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

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Date: Mon, 13 May 2024 12:39:42 +0000
From: Peng Fan <peng.fan@....com>
To: Rasmus Villemoes <rasmus.villemoes@...vas.dk>, "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>, Aisheng Dong
	<aisheng.dong@....com>
CC: "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
	"imx@...ts.linux.dev" <imx@...ts.linux.dev>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Abel Vesa <abel.vesa@...aro.org>, "S.J. Wang"
	<shengjiu.wang@....com>, Chancel Liu <chancel.liu@....com>
Subject: RE: [PATCH v2 05/17] clk: imx: pll14xx: Add constraint for fvco
 frequency

> 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?

VCO has a range, we need update reference manual.

Regards,
Peng.

 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