[libgpiod] How can I use PWM

[libgpiod] How can I use PWM

[Robert Baumgartner]

Hi team,

I try to migrate some python scripts for my Raspberry from RPi.GPIO to
libgpiod, but I miss PWM features.
Do I miss something? Or is it not available?

Some details: https://www.electronicwings.com/raspberry-pi/raspberry-pi-pwm-generation-using-python-and-c

Any help would be appreciated.
Mit freundlichen Grüßen / Kind regards

Robert Baumgartner
Senior Solution Architect OpenShift
Red Hat Austria GmbH, Firmenbuch Nr.: FN 479668w, Handelsgericht Wien

Millenniumtower 26. Stock
Handelskai 94-96
A-1200 Wien

robert.baumgartner@redhat.com
M: +43-660-5913883

Re: [libgpiod] How can I use PWM

[Andy Shevchenko]

On Fri, Aug 5, 2022 at 1:17 PM Robert Baumgartner <rbaumgar@redhat.com> wrote:
>
> Hi team,
>
> I try to migrate some python scripts for my Raspberry from RPi.GPIO to
> libgpiod,

This is great and everyone should be encouraged to follow your example!

> but I miss PWM features.
> Do I miss something? Or is it not available?

No, you haven't missed anything except Unix ideology. That ideology is
telling us that one tool for one thing, and in very featurable mode.
That said, lingpiod is exclusively for GPIO ABI between kernel and
user space. For PWM you need to access the PWM ABI in a way how it's
represented by the Linux kernel. I believe there are plenty of
libraries more or less okayish for that purpose, but I never heard
about any official library and/or Python bindings for it.

Thierry, any suggestion here?

> Some details: https://www.electronicwings.com/raspberry-pi/raspberry-pi-pwm-generation-using-python-and-c
>
> Any help would be appreciated.


-- 
With Best Regards,
Andy Shevchenko

Re: [libgpiod] How can I use PWM

[Kent Gibson]

On Sat, Aug 06, 2022 at 12:12:31PM +0200, Andy Shevchenko wrote:
> On Fri, Aug 5, 2022 at 1:17 PM Robert Baumgartner <rbaumgar@redhat.com> wrote:
> >
> > Hi team,
> >
> > I try to migrate some python scripts for my Raspberry from RPi.GPIO to
> > libgpiod,
> 
> This is great and everyone should be encouraged to follow your example!
> 
> > but I miss PWM features.
> > Do I miss something? Or is it not available?
> 
> No, you haven't missed anything except Unix ideology. That ideology is
> telling us that one tool for one thing, and in very featurable mode.
> That said, lingpiod is exclusively for GPIO ABI between kernel and
> user space. For PWM you need to access the PWM ABI in a way how it's
> represented by the Linux kernel. I believe there are plenty of
> libraries more or less okayish for that purpose, but I never heard
> about any official library and/or Python bindings for it.
> 

I'm assuming Robert is refering to software PWM.  AFAIAA Rpi.GPIO doesn't
support hardware PWM, and the referenced article also refers to software
PWM for the Python case.

libgpiod doesn't directly support software PWM, but it is fairly
straight forward to implement yourself using libgpiod.

Separate from Andy's point on Unix ideology, adding it to libgpiod would
involve threading or async, which in C would open cans of worms that are
best left to the specific application to decide on.  So I don't see it
ever going into the core libgpiod.

Not such an issue for Python with its included batteries, so I would
personally consider adding something along those lines to the Python
bindings, and possibly the Rust as well.  OTOH I suspect Bart's
preference is for the bindings to idiomatically wrap, not extend, the
libgpiod API.

Cheers,
Kent.

Re: [libgpiod] How can I use PWM

[Andy Shevchenko]

On Sat, Aug 6, 2022 at 3:40 PM Kent Gibson <warthog618@gmail.com> wrote:
> On Sat, Aug 06, 2022 at 12:12:31PM +0200, Andy Shevchenko wrote:
> > On Fri, Aug 5, 2022 at 1:17 PM Robert Baumgartner <rbaumgar@redhat.com> wrote:

...

