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

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Message-ID: <CAMgXwTieXNa13_xGp2mkw99+VJm-R1xrxMHSmRLZV-hG5oSdzA@mail.gmail.com>
Date:   Mon, 15 Oct 2018 16:19:20 -0700
From:   Wesley Terpstra <wesley@...ive.com>
To:     Atish Patra <atish.patra@....com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        linux-riscv@...ts.infradead.org, linux-pwm@...r.kernel.org,
        linux-gpio@...r.kernel.org, linus.walleij@...aro.org,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [RFC 1/4] pwm: sifive: Add DT documentation for SiFive PWM Controller.

On Mon, Oct 15, 2018 at 3:57 PM Atish Patra <atish.patra@....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
control.

> 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.

Correct.