linux-kernel - Re: [PATCH] ARM: dts: document pinctrl-single,pins when #pinctrl-cells = 2

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Message-ID: <CA+7tXihBdw9AOGL7Hp2cH9+ii8fUXaaZZDUP3icyeOkMuGm4qA@mail.gmail.com>
Date:   Tue, 29 Sep 2020 13:15:45 -0700
From:   Trent Piepho <tpiepho@...il.com>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Drew Fustini <drew@...gleboard.org>,
        Rob Herring <robh+dt@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Jason Kridner <jkridner@...gleboard.org>,
        Robert Nelson <robertcnelson@...il.com>,
        linux-omap@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org,
        linux-gpio <linux-gpio@...r.kernel.org>,
        Christina Quast <cquast@...overdisplays.com>
Subject: Re: [PATCH] ARM: dts: document pinctrl-single,pins when
 #pinctrl-cells = 2

On Thu, Sep 24, 2020 at 12:04 AM Tony Lindgren <tony@...mide.com> wrote:
>
> * Trent Piepho <tpiepho@...il.com> [200924 06:31]:
> > > >
> > > > The pinctrl-single driver?  How will that work with boards that are
> > > > not am335x and don't use conf and mux fields in the same manner as
> > > > am335x?
> > >
> > > For those cases we still have #pinctrl-cells = <1>.
> >
> > If pincntrl-single is going to be am335x specific, then shouldn't it
> > be a different compatible string?
>
> Certainly different compatible strings can be used as needed.
> But pinctrl-single is not going to be am335x specific though :)
> We have quite a few SoCs using it:

So what doesn't make sense to me, is to put something am335x specific
like two cells for conf and mux, into a generic driver like pinctrl
single.

This series adds two cells ORed into one.  Ok, that's generic, other
platforms could use it.  But it also accomplishes nothing, so what's
the point?  You've hinted there is more to come, which will accomplish
something, but what is it?  That can be:
Used by platforms other than am335x
Can't already be done with the pinctrl single pinconf features
Needs more than one data cell per pin

> > Are the driver changes something that can be not be done with the
> > pinconf-single properties?  They all include a mask.
>
> Sure but in the long term we're better off with using #pinctrl-cells
> along the lines what we have for example for #interrupt-cells and
> #gpio-cells.

So if you look at gpio-cells, virtually every driver uses 2, where one
cell is the gpio index and the other is a common set of flags.  It's
the standard layout of a gpio handle that almost all bindings use.
Only a handful of platform specific gpio drivers have another cell to
indicate bank (probably better to have made each bank its own device
node).

Interrupt controllers have different numbers of cells, but they are
all platform specific, and the cells have defined platform specific
meanings.  pci-host-cam-generic is a somewhat generic interrupt
controller and it uses 1 cell, since it lacks device specific fields
to put into additional cells.

So I don't see an example of multiple cells which do not have a
defined meaning that applies to all devices using the bindings.

Consider also that any future changes to the pinctrl-single bindings
would need to be backward compatible with a device tree binary where
two cells get combined.  So if the bindings being added here aren't
done, then adding them now creates an unnecessary additional version
to deal with for backward compatibility.