Re: [PATCH v2 2/7] pwm: core: Always require PWM flags to be provided

Andy Shevchenko
2021-06-07T10:15:29Z

On Mon, Jun 07, 2021 at 11:53:24AM +0200, Uwe Kleine-König wrote:
> On Mon, Jun 07, 2021 at 12:02:37PM +0300, Andy Shevchenko wrote:
> > On Sun, Jun 06, 2021 at 11:30:54PM +0200, Uwe Kleine-König wrote:
> > > On Mon, May 31, 2021 at 10:49:42PM +0300, Andy Shevchenko wrote:
> > > > It makes little sense to make PWM flags optional since in case
> > > > of multi-channel consumer the flags can be optional only for
> > > > the last listed channel.
> > >
> > > I think the same holds true for dt references.
> >
> > Can you elaborate this? I haven't got what you are talking about, not a DT
> > expert here.
>
> Ah no, I mixed that up. While the function that parses the phandle is
> flexible, for each pwm controller the number of arguments is fixed, so
>
> pwms = <&pwm1 100000 &pwm2 100000 &pwm3 1000000>;
>
> cannot be interpreted as 3-argument references to two PWMs. This is
> different to ACPI (I guess, not an ACPI expert here :-) because &pwm1
> "knows" if it needs 1 or 2 additional parameters (#pwm-cells).

It's not about ACPI, it's about "the ACPI glue layer in Linux kernel".
Used API is a part of it and it does allow only two cases, either NULL entry
(by having 0 as an argument) or full-length supplied tuple (in case of PWM it's
3, so, means 4 parameters.

Let's consider examples:

(0, 0, x3, y3, z3, t3) // NULL, NULL, PWM3
(x1, y1, z1, t1, 0, x3, y3, z3, t3) // PWM1, NULL, PWM3

So, making last parameter "flexible" will work only for the last tuple in the
array.

Read this [1] for further information.

[1]: https://elixir.bootlin.com/linux/latest/source/drivers/acpi/property.c#L629

--
With Best Regards,
Andy Shevchenko