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

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Date:   Thu, 17 Sep 2020 12:39:42 +0200
From:   Drew Fustini <drew@...gleboard.org>
To:     Trent Piepho <tpiepho@...il.com>
Cc:     Tony Lindgren <tony@...mide.com>, 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 17, 2020 at 03:00:36AM -0700, Trent Piepho wrote:
> On Thu, Sep 17, 2020 at 2:20 AM Drew Fustini <drew@...gleboard.org> wrote:
> >
> > On Thu, Sep 17, 2020 at 02:03:46AM -0700, Trent Piepho wrote:
> > > On Mon, Sep 14, 2020 at 3:44 AM Drew Fustini <drew@...gleboard.org> wrote:
> > > >
> > > > +
> > > > +When #pinctrl-cells = 2, then setting a pin for a device could be done with:
> > > > +
> > > > +       pinctrl-single,pins = <0xdc 0x30 0x07>;
> > > > +
> > > > +Where 0x30 is the pin configuration value and 0x07 is the pin mux mode value.
> > > > +See the device example and static board pins example below for more information.
> > >
> > > Pin configuration and mux mode don't mean anything in pinctrl-single.
> > > On another machine, mux mode might not be programmed this way or even
> > > exist.  Or the location of bits would probably be different, and this
> > > would seem to imply the 0x07 would get shifted to the correct location
> > > for where the pin mux setting was on that machine's pinctrl registers.
> > >
> > > It seems like it would be better to explain the values are ORed together.
> >
> > I descirbed it as seoerate values as I did not want to prescribe what
> > the pcs driver would do with those values.  But, yes, it is a just an OR
> > operation, so I could change the language to reflect tat.
> 
> If you don't say what the pinctrl-single driver does with the values,
> how would anyone know how to use it?
> 
> > > What is the purpose of this change anyway?  It seems like in the end
> > > it just does what it did before.  The data is now split into two cells
> > > in the device tree, but why?
> >
> > These changes were a result of desire to seperate pinconf and pinmux.
> > Tony raised the idea in a thread at the end of May [1].
> >
> > Tony wrote:
> > > Only slightly related, but we should really eventually move omaps to use
> > > #pinctrl-cells = <2> (or 3) instead of 1, and pass the pinconf seprately
> > > from the mux mode. We already treat them separately with the new
> > > AM33XX_PADCONF macro, so we'd only have to change one SoC at a time to
> > > use updated #pinctrl-cells. But I think pinctrl-single might need some
> > > changes before we can do that.
> 
> I still don't see what the goal is here.  Support generic pinconf?

My interest is came out of my desire to turn on generic pinconf for AM3358
and I had to fix a bug that was breaking compatible "pinconf,single":
f46fe79ff1b6 ("pinctrl-single: fix pcs_parse_pinconf() return value")

> Also note that while AM33XX_PADCONF() is changed, there is an in tree
> board that doesn't use it, so it's broken now.  I found this change
> when it broke my out of tree board, due to the dtsi change not being
> reflected in my board's pinctrl values.

Thanks, that is a good point that arch/arm/boot/dts/am335x-guardian.dts
needs to be converted from AM33XX_IOPAD to AM33XX_PADCONF.  I'll submit
a patch for that.

Regarding AM33XX_PADCONF() restructuring, the change to have seperate
arguments for direction and mux in AM33XX_PADCONF() predates my
invovlement, so I've CC'd Christina Quast.

    commit f1ff9be7652b716c7eea67c9ca795027d911f148
    Author: Christina Quast <cquast@...overdisplays.com>
    Date:   Mon Apr 8 10:01:51 2019 -0700

    ARM: dts: am33xx: Added AM33XX_PADCONF macro
    
    AM33XX_PADCONF takes three instead of two parameters, to make
    future changes to #pinctrl-cells easier.
    
    For old boards which are not mainlined, we left the AM33XX_IOPAD
    macro.
    
    Signed-off-by: Christina Quast <cquast@...overdisplays.com>
    Reviewed-by: Rob Herring <robh@...nel.org>
    Signed-off-by: Tony Lindgren <tony@...mide.com>

Hopefully, Tony can also chime in.

-Drew