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

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Date:   Thu, 22 Aug 2019 08:20:53 +0000
From:   Leonard Crestez <leonard.crestez@....com>
To:     Peng Fan <peng.fan@....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

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?

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.

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.

--
Regards,
Leonard