[RFC 1/4] pwm: sifive: Add DT documentation for SiFive PWM Controller.

Wesley Terpstra wesley at sifive.com
Mon Oct 15 16:19:20 PDT 2018

On Mon, Oct 15, 2018 at 3:57 PM Atish Patra <atish.patra at wdc.com> wrote:
> >> +SiFive PWM controller
> >> +
> >> +Unlike most other PWM controllers, the SiFive PWM controller currently only
> >> +supports one period for all channels in the PWM. This is set globally in DTS.
> >> +The period also has significant restrictions on the values it can achieve,
> >> +which the driver rounds to the nearest achievable frequency.
> >
> > What restrictions are these? If "nearest achievable" is too far off the
> > target period it might be preferable to error out.
> >
> @Wes: Any comments?

When I last looked at this driver and hardware, I briefly considered
throwing up my hands and pretending that this PWM device had no period
control at all, only duty-cycle. There are several examples of PWM
controllers in linux already that behave that way.

Most of the uses of this PWM are only going to care about the
duty-cycle, not the period. So failing when the period is unachievable
seems worse to me than just completely eliminating access to period

> I think yes. Since fu540 is the first Linux capable RISC-V core, SiFive
> Guys decided mark it as version 0.
> @Wesly: Please correct me if I am wrong.


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