linux-kernel - Re: [PATCH v3] pwm: imx27: workaround of the pwm output bug

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Message-ID: <20240103122005.GA3347@francesco-nb>
Date: Wed, 3 Jan 2024 13:20:05 +0100
From: Francesco Dolcini <francesco@...cini.it>
To: pratikmanvar09@...il.com
Cc: francesco@...cini.it, festevam@...il.com, jun.li@....com,
	kernel@...gutronix.de, linux-arm-kernel@...ts.infradead.org,
	linux-imx@....com, linux-kernel@...r.kernel.org,
	linux-pwm@...r.kernel.org, oe-kbuild-all@...ts.linux.dev,
	pratik.manvar@....com, s.hauer@...gutronix.de, shawnguo@...nel.org,
	thierry.reding@...il.com, u.kleine-koenig@...gutronix.de,
	xiaoning.wang@....com
Subject: Re: [PATCH v3] pwm: imx27: workaround of the pwm output bug

Hello Pratik,

On Wed, Jan 03, 2024 at 04:32:00PM +0530, pratikmanvar09@...il.com wrote:
> From: Clark Wang <xiaoning.wang@....com>
> 
> This fixes the pwm output bug when decrease the duty cycle.
> This is a limited workaround for the PWM IP issue TKT0577206.
> 
> Root cause:
> When the SAR FIFO is empty, the new write value will be directly applied
> to SAR even 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 filp the
> level. This will result in a pulse with a duty cycle of 100%.
> 
> Workaround:
> Add an old value SAR write before updating the new duty cycle to SAR.
> This will keep the new value is always in a not empty fifo, and can be
> wait to update after a period finished.
> 
> Limitation:
> This workaround can only solve this issue when the PWM period is longer
> than 2us(or <500KHz).
> 
> Reviewed-by: Jun Li <jun.li@....com>
> Signed-off-by: Clark Wang <xiaoning.wang@....com>
> Link: https://github.com/nxp-imx/linux-imx/commit/16181cc4eee61d87cbaba0e5a479990507816317
> Tested-by: Pratik Manvar <pratik.manvar@....com>
> Signed-off-by: Pratik Manvar <pratik.manvar@....com>

A very similar patch was already send in 2021 [1], did it had review
comments not addressed? Please have a look.

In general please refrain from sending a new patch version every other
day, while every Linux kernel subsystem has different rules and a
difference pace of development, in this specific case sending a v3 just
adding your signed-off-by without allowing a little bit of time to wait
for more feedback is just not sane.

[1] https://lore.kernel.org/all/?q=dfn%3Adrivers%2Fpwm%2Fpwm-imx27.c+AND+b%3A%22Clark+Wang%22

Francesco