linux-kernel - Re: [PATCH v2 05/17] clk: imx: pll14xx: Add constraint for fvco frequency

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Date: Mon, 13 May 2024 14:13:45 +0200
From: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To: "Peng Fan (OSS)" <peng.fan@....nxp.com>, Abel Vesa <abelvesa@...nel.org>,
 Michael Turquette <mturquette@...libre.com>, Stephen Boyd
 <sboyd@...nel.org>, Shawn Guo <shawnguo@...nel.org>,
 Sascha Hauer <s.hauer@...gutronix.de>,
 Pengutronix Kernel Team <kernel@...gutronix.de>,
 Fabio Estevam <festevam@...il.com>, Jacky Bai <ping.bai@....com>,
 Ye Li <ye.li@....com>, Dong Aisheng <aisheng.dong@....com>
Cc: linux-clk@...r.kernel.org, imx@...ts.linux.dev,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Abel Vesa <abel.vesa@...aro.org>, Peng Fan <peng.fan@....com>,
 Shengjiu Wang <shengjiu.wang@....com>, Chancel Liu <chancel.liu@....com>
Subject: Re: [PATCH v2 05/17] clk: imx: pll14xx: Add constraint for fvco
 frequency

On 10/05/2024 11.19, Peng Fan (OSS) wrote:
> From: Shengjiu Wang <shengjiu.wang@....com>
> 
> The fvco frequency range is between 1600MHz and 3200MHz, without
> this constraint the fvco may out of range, the real output
> frequency is no accurate.

Could you please point everybody in the direction of where that
requirement is stated? The imx8mp reference manual, for example, merely
lists constraints for p, m, s and k.


>  
>  	/* First try if we can get the desired rate from one of the static entries */
> @@ -193,6 +195,10 @@ static void imx_pll14xx_calc_settings(struct clk_pll14xx *pll, unsigned long rat
>  			kdiv = pll1443x_calc_kdiv(mdiv, pdiv, sdiv, rate, prate);
>  			fout = pll14xx_calc_rate(pll, mdiv, pdiv, sdiv, kdiv, prate);
>  
> +			fvco = fout << sdiv;
> +
> +			if (fvco < 1600000000 || fvco > 3200000000)
> +				continue;

If this is really a necessary constraint, it seems that one could just
up-front compute the only possible value of s, or at least change the
logic so that one loops over a smaller range of possible values of s.

Rasmus