Fix for PWM on Beaglebone black with a kernel > 3.8.13

[SyrianSpock]

Tested on hardware (Beaglebone black Rev B6)

[arfoll]

Thanks for the contribution! Is this a debian thing or global? I've never seen such a thing and edison for example has a 3.10 kernel? I'd like to understand if this is a beaglebone thing or general? Quark also uses a 3.8.x kernel and doesn't have this.

[SyrianSpock]

I have only seen this on the Beaglebone black with Debian and Ubuntu.
I don't know if the Beaglebone is the only board concerned or if it's an Linux on ARM thingy, sorry I don't have an answer.

[arfoll]

Ok I unfortunately don't have a BBB, @tingleby do you think you could take a look? My only arm platform is an rpi2 and that doesn't have pwm (least not natively)

[arfoll]

@cosscat is this something you could help with if you have a board?

[alext-mkrs]

I don't have BBB either, but I've found this nice description: https://github.com/szmoore/MCTX3420/wiki/Software:-PWM-Control and looks like the standard sysfs location is actually just a secondary mean to use PWMs on BBB and the one that doesn't follow the kernel documentation fully (dropping the pwmchip piece).

[alext-mkrs]

I believe this should rather be reworked using _replace approach, as this is specific to one board we support. @SyrianSpock, do you think you'd want to do that?

[SyrianSpock]

@alext-mkrs Sure, I'll do it ASAP.

[alext-mkrs]

Thanks and no rush 😄 I just believe this is a better way to go and looking at the code and @arfoll's comments I think it will increase its chances for merging.

[arfoll]

Needs to use _replace API before can be merged

[alext-mkrs]

So @SyrianSpock, would you be willing to modify this one as requested?