Re: [PATCH V4 2/2] pwm: Add GPIO PWM driver

[Sean Young <sean@mess.org>]

On Tue, Feb 27, 2024 at 05:59:40PM +0100, Stefan Wahren wrote:
> Hi Chris,
> 
> Am 27.02.24 um 16:32 schrieb Chris Morgan:
> > I have a series of devices with GPIO controlled force feedback that
> > this driver helps us control better. So I'm looking forward to this,
> > thank you.
> Thanks for testing. I didn't had much time recently and i was fighting
> with hr timer resolution stuff. But will try to send the next version in
> March.
> > Note that when I set the resolution too low (I got confused and set
> > the period to 255) my device locked up hard and only a forced
> > power cycle could bring it back.
> Unfortunately this is a general design issue by driving the GPIO by a
> kernel driver and "expected" behavior. I didn't have a good solution for
> it yet.

When we reprogram the timer with hrtimer_forward(), we could check whether
we have overrun - i.e. we are already beyond the expires time. This could
be a hint that a) we cannot generate the pwm signal and b) this might
be what causes the hang, because we are returning HRTIMER_RESTART yet
no new expires has been programmed.

Crashing the machine if the period is too short is not really good enough
for mainline, I think. There is talk of pwm chardevs in the future.


Sean