Re: [PATCH v8 15/26] pwm: jz4740: Add support for the JZ4725B

[Paul Cercueil]

Hi,

Le ven. 14 déc. 2018 à 15:26, Uwe Kleine-König 
<u.kleine-koenig@xxxxxxxxxxxxxx> a écrit :
> Hello,
>
> On Fri, Dec 14, 2018 at 02:50:20PM +0100, Linus Walleij wrote:
> >  On Thu, Dec 13, 2018 at 9:42 PM Uwe Kleine-König
> >  <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
> >  > [Adding Linus Walleij to Cc:]
> >  > On Thu, Dec 13, 2018 at 03:03:15PM +0100, Paul Cercueil wrote:
> >  > > Le jeu. 13 déc. 2018 à 10:24, Uwe Kleine-König
> >  > > <u.kleine-koenig@xxxxxxxxxxxxxx> a écrit :
> >  > > > On Wed, Dec 12, 2018 at 11:09:10PM +0100, Paul Cercueil wrote:
> >  > > > >  The PWM in the JZ4725B works the same as in the JZ4740, 
> > except that
> >  > > > > it
> >  > > > >  only has 6 channels available instead of 8.
> >  > > >
> >  > > > this driver is probed only from device tree? If yes, it might 
> > be
> >  > > > sensible to specify the number of PWMs there and get it from 
> > there.
> >  > > > There doesn't seem to be a generic binding for that, but 
> > there are
> >  > > > several drivers that could benefit from it. (This is a bigger 
> > project
> >  > > > though and shouldn't stop your patch. Still more as it 
> > already got
> >  > > > Thierry's ack.)
> >  > >
> >  > > I think there needs to be a proper guideline, as there doesn't 
> > seem to be
> >  > > a consensus about this. I learned from emails with Rob and 
> > Linus (Walleij)
> >  > > that I should not have in devicetree what I can deduce from the 
> > compatible
> >  > > string.
> >  >
> >  > I understood them a bit differently. It is ok to deduce things 
> > from the
> >  > compatible string. But if you define a generic property (say) 
> > "num-pwms"
> >  > that is used uniformly in most bindings this is ok, too. (And 
> > then the
> >  > two different devices could use the same compatible.)
> >  >
> >  > An upside of the generic "num-pwms" property is that the pwm core 
> > could
> >  > sanity check pwm phandles before passing them to the hardware 
> > drivers.
> >
> >  I don't know if this helps, but in GPIO we have "ngpios" which is
> >  used to augment an existing block as to the number of lines actually
> >  used with it.
> >
> >  The typical case is that an ASIC engineer synthesize a block for
> >  32 GPIOs but only 12 of them are routed to external pads. So
> >  we augment the behaviour of that driver to only use 12 of the
> >  32 lines.
> >
> >  I guess using the remaining 20 lines "works" in a sense but they
> >  have no practical use and will just bias electrons in the silicon
> >  for no use.
>
> This looks very similar to the case under discussion.
>
> >  So if the PWM case is something similar, then by all means add
> >  num-pwms.
>
> .. or "npwms" to use the same nomenclature as the gpio binding?

If we're going to do something like this, should it be the drivers or
the core (within pwmchip_add) that checks for this "npwms" property?

> Best regards
> Uwe
>
> --
> Pengutronix e.K.                           | Uwe Kleine-König        
>     |
> Industrial Linux Solutions                 | 
> http://www.pengutronix.de/  |