linux-kernel - Re: [PATCH] drm/panel: lvds: Handle the optional regulator case properly

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Date:   Thu, 11 Jan 2018 14:12:56 +0100
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Daniel Vetter <daniel.vetter@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH] drm/panel: lvds: Handle the optional regulator case
 properly

Hi,

On Thu, Jan 11, 2018 at 03:05:01PM +0200, Laurent Pinchart wrote:
> Hi Maxime,
> 
> (CC'ing Mark Brown)
> 
> Thank you for the patch.
> 
> On Wednesday, 10 January 2018 17:59:41 EET Maxime Ripard wrote:
> > The devm_regulator_get_optional function, unlike it was assumed in the
> > commit a1c55bccf600 ("drm/panel: lvds: Add support for the power-supply
> > property"), is actually returning an error pointer with -ENODEV instead of
> > NULL when there's no regulator to find.
> > 
> > Make sure we handle that case properly.
> > 
> > Fixes: a1c55bccf600 ("drm/panel: lvds: Add support for the power-supply
> > property") Signed-off-by: Maxime Ripard <maxime.ripard@...e-electrons.com>
> > ---
> >  drivers/gpu/drm/panel/panel-lvds.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/panel/panel-lvds.c
> > b/drivers/gpu/drm/panel/panel-lvds.c index 57e38a9e7ab4..9f46e7095c0e
> > 100644
> > --- a/drivers/gpu/drm/panel/panel-lvds.c
> > +++ b/drivers/gpu/drm/panel/panel-lvds.c
> > @@ -215,8 +215,13 @@ static int panel_lvds_probe(struct platform_device
> > *pdev) lvds->supply = devm_regulator_get_optional(lvds->dev, "power");
> >  	if (IS_ERR(lvds->supply)) {
> >  		ret = PTR_ERR(lvds->supply);
> > -		dev_err(lvds->dev, "failed to request regulator: %d\n", ret);
> > -		return ret;
> > +
> > +		if (ret != -ENODEV) {
> > +			dev_err(lvds->dev, "failed to request regulator: %d\n", ret);
> > +			return ret;
> 
> I wouldn't print an error message if ret == -EPROBE_DEFER.
> 
> > +		} else {
> > +			lvds->supply = NULL;
> > +		}
> >  	}
> 
> How about
> 
> 	lvds->supply = devm_regulator_get_optional(lvds->dev, "power");
> 	if (IS_ERR(lvds->supply)) {
> 		ret = PTR_ERR(lvds->supply);
> 		if (ret != -ENODEV) {
> 			if (ret == -EPROBE_DEFER)

I guess that would be != -EPROBE_DEFER

> 				dev_err(lvds->dev, "failed to request regulator: %d\n", ret);
> 			return ret;
> 		}
> 
> 		lvds->supply = NULL;
> 	}

Otherwise, it works for me.

> My preference, however, would be for devm_regulator_get_optional() to return 
> NULL when no regulator is present. The current implementation returns -ENODEV 
> in multiple cases, making it impossible to properly discriminate between 
> having no regulator and not being able to get the regulator due to an error.

It would feel more intuitive to me too, but it would also require to
fix most of the call sites that would have a similar pattern.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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