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

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Message-ID: <20210607141117.24bvqiafy4cndoj4@pengutronix.de>
Date:   Mon, 7 Jun 2021 16:11:17 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Flavio Suligoi <f.suligoi@...m.it>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pwm@...r.kernel.org, linux-acpi@...r.kernel.org,
        Jonathan Corbet <corbet@....net>,
        Lee Jones <lee.jones@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>
Subject: Re: [PATCH v2 2/7] pwm: core: Always require PWM flags to be provided

Hello Andy,

On Mon, Jun 07, 2021 at 02:49:01PM +0300, Andy Shevchenko wrote:
> On Mon, Jun 07, 2021 at 01:21:01PM +0300, Andy Shevchenko wrote:
> > On Mon, Jun 07, 2021 at 01:15:20PM +0300, Andy Shevchenko wrote:
> > > 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
> > 
> > Hmm... I have read the actual implementation and it seems it's possible to have
> > flexible array, so this patch needs to be reconsidered.
> 
> I was thinking more about it and what we have here is positional-dependent
> arguments. Either way we might end up in the same situation (when we need to
> parse arguments based on their positions, rather than always have them being
> present). So, while I won't change documentation example (to be more stricter
> there), I will drop this change.
> 
> Also, the PWM initial state doesn't include duty cycle. Any explanations why is
> that?

This isn't technically the initial state. It's a hint to the consumer
which period to pick. The duty-cycle is usually variable, but if I
designed the binding today I would not include the period in the pwm
handle. But to discuss this is moot---the binding is as it is.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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