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

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Date: Wed, 24 Jan 2024 12:48:21 +0100
From: Stefan Wahren <wahrenst@....net>
To: Francesco Dolcini <francesco@...cini.it>, pratikmanvar09@...il.com,
 u.kleine-koenig@...gutronix.de
Cc: 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,
 xiaoning.wang@....com
Subject: Re: [PATCH v3] pwm: imx27: workaround of the pwm output bug

Hi,

Am 03.01.24 um 13:20 schrieb Francesco Dolcini:
> 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
thank you, this is very helpful. Unfortunately i don't have the
knowledge and resources to continue this work.

@Uwe It seems that you were able to reproduce this issue. Is it possible
to trigger this via sysfs and some kind of script?