Re: PWM and rounding for pwm-ir-tx

[Sean Young <sean@mess.org>]

Hello Uwe,

On Fri, Nov 21, 2025 at 05:04:41PM +0100, Uwe Kleine-König wrote:
> Hello Sean,
> 
> I'm currently working on the pwm-bcm2835 driver, converting it to the
> waveform PWM API.
> 
> A result of that conversation is that calling pwm_apply_atomic() will
> result in the period being rounded down instead of rounded to the
> nearest possible value. Looking at commit 11fc4edc483b ("pwm: bcm2835:
> Improve precision of PWM") I guess you will be unhappy about that.
> 
> The obvious way forward is to let the pwm-ir-tx driver make use of the
> waveform API which allows better control over the actual hardware
> output.

That's very interesting, I must have missed the new waveform API. I agree,
this is a great way forward.

> I looked into the pwm-ir-tx driver to check if I can do the conversion,
> but didn't understand the protocol good enough to try that.
> 
> I wonder if you are still interested enough in the driver to do the
> conversion yourself? Alternatively can you point me to some documention
> resource explaining the protocol? Maybe there is even an easy way to
> test changes?

I'm more than happy to do the conversion, then I can also test that works
correctly with a logic analyizer.

> One thing that I noticed is that
> 
> 	pwm_ir->state->enabled = !(pwm_ir->txbuf_index % 2);
> 	pwm_apply_atomic(pwm_ir->pwm, pwm_ir->state);
> 
> is conceptually broken because the output of a disabled PWM is
> undefined. Many hardwares emit a constant inactive level, but others
> might emit active instead, or a random level or might even continue to
> toggle. If you want to be sure to drive the inactive level, keep the PWM
> enabled but use duty_cycle = 0.

That's very useful, I did not know that. I'll write a fix for that.

For the pwm-bcm2835 driver: do you have a draft commit that enables the
waveform API? That way I can start testing any changes I make to pwm-ir-tx
on RPi.

Thank you for keeping in the loop on this, I should pay closer attention
to the list.


Sean