> > No, you haven't missed anything except Unix ideology. That ideology is
> > telling us that one tool for one thing, and in very featurable mode.
> > That said, lingpiod is exclusively for GPIO ABI between kernel and
> > user space. For PWM you need to access the PWM ABI in a way how it's
> > represented by the Linux kernel. I believe there are plenty of
> > libraries more or less okayish for that purpose, but I never heard
> > about any official library and/or Python bindings for it.
>
> I'm assuming Robert is refering to software PWM.

I don't think so. The article he referenced clearly tells about two
(hardware!) PWMs.

> AFAIAA Rpi.GPIO doesn't
> support hardware PWM, and the referenced article also refers to software
> PWM for the Python case.
>
> libgpiod doesn't directly support software PWM, but it is fairly
> straight forward to implement yourself using libgpiod.

Hmm... I was under the impression that we have pwm-gpio in the
kernel... Can't find quickly if there is one.

-- 
With Best Regards,
Andy Shevchenko

Re: [libgpiod] How can I use PWM

[Robert Baumgartner]

Andy, Kent,

I believe I am on the right track.

I added following line to my config.txt
# enable pwm on pin 18
dtoverlay=pwm,pin=18,func=2

Now I have
# ls /sys/class/pwm/pwmchip0
device  export  npwm  power  subsystem  uevent  unexport
# lsmod|grep pwm
pwm_bcm2835            16384  0

and an example I found is working
https://github.com/jdimpson/syspw


Kind regards
Robert

On Sat, Aug 6, 2022 at 4:04 PM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Sat, Aug 6, 2022 at 3:40 PM Kent Gibson <warthog618@gmail.com> wrote:
> > On Sat, Aug 06, 2022 at 12:12:31PM +0200, Andy Shevchenko wrote:
> > > On Fri, Aug 5, 2022 at 1:17 PM Robert Baumgartner <rbaumgar@redhat.com> wrote:
>
> ...
>
> > > No, you haven't missed anything except Unix ideology. That ideology is
> > > telling us that one tool for one thing, and in very featurable mode.
> > > That said, lingpiod is exclusively for GPIO ABI between kernel and
> > > user space. For PWM you need to access the PWM ABI in a way how it's
> > > represented by the Linux kernel. I believe there are plenty of
> > > libraries more or less okayish for that purpose, but I never heard
> > > about any official library and/or Python bindings for it.
> >
> > I'm assuming Robert is refering to software PWM.
>
> I don't think so. The article he referenced clearly tells about two
> (hardware!) PWMs.
>
> > AFAIAA Rpi.GPIO doesn't
> > support hardware PWM, and the referenced article also refers to software
> > PWM for the Python case.
> >
> > libgpiod doesn't directly support software PWM, but it is fairly
> > straight forward to implement yourself using libgpiod.
>
> Hmm... I was under the impression that we have pwm-gpio in the
> kernel... Can't find quickly if there is one.
>
> --
> With Best Regards,
> Andy Shevchenko
>

Re: [libgpiod] How can I use PWM

[Kent Gibson]

On Sun, Aug 07, 2022 at 03:20:33PM +0200, Robert Baumgartner wrote:
> Andy, Kent,
> 
> I believe I am on the right track.
> 
> I added following line to my config.txt
> # enable pwm on pin 18
> dtoverlay=pwm,pin=18,func=2
> 
> Now I have
> # ls /sys/class/pwm/pwmchip0
> device  export  npwm  power  subsystem  uevent  unexport
> # lsmod|grep pwm
> pwm_bcm2835            16384  0
> 

Excellent - if the hardware PWMs cover the lines you are interested in
then that is certainly the way to go.

> and an example I found is working
> https://github.com/jdimpson/syspw
> 

I believe that should be syspwm.

Cheers,
Kent.