linux-kernel - Re: [PATCH v2] ARM: dts: am335x-pocketbeagle: set default mux for gpio pins

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Message-ID: <20200630182037.GF37466@atomide.com>
Date:   Tue, 30 Jun 2020 11:20:37 -0700
From:   Tony Lindgren <tony@...mide.com>
To:     Drew Fustini <drew@...gleboard.org>
Cc:     Rob Herring <robh+dt@...nel.org>, bcousson@...libre.com,
        linux-omap@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Jason Kridner <jkridner@...gleboard.org>,
        Robert Nelson <robertcnelson@...il.com>,
        "H . Nikolaus Schaller" <hns@...delico.com>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH v2] ARM: dts: am335x-pocketbeagle: set default mux for
 gpio pins

* Drew Fustini <drew@...gleboard.org> [200630 02:02]:
> On Mon, Jun 29, 2020 at 10:03:58AM -0700, Tony Lindgren wrote:
> > Hi,
> > 
> > * Drew Fustini <drew@...gleboard.org> [200628 08:26]:
> > > v2 changes:
> > > - change default mux from output to input.  Input is safer as it does
> > >   not drive the line.  If the user wants output, they will need to edit
> > >   this device tree.
> > 
> > So can you please clarify this a bit, are you saying that gpio output can't
> > be done via userspace?
> 
> As it stands, there is no way for userspace to change a pin from gpio 
> input to gpio output as it is based on the reciever active bit in the
> pin control register (table 9-60 in the AM3358 datasheet [0]).

OK. I guess one not so nice option would be to enable receiver active
for all the gpio pins.. But this also affects power consumption a little.

> I thought it would be useful to at least define the GPIOs available for
> use on the PocketBeagle by default (and eventually the other BeagleBone
> device trees).  Input seems to be the safest value.  This patch at least
> makes it more clear where the user should edit to switch from input to
> output.

Some TI TRMs actually have tables listing the preferred mux logic
for unused pins. I recall safe mode input pull down being one of the
preferred modes. Sorry not sure which TRM that is, maybe omap3 or
omap4 or omap5?

> > Needing to change the dts does not sound good to me.. But maybe you mean
> > this is needed until the gpio-omap and pinctrl-single patches are merged?
>
> I agree that I would like for userspace to be able to do run-time
> changes.  However, I think something would need to be added to the
> pinconf support in pinctrl-single for that to be possible.  There are
> bias properties but non for receiver enable.
> 
> Does it seem sensible to add that?

Well let's see with Linus W says. To me it seems this might be a good
reason to allow a sysfs interface to change the pinctrl if we don't
have it yet? With the proper gpio line naming it should be quite simple
to use too :)

Regards,

Tony