linux-kernel - [PATCH v3 4/4] clk: fractional-divider: Document the arithmetics used behind the code

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Message-Id: <20210722195010.45940-4-andriy.shevchenko@linux.intel.com>
Date:   Thu, 22 Jul 2021 22:50:10 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Elaine Zhang <zhangqing@...k-chips.com>,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-imx@....com,
        linux-arm-kernel@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>, Abel Vesa <abel.vesa@....com>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        Lee Jones <lee.jones@...aro.org>, Liu Ying <victor.liu@....com>
Subject: [PATCH v3 4/4] clk: fractional-divider: Document the arithmetics used behind the code

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@....com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
---
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 <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