Re: (EXT) Re: [PATCH 1/4] pwm: pca9685: remove unused duty_cycle struct element

Sven Van Asbroeck
2020-04-04T20:17:14Z

On Sat, Apr 4, 2020 at 1:35 PM Clemens Gruber
<clemens.gruber@pqgruber.com> wrote:
>
> As the user is setting the duty cycle in nanoseconds, it makes sense
> that the relative duty cycle decreases in an absolute period increase.
> As for the behavior that the other channels remain at the same relative
> duty cycle: Not sure how we can avoid this, other than reprogramming all
> 15 other channels if one of them is changed and that's not really
> acceptable, I think.

Thank you for the explanation, Clemens.

Yes, it does make sense that the relative duty cycle changes when we change
the period. A relative duty cycle of duty_cycle / period is what the user would
expect to see.

It also kind-of makes sense that the relative duty cycles of the other
pwm channels
do not change: after all, the user is not touching them, so would not expect
them to change.

However, the following does not make sense to me. Imagine pwm0 and pwm1
are both active and at 50%: period=5000000, duty_cycle=2500000. Then, change
the period on pwm0:

$ echo 10000000 > pwm0/period

Then pwm0 gets dimmer (makes sense) and pwm1 keeps the same relative duty
cycle (makes sense). However, if we now read out sysfs for pwm1, we get:

$ echo pwm1/period
5000000 (wrong!)
$ echo pwm1/duty_cycle
2500000 (wrong! although relative duty cycle is correct)

Although the pwm1 period has changed, the API calls do not reflect this.
This makes it next to impossible for users to know what the current period
is set to.

Moving to the atomic API won't help, because .get_state is called only
once, when the chip is registered.

It does look like we have a square peg (this chip) in a round hole (the
standard assumptions the pwm core makes) ?