Re: PWM...

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Sun, Jan 19, 2014 at 11:11:41AM -0800, Olof Johansson wrote:
> On Sun, Jan 19, 2014 at 11:08 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> > On Sunday 19 January 2014 11:03:24 Olof Johansson wrote:
> >> >
> >> > Yes, because iMX6 specifies #pwm-cells as 2, there's no flags able to
> >> > be specified in the pwms declaration in pwmleds.  So that doesn't work.
> >> > There's no property to tell pwmleds that it should use inverted sense
> >> > either.
> >>
> >> Adding a property for active-low to the pwm-leds binding would be
> >> easy, and backwards compatible. I'm surprised the original binding
> >> didn't specify it. The leds-pwm driver already seems to support it for
> >> C-configured instances.
> >>
> >> I'm also surprised that the imx pwm driver even has a #pwm-cells of
> >> two, since the driver only supports one output. It'd be nice if they
> >> had allocated the extra cell for flags, but it's hard to change now,
> >> unless you do a new binding/compatible value and deprecate the old one.
> >
> > Actually I think it's not that hard to change: The binding can specify
> > that either #pwm-cells=<2> or #pwm-cells=<3> is supported, and the
> > driver extended to handle both cases. This would maintain backwards
> > compatibility for old dtb files, though no forward compatibility for
> > new dtb files with old kernels.
> 
> Ah, yes, if you add a cell that can be done. There'll still be the
> "dead" first cell that will always be 0, but that's alright.

Does it not mean that PWM specifications of:

	<&pwm1 0 n> <&pwm2 0 n>

would need to be converted to:

	<&pwm1 0 n 0> <&pwm2 0 n 0>

in every DT file referring to these PWMs - because isn't this just
treated in DT as one single array of values?  (If DT knew how many
were in each specification, we wouldn't need the #foo-cells...)

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