Message-ID: <4FDB6000.5080703@wwwdotorg.org>
Date:	Fri, 15 Jun 2012 10:17:04 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Tony Lindgren <tony@...mide.com>
CC:	Linus Walleij <linus.walleij@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-omap@...r.kernel.org, devicetree-discuss@...ts.ozlabs.org,
	Stephen Warren <swarren@...dia.com>,
	Arnd Bergmann <arnd@...db.de>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>
Subject: Re: [PATCH] pinctrl: Add one-register-per-pin type device tree based
 pinctrl driver

On 06/15/2012 03:49 AM, Tony Lindgren wrote:

(Arnd, Grant, Rob, CC'ing you mainly re: the very last set of comments
in this email; can you take a look at Tony's patch and comment on the
binding)

> * Stephen Warren <swarren@...dotorg.org> [120614 16:16]:
>> On 06/11/2012 07:58 AM, Tony Lindgren wrote:
>>> Add one-register-per-pin type device tree based pinctrl driver.
>>>
>>> Currently this driver only works on omap2+ series of processors,
>>> where there is either an 8 or 16-bit padconf register for each pin.
>>> Support for other similar pinmux controllers can be added.
>>
>>> diff --git a/Documentation/devicetree/bindings/pinctrl/pinctrl-omap2plus.txt b/Documentation/devicetree/bindings/pinctrl/pinctrl-omap2plus.txt
>>
>> I'm not sure why you'd need an explicit OMAP2 binding document or
>> compatible values if it just uses the plain pinctrl-single binding
>> unmodified.
> 
> Hmm I thought that's what you wanted with your earlier comments at [1]:
> 
> * Stephen Warren <swarren@...dotorg.org> [120504 12:27]:
>>
>> If this is truly intended to be generic, I would not document any of the
>> ti compatible values here. Instead, I'd create a binding document for
>> the TI controllers that basically just says "this uses the bindings in
>> pinctrl-simple.txt, with the following additions", and list the
>> compatible values as an addition.
> 
> Care to clarify a bit what you had in mind there?

Basically I meant:

1) Remove anything TI-/OMAP-specific from the generic binding document

2) For any TI-/OMAP-specific additions, create a binding document that
describes those separately to the generic binding documentation.

However, given that there are no TI-/OMAP-specific additions; OMAP2 just
uses the base generic bindings exactly as defined, I don't think you
actually need to create an TI-/OMAP-specific document, since there's
nothing for it to document.

>>> diff --git a/drivers/pinctrl/pinctrl-single.c b/drivers/pinctrl/pinctrl-single.c

>>> +static struct of_device_id pcs_of_match[] __devinitdata = {
>>> +	{ .compatible = DRIVER_NAME, },
>>> +	{ .compatible = "ti,omap2420-padconf", },
>>> +	{ .compatible = "ti,omap2430-padconf", },
>>> +	{ .compatible = "ti,omap3-padconf", },
>>> +	{ .compatible = "ti,omap4-padconf", },
>>> +	{ },
>>> +};
>>
>> Shouldn't that contain just one entry for "pinctrl-single", and no others?
> 
> It's best to describe the hardware in case we need to set up some hardware
> specific things in the driver.

There's a difference between the (set of) compatible value(s) that
appears in the .dts file, and the compatible value(s) that the driver
binds to.

Since this driver is purely implementing the pinctrl-single binding, I
believe only that should be listed in the driver's of_match table.

However, since the .dts file is describing HW that is both a specific
OMAP instance, and compatible with pinctrl-single, the .dts file can
certainly list both. If in the future, the driver ever needs to
specifically know the difference between the specific OMAP versions, or
OMAP-vs-something-else, the compatible value will be there already in
the .dts file, so this will all work. However, I'd argue that if the
driver has to ever know that, it's probably not appropriate to say it's
compatible with pinctrl-single any more...

>> One final comment: A little while before Linaro Connect in San
>> Francisco, we were all having difficulty coming up with any kind of DT
>> binding for pinctrl. I had suggested the possibility of creating a
>> binding which just said "write value X to register Y, write value P to
>> register Q, ...". This was rejected at connect, because a raw list of
>> register writes didn't document anything or describe semantics - it was
>> too much of a binary blob. This driver seems to be doing almost the
>> exact same thing. In order to avoid that assertion, it'd need to truly
>> have individual representations of which pins exist, which pingroups
>> exist, which functions exist, and which functions can be selected onto
>> which pins/pingroups. That would be a radically different binding. I
>> wonder what's changed such that this kind of driver wasn't acceptable
>> then, but is now?
> 
> Well that's why I had two bindings earlier: The current binding for the
> bulk pins that's faster to parse, and a more verbose binding for drivers that
> allows naming the functions.
> 
> In your previous comments at [1] you seemed to prefer this current binding
> based on the "Why not only allow (1)?" comment. Maybe you had something else
> in mind, care to clarify that a bit too?

Well, first I thought about semantics vs. just banging a register list a
tiny bit more, and remembered the previous discussion.

But my comment at [1] wasn't so much about "why not just have a raw
register list", but more regarding why support two different
representations within the same binding; it wasn't really clear to me
from reading the original email what the difference between the two
representations; why have two representations, why/when-to use one vs.
the other, that the difference was about providing a "semantic" list of
pins/pingroups/functions vs. providing a "raw" register list.

IIRC, the pushback on a raw "bang these registers" representation came
from Grant and/or Arnd. It'd be a good idea if they (and indeed Rob)
could comment on this binding proposal. If they're OK with this kind of
pretty raw representation, I'm not going to object, but I have a feeling
they might not like it?

> [1] http://article.gmane.org/gmane.linux.kernel/1291838
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