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

[Stephen Boyd]

Quoting quanyang.wang@xxxxxxxxxxxxx (2022-08-26 07:20:30)
> From: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
> 
> 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@xxxxxxxxxxxxx>
> ---

Applied to clk-next