linux-kernel - Re: [PATCH] clk: zynqmp: pll: rectify rate rounding in zynqmp_pll_round

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Date:   Tue, 30 Aug 2022 15:06:25 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Michael Turquette <mturquette@...libre.com>,
        Michal Simek <michal.simek@...inx.com>,
        quanyang.wang@...driver.com
Cc:     Michael Tretter <m.tretter@...gutronix.de>,
        Quanyang Wang <quanyang.wang@...driver.com>,
        linux-clk@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clk: zynqmp: pll: rectify rate rounding in zynqmp_pll_round_rate

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

Applied to clk-next