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Subject: Re: [PATCH] clk: zynqmp: pll: rectify rate rounding in zynqmp_pll_round_rate
From:   Stephen Boyd <sboyd@kernel.org>
Cc:     Michael Tretter <m.tretter@pengutronix.de>,
        Quanyang Wang <quanyang.wang@windriver.com>,
        linux-clk@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
To:     Michael Turquette <mturquette@baylibre.com>,
        Michal Simek <michal.simek@xilinx.com>,
        quanyang.wang@windriver.com
Date:   Tue, 30 Aug 2022 15:06:25 -0700
User-Agent: alot/0.10
Message-Id: <20220830220627.07440C433B5@smtp.kernel.org>
Precedence: bulk

Quoting quanyang.wang@windriver.com (2022-08-26 07:20:30)
> From: Quanyang Wang <quanyang.wang@windriver.com>
>=20
> 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).
>=20
> There are two problems with the current code of zynqmp_pll_round_rate:
>=20
> 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 word=
s,
> 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.
>=20
> 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 =3D *prate * DIV_ROUND_CLOSEST(rate, *prate)).
> This value is also invalid since it's slightly smaller than PS_PLL_VCO_MI=
N.
> because DIV_ROUND_CLOSEST makes some loss in the fractional part.
>=20
> Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
> ---

Applied to clk-next