Hello Stefan,

On Wed, Jan 24, 2024 at 12:48:21PM +0100, Stefan Wahren wrote:

Am 03.01.24 um 13:20 schrieb Francesco Dolcini:

quoted

Hello Pratik,

On Wed, Jan 03, 2024 at 04:32:00PM +0530, pratikmanvar09@gmail.com wrote:

quoted

From: Clark Wang <xiaoning.wang@nxp.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 <redacted>
Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Link: https://github.com/nxp-imx/linux-imx/commit/16181cc4eee61d87cbaba0e5a479990507816317
Tested-by: Pratik Manvar <redacted>
Signed-off-by: Pratik Manvar <redacted>

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?

When I read through the linked thread I was surprised to read that I
could reproduce it. My best guess is that I did that using a long period
and an LED connected to the PWM's output to make the effect easily visible.
Something like: Configure for

	.duty_cycle = 1s, .period = 5s

and change that to

	.duty_cycle = 0s, .period = 5s

(e.g. using sysfs) when the output got high.

Best regards
Uwe

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Pengutronix e.K.                           | Uwe Kleine-König            |
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