linux-kernel - Re: [RFC PATCH 5/6] extcon-gpio: Describe devicetree bindings

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Message-ID: <20130916142147.GF30650@e106331-lin.cambridge.arm.com>
Date:	Mon, 16 Sep 2013 15:21:47 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Pawel Moll <Pawel.Moll@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Ian Campbell <ian.campbell@...rix.com>,
	MyungJoo Ham <myungjoo.ham@...sung.com>,
	Chanwoo Choi <cw00.choi@...sung.com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>
Subject: Re: [RFC PATCH 5/6] extcon-gpio: Describe devicetree bindings

On Thu, Sep 12, 2013 at 05:53:04PM +0100, Guenter Roeck wrote:
> On Thu, Sep 12, 2013 at 05:41:00PM +0100, Mark Rutland wrote:
> > On Fri, Aug 30, 2013 at 05:29:37AM +0100, Guenter Roeck wrote:
> > > Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> > > ---
> > >  .../devicetree/bindings/extcon/extcon-gpio         |   23 ++++++++++++++++++++
> > >  1 file changed, 23 insertions(+)
> > >  create mode 100644 Documentation/devicetree/bindings/extcon/extcon-gpio
> > > 
> > > diff --git a/Documentation/devicetree/bindings/extcon/extcon-gpio b/Documentation/devicetree/bindings/extcon/extcon-gpio
> > > new file mode 100644
> > > index 0000000..091ddc6
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/extcon/extcon-gpio
> > > @@ -0,0 +1,23 @@
> > > +Device-Tree bindings for extcon/extcon-gpio driver
> > 
> > Bindings shouldn't refer to Linux-specifics like particular drivers.
> > What class of hardware are you actually trying to describe?
> > 
> Agreed. Question is where to put the bindings, as they are not really 
> specific to the extcon driver. The extcon driver merely implements 
> the bindings. This is why the "compatible" statement reads "gpio-connector"
> and not "extcon-something".
> 
> The bindings describe a connector managed through gpio pins.

Ok, then that's what the binding document should state.

> 
> > > +
> > > +Required properties:
> > > +	- compatible = "gpio-connector";
> > > +	- presence-detect-gpios - presence detect gpio pin
> > > +
> > > +Optional properties:
> > > +	- debounce-interval - debounce interval in milli-seconds
> > > +	- state-on - on (connected) state
> > > +	- state-off - off (disconnected) state
> > > +	  Depending on the type of connector or cable, states may
> > > +	  for example be reported as "connected"/"disconnected"
> > > +	  or "inserted"/"removed".
> > 
> > I don't understand what the state-* properties describe. Do these
> > provide semantic information to drivers? What is the full set of valid
> > values?
> > 
> That is merely text which is ultimately passed on to the user.
> Guess 'semantic information' might be a way to phrase it.

Where do these end up appearing to the user? Through names in the
filesystem? That's an ABI, which should be thoroughly described...

If it's arbitrary, why is it necessary at all? Surely sensible names for
the state of the connector can be coded in the driver for the device
attached to said connectors (which can be consistent and later changed
if necessary)...

> 
> > > +
> > > +Example node:
> > > +
> > > +	some-connector {
> > > +		compatible = "gpio-connector";
> > > +		presence-detect-gpios = <&gpio1 7 1>;
> > > +		debounce-interval = <1>;
> > > +		state-on = "connected";
> > > +		state-on = "disconnected";

I don't think that's a valid dts. I assume the second is meant to be
"state-off"?

> > > +	};
> > 
> > I'm not sure how much value this adds to bindings over describing the
> > gpios directly. This seems to add a layer of indirection because of
> > Linux internals.
> > 
> Not sure I understand what you mean with "describing the gpios directly".
> Can you elaborate and/or provide an example ?

Take a look at the MMC bindings [1], specifically the cd-gpios and
wp-gpios properties. I don't see why the connection of the GPIO needs to
be described by a wrapper device that doesn't really exist, when it can
be described directly.

> 
> How would you describe the use of gpio pins used to detect board presence ?
> That doesn't seem very Linux specific to me.

As above, with cd-gpios.

Thanks,
Mark.

[1] Documentation/devicetree/bindings/mmc/mmc.txt
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