[PATCH] clk: zynqmp: pll: rectify rate rounding in zynqmp_pll_round_rate

* [PATCH] clk: zynqmp: pll: rectify rate rounding in zynqmp_pll_round_rate
@ 2022-08-26 14:20 quanyang.wang
  2022-08-30  6:18 ` Datta, Shubhrajyoti
  2022-08-30 22:06 ` Stephen Boyd
  0 siblings, 2 replies; 3+ messages in thread
From: quanyang.wang @ 2022-08-26 14:20 UTC (permalink / raw)
  To: Michael Turquette, Stephen Boyd, Michal Simek
  Cc: Michael Tretter, Quanyang Wang, linux-clk, linux-arm-kernel,
	linux-kernel

From: Quanyang Wang <quanyang.wang@windriver.com>

The function zynqmp_pll_round_rate is used to find a most appropriate
PLL frequency which the hardware can generate according to the desired
frequency. For example, if the desired frequency is 297MHz, considering
the limited range from PS_PLL_VCO_MIN (1.5GHz) to PS_PLL_VCO_MAX (3.0GHz)
of PLL, zynqmp_pll_round_rate should return 1.872GHz (297MHz * 5).

There are two problems with the current code of zynqmp_pll_round_rate:

1) When the rate is below PS_PLL_VCO_MIN, it can't find a correct rate
when the parameter "rate" is an integer multiple of *prate, in other words,
if "f" is zero, zynqmp_pll_round_rate won't return a valid frequency which
is from PS_PLL_VCO_MIN to PS_PLL_VCO_MAX. For example, *prate is 33MHz
and the rate is 660MHz, zynqmp_pll_round_rate will not boost up rate and
just return 660MHz, and this will cause clk_calc_new_rates failure since
zynqmp_pll_round_rate returns an invalid rate out of its boundaries.

2) Even if the rate is higher than PS_PLL_VCO_MIN, there is still a risk
that zynqmp_pll_round_rate returns an invalid rate because the function
DIV_ROUND_CLOSEST makes some loss in the fractional part. If the parent
clock *prate is 33333333Hz and we want to set the PLL rate to 1.5GHz,
this function will return 1499999985Hz by using the formula below:
    value = *prate * DIV_ROUND_CLOSEST(rate, *prate)).
This value is also invalid since it's slightly smaller than PS_PLL_VCO_MIN.
because DIV_ROUND_CLOSEST makes some loss in the fractional part.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
---
 drivers/clk/zynqmp/pll.c | 31 +++++++++++++++----------------
 1 file changed, 15 insertions(+), 16 deletions(-)

diff --git a/drivers/clk/zynqmp/pll.c b/drivers/clk/zynqmp/pll.c
index 91a6b4cc910e..0d3e1377b092 100644
--- a/drivers/clk/zynqmp/pll.c
+++ b/drivers/clk/zynqmp/pll.c
@@ -102,26 +102,25 @@ static long zynqmp_pll_round_rate(struct clk_hw *hw, unsigned long rate,
 				  unsigned long *prate)
 {
 	u32 fbdiv;
-	long rate_div, f;
+	u32 mult, div;
 
-	/* Enable the fractional mode if needed */
-	rate_div = (rate * FRAC_DIV) / *prate;
-	f = rate_div % FRAC_DIV;
-	if (f) {
-		if (rate > PS_PLL_VCO_MAX) {
-			fbdiv = rate / PS_PLL_VCO_MAX;
-			rate = rate / (fbdiv + 1);
-		}
-		if (rate < PS_PLL_VCO_MIN) {
-			fbdiv = DIV_ROUND_UP(PS_PLL_VCO_MIN, rate);
-			rate = rate * fbdiv;
-		}
-		return rate;
+	/* Let rate fall inside the range PS_PLL_VCO_MIN ~ PS_PLL_VCO_MAX */
+	if (rate > PS_PLL_VCO_MAX) {
+		div = DIV_ROUND_UP(rate, PS_PLL_VCO_MAX);
+		rate = rate / div;
+	}
+	if (rate < PS_PLL_VCO_MIN) {
+		mult = DIV_ROUND_UP(PS_PLL_VCO_MIN, rate);
+		rate = rate * mult;
 	}
 
 	fbdiv = DIV_ROUND_CLOSEST(rate, *prate);
-	fbdiv = clamp_t(u32, fbdiv, PLL_FBDIV_MIN, PLL_FBDIV_MAX);
-	return *prate * fbdiv;
+	if (fbdiv < PLL_FBDIV_MIN || fbdiv > PLL_FBDIV_MAX) {
+		fbdiv = clamp_t(u32, fbdiv, PLL_FBDIV_MIN, PLL_FBDIV_MAX);
+		rate = *prate * fbdiv;
+	}
+
+	return rate;
 }
 
 /**
-- 
2.36.1


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