[PATCH v2 2/3] clk: nuvoton: ma35d1: fix PLL_CTL1_FRAC bit field width and fractional calc

[Joey Lu]

PLL_CTL1_FRAC was defined as GENMASK(31, 24), covering only 8 bits.
The hardware fractional field occupies bits [31:8] (24 bits), so the
mask must be GENMASK(31, 8).

The previous fractional-mode calculation used FIELD_MAX(PLL_CTL1_FRAC)
as the denominator to obtain 2 decimal places.  With the corrected 24-bit
mask the old divisor is wrong; replace the arithmetic with a proper
24-bit fixed-point rounding to 3 decimal places:

  n_frac = n * 1000 + (x * 1000 + 500) >> 24

The +500 term provides round-to-nearest before the right shift.

Fixes: 691521a367cf ("clk: nuvoton: Add clock driver for ma35d1 clock controller")
Signed-off-by: Joey Lu <a0987203069 at gmail.com>
---
 drivers/clk/nuvoton/clk-ma35d1-pll.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/clk/nuvoton/clk-ma35d1-pll.c b/drivers/clk/nuvoton/clk-ma35d1-pll.c
index bfedd45bd04b..7e6b30d20c01 100644
--- a/drivers/clk/nuvoton/clk-ma35d1-pll.c
+++ b/drivers/clk/nuvoton/clk-ma35d1-pll.c
@@ -48,7 +48,7 @@
 #define PLL_CTL1_PD		BIT(0)
 #define PLL_CTL1_BP		BIT(1)
 #define PLL_CTL1_OUTDIV		GENMASK(6, 4)
-#define PLL_CTL1_FRAC		GENMASK(31, 24)
+#define PLL_CTL1_FRAC		GENMASK(31, 8)
 #define PLL_CTL2_SLOPE		GENMASK(23, 0)
 
 #define INDIV_MIN		1
@@ -113,9 +113,9 @@ static unsigned long ma35d1_calc_pll_freq(u8 mode, u32 *reg_ctl, unsigned long p
 		pll_freq = div_u64(pll_freq, m * p);
 	} else {
 		x = FIELD_GET(PLL_CTL1_FRAC, reg_ctl[1]);
-		/* 2 decimal places floating to integer (ex. 1.23 to 123) */
-		n = n * 100 + ((x * 100) / FIELD_MAX(PLL_CTL1_FRAC));
-		pll_freq = div_u64(parent_rate * n, 100 * m * p);
+		/* x is 24-bit fractional part, convert to 3 decimal digits */
+		n = n * 1000 + (u32)(((u64)x * 1000 + 500) >> 24);
+		pll_freq = div_u64((u64)parent_rate * n, 1000 * m * p);
 	}
 	return pll_freq;
 }
-- 
2.43.0