On 09/04/2022 15:40, Martin Jücker wrote:
> Hello,
>
> I have some trouble with a pwm backlight in the p4note and I'm not sure
> what the cause is. The problem is with setting the backlight brightness
> to its maximum value, which will actually turn off the display. All the
> other values seem to be working just fine.
A bit similar issues was fixed by Mårten (5d82e661398e ("pwm:
pwm-samsung: Trigger manual update when disabling PWM")
). I assume you test on a tree having that commit.
>
> Looking at the documentation for the pwm driver in the exynos4412
> manual, I found that for pwm, the higher the tcmp value, the lower the
> pwm value is. Enabling debug logging, this seems to be exactly what's
> happening for values below 100% duty, but when the duty reaches 100%,
> the tcmp is zero and then gets decreased which causes it to wrap around.
> Looking at the rest of the code, this seems to be expected behaviour and
> I would assume that it should cause the pwm to never switch the value
> from high to low which would also fit.
I miss here some details.
The tcmp=-1 is written to registers for the next duty cycle. When
current duty cycle finishes, this new tcmp=-1 should be loaded into the PWM.
It seems that this loading does not happen in your case. Correct?
I tried this on Odroid HC1 and works correctly:
duty_ns=2000000, period_ns=2000000
tin_ns=240
tcmp=4294967295
tcnt=8332
What values do you have? How can I reproduce your case?
Best regards,
Krzysztof
