Re: [PATCH RFC] gpio: of: document gpio-init nodes

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

On Wed, Oct 04, 2017 at 03:53:06PM -0500, Rob Herring wrote:
> On Fri, Sep 22, 2017 at 10:41:38PM +0200, Uwe Kleine-König wrote:
> > Sometimes it is desirable to define a "safe" configuration for a GPIO in
> > the device tree but let the operating system later still make use of
> > this pin.
> > 
> > This might for example be useful to initially configure a debug pin that
> > is usually unconnected as output to prevent floating until it is used
> > later.
> > 
> > Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> > ---
> > Hello,
> > 
> > this picks up a discussion that pops up now and then with our customers.
> > 
> > Last time I discussed this topic with Linus Walleij my suggestion was to
> > merge this usecase with gpio-hogs, but he wasn't happy with it because
> > hogging implies that the pin is not free for other usage and he
> > suggested to use "gpio-init" instead.
> > 
> > Maybe it's arguable if this "initial configuration" belongs into the
> > device tree, but IMHO defining a "safe configuration" should have a
> > place and the requirements are identical. This isn't implied by the name
> > however, but I don't have a better idea for a different name.
> 
> It can be argued that by the time the kernel boots, it is way to late to 
> configure pins to a safe state. Of course, even secure world reads the 
> DT these days (or are at least talking about doing so). Still any s/w 
> handling this could be too slow to get to a safe state.

Note I didn't target the kernel to implement this. I already have
patches that implement this in barebox which is also using dt. (After
all dt is about hardware description and not about what Linux should do,
right? :-)

> Maybe "optimal default" state would be more accurate. 
> 
> > Thinking further (which was also discussed last time) it would also be
> > nice to restrict usage. For example that a given pin that has
> > "output-low" as its safe setting might be configured later als high
> > output but not as input. Maybe:
> 
> I can't imagine that an output can't be an input.

It might make that line float which I'd consider "unsafe" (or "not
optimal").

> Regardless, what you're describing is constraints and that seems like
> a whole other problem than default/initial state.
> 
> Plus, for constraints I'd think we want this done at the pin level, not 
> GPIO. And we kind of already have that with pin states.

Not 100% sure I'm up to date here, if you mean

	pinctrl-names = "default", "idle"
	pinctrl-0 = ... /* that's default */
	pinctrl-1 = ... /* that's idle */

this doesn't help completely. If the idle/save state means that the pin
should be configured as low-output, you cannot define that in general.
You can only configure the pin into its gpio function but not say it
should be an output driving the pin low.

> > 	companion-reset {
> > 		gpio-somethingwithsafe;
> > 		gpios = <12 0>;
> 
> "gpios" is already a defined property with a type (phandle + args). dtc 
> checks for this now though gpio-hogs is already one exception, and I 
> don't want to add another. Maybe it could be generalized to be allowed 
> when the parent is a gpio-controller, but really I'd like to avoid this 
> pattern from spreading.

I choosed the same way as gpio-hogs because IMHO they are quite similar.
Also if the propery is supposed to be located in a child node of a
gpio-controller, repeating &gpioX seems to be at least arguable.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |