[RFT PATCH] clk: ls1c: Fix PLL rate calculation

[Sean Anderson <seanga2@xxxxxxxxx>]

While reviewing Dhu's patch adding ls1c300 clock support to U-Boot [1], I
noticed the following calculation, which is copied from
drivers/clk/loongson1/clk-loongson1c.c:

ulong ls1c300_pll_get_rate(struct clk *clk)
{
	unsigned int mult;
	long long parent_rate;
	void *base;
	unsigned int val;

	parent_rate = clk_get_parent_rate(clk);
	base = (void *)clk->data;

	val = readl(base + START_FREQ);
	mult = FIELD_GET(FRAC_N, val) + FIELD_GET(M_PLL, val);
	return (mult * parent_rate) / 4;
}

I would like to examine the use of M_PLL and FRAC_N to calculate the multiplier
for the PLL. The datasheet has the following to say:

START_FREQ 位    缺省值      描述
========== ===== =========== ====================================
FRAC_N     23:16 0           PLL 倍频系数的小数部分

                 由          PLL 倍频系数的整数部分
M_PLL      15:8  NAND_D[3:0] (理论可以达到 255，建议不要超过 100)
                 配置

which according to google translate means

START_FREQ Bits  Default       Description
========== ===== ============= ================================================
FRAC_N     23:16 0             Fractional part of the PLL multiplication factor

                 Depends on    Integer part of PLL multiplication factor
M_PLL      15:8  NAND_D[3:0]   (Theoretically it can reach 255, [but] it is
                 configuration  recommended not to exceed 100)

So just based on this description, I would expect that the formula to be
something like

	rate = parent * (255 * M_PLL + FRAC_N) / 255 / 4

However, the datasheet also gives the following formula:

	rate = parent * (M_PLL + FRAC_N) / 4

which is what the Linux driver has implemented. I find this very unusual.
First, the datasheet specifically says that these fields are the integer and
fractional parts of the multiplier. Second, I think such a construct does not
easily map to traditional PLL building blocks. Implementing this formula in
hardware would likely require an adder, just to then set the threshold of a
clock divider.

I think it is much more likely that the first formula is correct. The author of
the datasheet may think of a multiplier of (say) 3.14 as

	M_PLL = 3
	FRAC_N = 0.14

which together sum to the correct multiplier, even though the actual value
stored in FRAC_N would be 36.

I suspect that this has slipped by unnoticed because when FRAC_N is 0, there is
no difference in the formulae. The following patch is untested, but I suspect
it will fix this issue. I would appreciate if anyone with access to the
hardware could measure the output of the PLL (or one of its derived clocks) and
determine the correct formula.

[1] https://lore.kernel.org/u-boot/20220418204519.19991-1-dhu@xxxxxxxxxxxxxx/T/#u

Fixes: b4626a7f4892 ("CLK: Add Loongson1C clock support")
Signed-off-by: Sean Anderson <seanga2@xxxxxxxxx>
---

 drivers/clk/loongson1/clk-loongson1c.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/clk/loongson1/clk-loongson1c.c b/drivers/clk/loongson1/clk-loongson1c.c
index 703f87622cf5..2b98a116c1ea 100644
--- a/drivers/clk/loongson1/clk-loongson1c.c
+++ b/drivers/clk/loongson1/clk-loongson1c.c
@@ -21,9 +21,9 @@ static unsigned long ls1x_pll_recalc_rate(struct clk_hw *hw,
 	u32 pll, rate;
 
 	pll = __raw_readl(LS1X_CLK_PLL_FREQ);
-	rate = ((pll >> 8) & 0xff) + ((pll >> 16) & 0xff);
+	rate = (pll & 0xff00) + ((pll >> 16) & 0xff);
 	rate *= OSC;
-	rate >>= 2;
+	rate >>= 10;
 
 	return rate;
 }
-- 
2.35.1