ARM: imx: correct Audio/Video PLL rate calculation formula

The audio/video PLL's rate calculation formula is as below in RM:

Fref * (DIV_SELECT + NUM / DENOM),

in original clk-pllv3's code, below code is used:

(parent_rate * div) + ((parent_rate / mfd) * mfn)

as it does NOT consider the float data using div, so below calculation formula should be used as a decent method:

(parent_rate * div) + ((parent_rate * mfn) / mfd)

and we also need to consider parent_rate * mfd may exceed a 32 bit value, 64 bit value should be used.

Below is one example of old/new formula's difference:

On i.MX7D, DRAM PLL is a Audio/Video type PLL, the target freq is 1066MHz, the register settings are as below:

PLL_DDRn: 8060202C -> div = 0x2C
DDR_NUM: 06AAAC4D -> mfn = 0x6AAAC4D
DDR_DENOM: 100003EC -> mfd = 0x100003EC

parent_rate = 24MHz.

with old formula, the (parent_rate / mfd) * mfn = 0, with new formula, the (parent_rate * mfn) / mfd = 10MHz, so old formula gets PLL_DDR = 1056MHz, while the updated formuls gets PLL_DDR = 1066MHz which exactly matches the real rate.

---
changes since V2:

	improve commit message;
	replace s64 with u64 to match do_div.
 drivers/clk/imx/clk-pllv3.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

index 641ebc5..e34a925 100644
@@ -203,8 +203,13 @@ static unsigned long clk_pllv3_av_recalc_rate(struct clk_hw *hw,
 	u32 mfn = readl_relaxed(pll->base + PLL_NUM_OFFSET);
 	u32 mfd = readl_relaxed(pll->base + PLL_DENOM_OFFSET);
 	u32 div = readl_relaxed(pll->base) & pll->div_mask;
+	u64 temp64 = parent_rate;
+
+	temp64 *= mfn;
+	do_div(temp64, mfd);
+
+	return (parent_rate * div) + temp64;
 
-	return (parent_rate * div) + ((parent_rate / mfd) * mfn);
 }
 
 static long clk_pllv3_av_round_rate(struct clk_hw *hw, unsigned long rate,

1.9.1

Hello,

apart from a few minor issues regarding the wording of the commit log I'm happy now. See below.

s/float data using div/non-integer part of the division/

"decent" sounds strange to my (German) ears. Maybe better write:

.. should be better used instead:

s/exceed/overflow/

s/formuls/formula/

Best regards
Uwe

Pengutronix e.K.                           | Uwe Kleine-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |