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clk: zynqmp: pll: rectify rate rounding in zynqmp_pll_round_rate

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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>
This commit is contained in:
Quanyang Wang 2022-08-26 22:20:30 +08:00 committed by Stephen Boyd
parent 8bdb15cd05
commit 30eaf02149
1 changed files with 15 additions and 16 deletions
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31
drivers/clk/zynqmp/pll.c
View File

@ -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;
}
/**