:[~2021-08-12 17:00 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-12 17:00 [PATCH v4 1/4] clk: fractional-divider: Export approximation algorithm to the CCF users Andy Shevchenko
2021-08-12 17:00 ` [PATCH v4 2/4] clk: fractional-divider: Hide clk_fractional_divider_ops from wide audience Andy Shevchenko
2021-08-12 17:00 ` [PATCH v4 3/4] clk: fractional-divider: Introduce POWER_OF_TWO_PS flag Andy Shevchenko
2021-08-12 17:00 ` Andy Shevchenko [this message]
     [not found] ` <162879819529.19113.6409882476721828944@swboyd.mtv.corp.google.com>
2021-08-13  9:43   ` [PATCH v4 1/4] clk: fractional-divider: Export approximation algorithm to the CCF users Andy Shevchenko
2021-08-17 12:45     ` Andy Shevchenko
2021-09-07 15:44 ` Chris Morgan
2021-09-07 17:54   ` Andy Shevchenko
2021-09-07 18:06     ` Andy Shevchenko
     [not found]       ` <SN6PR06MB534203466EA3D4E22858479FA5D49@SN6PR06MB5342.namprd06.prod.outlook.com>
2021-09-08 10:52         ` Andy Shevchenko

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210812170025.67074-4-andriy.shevchenko@linux.intel.com \
    --to=andriy.shevchenko@linux.intel.com \
    --cc=abel.vesa@nxp.com \
    --cc=festevam@gmail.com \
    --cc=heiko@sntech.de \
    --cc=kernel@pengutronix.de \
    --cc=lee.jones@linaro.org \
    --cc=lenb@kernel.org \
    --cc=linux-acpi@vger.kernel.org \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-clk@vger.kernel.org \
    --cc=linux-imx@nxp.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-rockchip@lists.infradead.org \
    --cc=mturquette@baylibre.com \
    --cc=rafael.j.wysocki@intel.com \
    --cc=rjw@rjwysocki.net \
    --cc=s.hauer@pengutronix.de \
    --cc=sboyd@kernel.org \
    --cc=shawnguo@kernel.org \
    --cc=victor.liu@nxp.com \
    --cc=zhangqing@rock-chips.com \
    --subject='Re: [PATCH v4 4/4] clk: fractional-divider: Document the arithmetics used behind the code' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).