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

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Message-ID: <20120705080234.GQ30009@pengutronix.de>
Date:	Thu, 5 Jul 2012 10:02:34 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Alex Courbot <acourbot@...dia.com>
Cc:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Thierry Reding <thierry.reding@...onic-design.de>,
	"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?

sounds good.

> 
> >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?

It could. In regulator_dev_lookup we have:

		if (node) {
			...
		} else {
			/*
			 * If we couldn't even get the node then it's
			 * not just that the device didn't register
			 * yet, there's no node and we'll never
			 * succeed.
			 */
			*ret = -ENODEV;
		}

So here the regulator core knows that there is no regulator and never
will be. All that needs to be done is to make _regulator_get look at
that value.
There may be some side effects if we just return ERR_PTR(-ENODEV) when
regulator_dev_lookup returns -ENODEV. Maybe Mark has some comments to
this.

Sascha

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