Re: [RFC PATCH 0/3] pwm: add support for duty_offset

[Trevor Gamblin]

On 2024-04-05 08:23, Uwe Kleine-König wrote:
> Hello Trevor,
>
> In general I really like your effort to generalize the pwm framework.
> Thanks a lot!
Thanks! I'm glad that it (mostly) fits.
> On Thu, Apr 04, 2024 at 08:30:22PM -0400, Trevor Gamblin wrote:
> > This series extends the PWM subsystem to support the duty_offset feature
> > found on some PWM devices. It includes a patch to enable this feature
> > for the axi-pwmgen driver, which can also serve as an example of how to
> > implement it for other devices. It also contains a patch adding a new
> > pwm_config_full() function mirroring the behavior of pwm_config() but
> Please don't. pwm_config() is a function I want to get rid of in the
> long term. Consumers that want to make use of it should use
> pwm_apply_*().
Okay, I'll drop this patch.
> > with duty_offset included, to help maintain compatibility for drivers
> > that don't support the feature.
> >
> > The series was tested on actual hardware using a Zedboard. An
> > oscilloscope was used to validate that the generated PWM signals matched
> > the requested ones. The libpwm [1] tool was also used for testing the
> > char device functionality.
> >
> > The series is marked RFC as there are some outstanding questions about
> > implementation:
> >
> > 1. In drivers/pwm/core.c, __pwm_apply() was modified to check that the
> > sum of state->duty_offset + state->duty_cycle does not exceed
> > state->period, but in the character device section these values are
> > being checked separately. Is this intentional? What is the intended
> > behavior?
> state->duty_offset + state->duty_cycle doesn't necessarily need to be
> less or equal to state->period. Consider this waveform, where ^ marks
> the start of a period.
>
>
>          ___       _________       __...
>             \_____/         \_____/
>          ^               ^
>
> This one has duty_offset = 9 and duty_cycle = 10 while
> period = 16 < 10 + 9.
Alright, I'll make a change here.
> > 2. Should __pwm_apply() explicitly disallow PWM_POLARITY_INVERSED and
> > duty_offset together?
> While there is no technical need for that, a configuration with both
> PWM_POLARITY_INVERSED and duty_offset > 0 is irritating. So I'd say yes,
> it should be disallowed. When I created the cdev API I even considered
> dropping .polarity for lowlevel drivers and convert them all to
> .duty_offset.
Will do.
> Having said that I don't like the addition of .supports_offset to
> struct pwm_chip, which only signals a new incomplete evolution of the
> pwm framework. Better adapt all drivers and then assume all of them
> support it.
Can you clarify what you mean here - is the intent to put basic handling 
of duty_offset (even if that means simply setting it to 0) in each driver?
> > 3. Are there other places that would need duty_offset handling which
> > have been missed?
> I'm happy you adapted the tracing stuff. I didn't look closely, but I
> don't think something important was missed.
>
> > Note that in addition to the other patches in this series, the changes
> > to the axi-pwmgen driver rely on [2] and [3], which haven't been picked
> > up yet.
> Best regards
> Uwe