[PATCH v4 4/4] clk: fractional-divider: Document the arithmetics used behind the code

From: Andy Shevchenko
Date: Thu Aug 12 2021 - 13:00:53 EST

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@xxxxxxx>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
---
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