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

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Message-ID: <VI1PR04MB70234B225BC4AC8E909A4C2AEEA50@VI1PR04MB7023.eurprd04.prod.outlook.com>
Date:   Thu, 22 Aug 2019 12:34:16 +0000
From:   Leonard Crestez <leonard.crestez@....com>
To:     Peng Fan <peng.fan@....com>, "sboyd@...nel.org" <sboyd@...nel.org>
CC:     Jacky Bai <ping.bai@....com>,
        "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 22.08.2019 12:18, Peng Fan wrote:
>> 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.

You're right, sorry.

>> 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.

Might make sense to post as a series.