linux-kernel - Re: [PATCH] leds: rgb: leds-qcom-lpg: Fix pwm resolution for Hi-Res PWMs

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Message-ID: <j55de6bbipoavqx25w2s6qr7n6fv6w7bj3lrgyag4dlvvddbqv@shn22aqcqeci>
Date: Thu, 27 Feb 2025 16:25:06 +0100
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: Abel Vesa <abel.vesa@...aro.org>
Cc: Sebastian Reichel <sre@...nel.org>, Lee Jones <lee@...nel.org>, 
	Pavel Machek <pavel@...nel.org>, Anjelique Melendez <quic_amelende@...cinc.com>, 
	Kamal Wadhwa <quic_kamalw@...cinc.com>, Jishnu Prakash <jishnu.prakash@....qualcomm.com>, 
	Bjorn Andersson <andersson@...nel.org>, Konrad Dybcio <konradybcio@...nel.org>, 
	Johan Hovold <johan@...nel.org>, Pavel Machek <pavel@....cz>, linux-leds@...r.kernel.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] leds: rgb: leds-qcom-lpg: Fix pwm resolution for Hi-Res
 PWMs

Hello Abel,

On Thu, Feb 27, 2025 at 04:26:14PM +0200, Abel Vesa wrote:
> On 25-02-27 10:58:47, Uwe Kleine-König wrote:
> > Can you please enable CONFIG_PWM_DEBUG, enable pwm tracing (
> > 
> > 	echo 1 > /sys/kernel/debug/tracing/events/pwm/enable
> > 
> > ) then reproduce the problem and provide the output of
> > 
> > 	cat /sys/kernel/debug/tracing/trace
> > 
> > .
> 
> $ cat trace
> # tracer: nop
> #
> # entries-in-buffer/entries-written: 13/13   #P:12
> #
> #                                _-----=> irqs-off/BH-disabled
> #                               / _----=> need-resched
> #                              | / _---=> hardirq/softirq
> #                              || / _--=> preempt-depth
> #                              ||| / _-=> migrate-disable
> #                              |||| /     delay
> #           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
> #              | |         |   |||||     |         |
>         modprobe-203     [000] .....     0.938668: pwm_get: pwmchip0.0: period=1066407 duty_cycle=533334 polarity=0 enabled=1 err=0
>         modprobe-203     [000] .....     0.938775: pwm_apply: pwmchip0.0: period=5000000 duty_cycle=0 polarity=0 enabled=1 err=0
>         modprobe-203     [000] .....     0.938821: pwm_get: pwmchip0.0: period=4266537 duty_cycle=0 polarity=0 enabled=1 err=0
>         modprobe-203     [000] .....     0.938936: pwm_apply: pwmchip0.0: period=4266537 duty_cycle=0 polarity=0 enabled=1 err=0
>         modprobe-203     [000] .....     0.938982: pwm_get: pwmchip0.0: period=4266537 duty_cycle=0 polarity=0 enabled=1 err=0
>         modprobe-203     [000] .....     0.939274: pwm_apply: pwmchip0.0: period=5000000 duty_cycle=921458 polarity=0 enabled=1 err=0
>         modprobe-203     [000] .....     0.939320: pwm_get: pwmchip0.0: period=4266537 duty_cycle=921355 polarity=0 enabled=1 err=0
>         modprobe-203     [000] .....     0.939434: pwm_apply: pwmchip0.0: period=4266537 duty_cycle=921355 polarity=0 enabled=1 err=0
>         modprobe-203     [000] .....     0.939480: pwm_get: pwmchip0.0: period=4266537 duty_cycle=921355 polarity=0 enabled=1 err=0
>  systemd-backlig-724     [006] .....     9.079538: pwm_apply: pwmchip0.0: period=5000000 duty_cycle=5000000 polarity=0 enabled=1 err=0
>  systemd-backlig-724     [006] .....     9.079585: pwm_get: pwmchip0.0: period=4266537 duty_cycle=4266537 polarity=0 enabled=1 err=0
>  systemd-backlig-724     [006] .....     9.079698: pwm_apply: pwmchip0.0: period=4266537 duty_cycle=4266537 polarity=0 enabled=1 err=0
>  systemd-backlig-724     [006] .....     9.079750: pwm_get: pwmchip0.0: period=4266537 duty_cycle=4266537 polarity=0 enabled=1 err=0
> $
> 
> > 
> > I didn't take a deeper dive in this driver combination, but here is a
> > description about what *should* happen:
> > 
> > You're talking about period in MHz, the PWM abstraction uses
> > nanoseconds. So your summary translated to the PWM wording is (to the
> > best of my understanding):
> > 
> >   1. PWM backlight driver requests PWM with .period = 200 ns and
> >      .duty_cycle = 200 ns.
> > 
> >   2. leds-qcom-lpg cannot pick 200 ns exactly and then chooses .period =
> >      1000000000 / 4.26666 MHz = 234.375 ns
> >      
> >   3. leds-qcom-lpg then determines setting for requested .duty_cycle
> >      based on .period = 200 ns which then ends up with something bogus.

The trace looks better than what I expected. 2. is fine here because it
seems when Sebastian wrote "driver requests PWM with 5 MHz period" that
meant period = 5000000 ns. That was then rounded down to 4266537 ns. And
the request for period = 5000000 ns + duty_cycle = 5000000 ns was
serviced by configuring period = 4266537 ns + duty_cycle = 4266537 ns.
So that's a 100 % relative duty configuration as intended.

So just from the traces I don't spot a problem. Do these logs not match
what actually happens on the signal?

Best regards
Uwe

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