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

Uwe Kleine-König
2025-02-27T18:09:44Z

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Hello,

On Thu, Feb 27, 2025 at 07:05:14PM +0200, Abel Vesa wrote:
> On 25-02-27 18:32:41, Abel Vesa wrote:
> > On 25-02-27 17:44:35, Abel Vesa wrote:
> > > 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 ?
> >
> > OK, so the second pwm_apply() is due to CONFIG_PWM_DEBUG.

ack. This is done just for the tests implemented in CONFIG_PWM_DEBUG, as
are the two pwm_get()s.

> > But still, the first pwm_apply() requests duty cycle of 5MHz:

5 ms, yes. But it cannot give you 5 ms and so you get 4.266 ns.

> > systemd-backlig-724 [006] ..... 9.079538: pwm_apply: pwmchip0.0: period=5000000 duty_cycle=5000000 polarity=0 enabled=1 err=0
> >
> > So since the period is 4.26MHz, due to the knobs selected by the
> > provider, this duty cycle will result in a PWM value that is above the
> > selected resolution, as I already mentioned.

"above the selected resolution"? Do you mean you don't get the exact
value that you requested?

> On top of that, the duty cycle in debugfs is also reported as 5000000ns
> when in fact it is 4266666ns, as the trace shows.

Yes. Consider that a relict from the times when there was no
pwm_get_state_hw(). Both values are interesting in different situations.
So just telling the real parameters isn't the optimal way forward
either.

Something like the patch I showed in
https://lore.kernel.org/all/7bcnckef23w6g47ll5l3bktygedrcfvr7fk3qjuq2swtoffhec@zs4w4tuh6qvm/
would make you a bit luckier I guess. Feel free to polish that one a bit
(e.g. by checking the return value of pwm_get_state_hw() and acting
sensible in reply to it) and send a proper patch. (A Suggested-by for me
is enough for such a patch, grab authorship yourself.)

> > > 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.

No it will not. The .duty_cycle value (also 5000000 ns) will reach the
lowlevel PWM driver together with .period = 5000000 ns. Both are rounded
down to 4266666ns. I see no overflow.

Best regards
Uwe

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