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

Abel Vesa
2025-02-27T15:44:39Z

On 25-02-27 16:25:06, Uwe Kleine-König wrote:
> 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?

What I do not get is why do we expect 2 pwm_get() and 2 pwm_apply()
calls each time ?

Need to dig a bit further.

But meanwhile, if the first pwm_apply() call goes all the way to the
provider, then the duty cycle value, when translated to the actual PWM
value that gets written to reg, will overflow. So this is what is wrong.
And this is what actually happens.

>
> Best regards
> Uwe