linux-kernel - Re: [PATCH] clk: imx: composite-8m: avoid glitches when set

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Message-ID: <20230801194516.h6gxj2lqmmmls62k@pengutronix.de>
Date:   Tue, 1 Aug 2023 21:45:16 +0200
From:   Marco Felsch <m.felsch@...gutronix.de>
To:     Ahmad Fatoum <a.fatoum@...gutronix.de>
Cc:     Abel Vesa <abelvesa@...nel.org>, Peng Fan <peng.fan@....com>,
        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>,
        NXP Linux Team <linux-imx@....com>,
        linux-clk@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clk: imx: composite-8m: avoid glitches when set_rate
 would be a no-op

On 23-08-01, Ahmad Fatoum wrote:
> Reconfiguring the clock divider to the exact same value is observed
> on an i.MX8MN to often cause a short clock pause, probably because
> the divider restarts counting from the time the register is written.
> 
> This issue doesn't show up normally, because the clock framework will
> take care to not call set_rate when the clock rate is the same.
> However, when we configure an upstream clock (e.g. an audio_pll), the
> common code will call set_rate with the newly calculated rate on all
> children. As the new rate is different, we enter set_rate and compute
> the same divider values, write them back and cause the glitch (e.g.
> on a SAI's MCLK).
> 
> To avoid the glitch, we skip writing the same value back again.
> 
> Fixes: d3ff9728134e ("clk: imx: Add imx composite clock")
> Signed-off-by: Ahmad Fatoum <a.fatoum@...gutronix.de>

Reviewed-by: Marco Felsch <m.felsch@...gutronix.de>