linux-kernel - Re: [PATCH 3/3] pwm: imx27: workaround of the pwm output bug when decrease the duty cycle

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Message-ID: <78119193-98b7-446f-82d6-37884a5b03ad@gmx.net>
Date: Fri, 12 Jul 2024 08:26:17 +0200
From: Stefan Wahren <wahrenst@....net>
To: Frank Li <Frank.Li@....com>, Uwe Kleine-König
 <ukleinek@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Shawn Guo <shawnguo@...nel.org>,
 Sascha Hauer <s.hauer@...gutronix.de>,
 Pengutronix Kernel Team <kernel@...gutronix.de>,
 Fabio Estevam <festevam@...il.com>, Philipp Zabel <p.zabel@...gutronix.de>
Cc: linux-pwm@...r.kernel.org, devicetree@...r.kernel.org,
 imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, Clark Wang <xiaoning.wang@....com>,
 Jun Li <jun.li@....com>, pratikmanvar09@...il.com
Subject: Re: [PATCH 3/3] pwm: imx27: workaround of the pwm output bug when
 decrease the duty cycle

Hi Frank,

Am 11.07.24 um 23:08 schrieb Frank Li:
> From: Clark Wang <xiaoning.wang@....com>
>
> When the SAR FIFO is empty, the write value is directly applied to SAR even
> though the current period is not over. If the new SAR value is less than
> the old one and the counter is greater than the new SAR value, the current
> period will not flip the level. This result in a pulse with a 100% duty
> cycle.
>
> Write the old SAR value before updating the new duty cycle to SAR. This
> avoids writing the new value into an empty FIFO.
>
> This only resolves the issue when the PWM period is longer than 2us
> (or <500KHz) because write register is not quick enough when PWM period is
> very short.
>
> Reviewed-by: Jun Li <jun.li@....com>
> Signed-off-by: Clark Wang <xiaoning.wang@....com>
> Signed-off-by: Frank Li <Frank.Li@....com>
the same patch has been submitted from other people in the past and they
received many review comments [1], [2].

Can you please explain which version of the patch this is and does it
address any review comments?

Best regards

[1] -
https://lore.kernel.org/linux-pwm/20211220073130.1429723-1-xiaoning.wang@nxp.com/
[2] -
https://lore.kernel.org/linux-pwm/20231229063013.1786-1-pratikmanvar09@gmail.com/