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

Uwe Kleine-König
2025-02-27T15:25:10Z

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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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