linux-kernel - RE: [PATCH] clk: imx: pll14xx: avoid glitch when set rate

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Date:   Thu, 22 Aug 2019 09:18:10 +0000
From:   Peng Fan <peng.fan@....com>
To:     Leonard Crestez <leonard.crestez@....com>,
        "sboyd@...nel.org" <sboyd@...nel.org>, Jacky Bai <ping.bai@....com>
CC:     "mturquette@...libre.com" <mturquette@...libre.com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "festevam@...il.com" <festevam@...il.com>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        dl-linux-imx <linux-imx@....com>,
        Anson Huang <anson.huang@....com>,
        Abel Vesa <abel.vesa@....com>,
        "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] clk: imx: pll14xx: avoid glitch when set rate

> Subject: Re: [PATCH] clk: imx: pll14xx: avoid glitch when set rate
> 
> On 20.08.2019 05:17, Peng Fan wrote:
> > According to PLL1443XA and PLL1416X spec, "When BYPASS is 0 and RESETB
> > is changed from 0 to 1, FOUT starts to output unstable clock until
> > lock time passes. PLL1416X/PLL1443XA may generate a glitch at FOUT."
> >
> > So set BYPASS when RESETB is changed from 0 to 1 to avoid glitch.
> > In the end of set rate, BYPASS will be cleared.
> >
> > @@ -191,6 +191,10 @@ static int clk_pll1416x_set_rate(struct clk_hw *hw,
> unsigned long drate,
> >   	tmp &= ~RST_MASK;
> >   	writel_relaxed(tmp, pll->base);
> >
> > +	/* Enable BYPASS */
> > +	tmp |= BYPASS_MASK;
> > +	writel(tmp, pll->base);
> > +
> 
> Shouldn't BYPASS be set before reset?

No. the glitch happens when RESET changes from 0 to 1, not from 1 to 0.

> 
> Also, isn't a similar bypass/unbypass dance also needed in
> clk_pll14xx_prepare? As far as I understand that could also output glitches
> until the PLL is locked. It could be a separate patch.

Yes, that might also output glitch. Fix in v2.

> 
> It's strange that this BYPASS bit is also handled by muxes like
> audio_pll1_bypass in clk-imx8mm.c but that's a separate issue not strictly
> related to the glitches you're trying to fix here.

Yes, need use EXT_BYPASS for the mux usage.

Regards,
Peng.

> 
> --
> Regards,
> Leonard