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

[Atish Patra]

On 10/16/18 3:04 PM, Thierry Reding wrote:
> On Tue, Oct 16, 2018 at 10:31:42AM -0700, Paul Walmsley wrote:
> > On 10/16/18 4:01 AM, Thierry Reding wrote:
> > > On Mon, Oct 15, 2018 at 03:57:35PM -0700, Atish Patra wrote:
> > > > On 10/10/18 6:49 AM, Thierry Reding wrote:
> > > > > On Tue, Oct 09, 2018 at 11:51:22AM -0700, Atish Patra wrote:
> > > > > > +Required properties:
> > > > > > +- compatible: should be one of
> > > > > > +	"sifive,fu540-c000-pwm0","sifive,pwm0".
> > > > > What's the '0' in here? A version number?
> > > > 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.
> > > It seems fairly superfluous to me to have a version number in additon to
> > > the fu540-c000, which already seems to be the core plus some sort of
> > > part number. Do you really expect there to be any changes in the SoC
> > > that would require a different compatible string at this point? If the
> > > SoC has taped out, how will you ever get a different version of the PWM
> > > IP in it?
> > >
> > > I would expect any improvements or changes to the PWM IP to show up in a
> > > different SoC generation, at which point it would be something like
> > > "sifive,fu640-c000" maybe, or perhaps "sifive,fu540-d000", or whatever
> > > the numbering is.
> >
> >
> > The "0" suffix refers to a revision number for the underlying PWM IP block.
> >
> > It's certainly important to keep that version number on the "sifive,pwm0"
> > compatible string that doesn't have the chip name associated with it.
>
> Isn't the hardware identified by "sifive,pwm0" and "sifive,fu540-c000"
> effectively identical? 

Yes.

Is there a need to have two compatible strings
> that refer to the exact same hardware?

The DT in the hardware has only sifive,pwm0. I have added 
"sifive,fu540-c000" as that was concluded as the correct compatible 
string from platform level interrupt controller patch(PLIC) discussion.

(http://lists.infradead.org/pipermail/linux-riscv/2018-August/001135.html)

"sifive,pwm0" is required to until all the Unleashed SoC gets an updated 
firmware with correct compatible string "sifive,fu540-c000". I agree 
this is a mess. But we have to carry it until all every DT(corresponding 
to each driver) is finalized. I guess SiFive will release a firmware 
update that contains all the updated DT once that is done. We can get 
rid of all the redundant compatible strings at that time.

Regards,
Atish
> > As to whether there could ever be a FU540-C000 part with different IP block
> > versions on it: FU540-C000 is ultimately a marketing name. While
> > theoretically we shouldn't have another "FU540-C000" chip with different
> > peripheral IP block versions on it, I don't think any engineer can guarantee
> > that it won't happen.
>
> I would argue that if at some point there was indeed a chip with the
> same name but a different IP block version in it, we can figure out what
> to call it. Sure there are no guarantees, but it's still fairly unlikely
> in my opinion, so I personally wouldn't worry about this up front.
>
> Anyway, I don't feel strongly either way, I'm just pointing out that
> this is somewhat unusual. If you want to keep it, feel free to.
>
> Thierry