[PATCH 5.19 653/717] clk: zynqmp: pll: rectify rate rounding in zynqmp_pll_round_rate

Greg Kroah-Hartman
2022-10-22T08:45:26Z

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

[ Upstream commit 30eaf02149ecc3c5815e45d27187bf09e925071d ]

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>
Link: https://lore.kernel.org/r/20220826142030.213805-1-quanyang.wang@windriver.com
Reviewed-by: Shubhrajyoti Datta <shubhrajyoti.datta@amd.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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;
}

/**
--
2.35.1