linux-kernel - Re: [PATCH] pwm-backlight: add regulator and GPIO support

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Date:	Thu, 5 Jul 2012 09:57:14 +0200
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Alex Courbot <acourbot@...dia.com>
Cc:	Sascha Hauer <s.hauer@...gutronix.de>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>
Subject: Re: [PATCH] pwm-backlight: add regulator and GPIO support

On Thu, Jul 05, 2012 at 04:43:27PM +0900, Alex Courbot wrote:
> On 07/05/2012 03:47 PM, Sascha Hauer wrote:
> >>I thought about just checking if devm_get_regulator returned -ENODEV
> >>and happily continue if that was the case, assuming no regulator was
> >>declared.
> >
> >And that's the problem. The get_regulator won't return -ENODEV. It will
> >return -EPROBE_DEFER which tells you nothing about whether a regulator
> >will ever be available or not.
> >
> >Having a flag in platform data would be fine with me, but I know other
> >people think differently.
> >
> >BTW in devicetree this flag implicitely exists with the power-supply
> >property.
> 
> One could actually question whether the whole regulator/gpio thing
> should be supported at all with platform data. The platform
> interface can use the function hooks in order to implement whatever
> behavior it wants when the light needs to be powered on and off. The
> reason for introducing optional regulator/gpio parameters is because
> the DT cannot use these. Since I have no plan to remove these
> function hooks, making the regulator/gpio option available in
> platform data might be redundant. Any thought about this?

I agree. Non-DT platforms have always used the callbacks to execute this
kind of code. As you've said before there are situations where it isn't
just about setting a GPIO or enabling a regulator but it also requires a
specific timing. Representing this in the platform data would become
tedious.

So I think for the DT case you can parse the power-on and power-off
sequences directly and execute code based on it, while in non-DT cases
the init and exit callbacks should be used instead. I think it even
makes sense to reuse the platform data's init and exit functions in the
DT case and implement the parser/interpreter within those.

> >Right now the regulator core will just return -EPROBE_DEFER in both
> >cases. This could easily be changed in the regulator core.
> 
> Could this be because the regulator core cannot make the difference
> between a not-yet-available regulator and a missing one?

I case where the regulator comes from a DT it should assume that it will
become available at some point, so -EPROBE_DEFER is correct. However if
the DT doesn't even contain the power-supply property, then EPROBE_DEFER
will never work because there's no regulator to become available.

Thierry

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