linux-kernel - Re: [PATCH] drm: panel: simple-panel: get the enable gpio as-is

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Message-ID: <1478528816.2461.25.camel@pengutronix.de>
Date:   Mon, 07 Nov 2016 15:26:56 +0100
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Chen-Yu Tsai <wens@...e.org>, Icenowy Zheng <icenowy@...c.xyz>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm: panel: simple-panel: get the enable gpio as-is

Am Montag, den 07.11.2016, 14:17 +0100 schrieb Thierry Reding:
> On Mon, Nov 07, 2016 at 06:12:43PM +0800, Chen-Yu Tsai wrote:
> > On Sun, Nov 6, 2016 at 7:09 PM, Icenowy Zheng <icenowy@...c.xyz> wrote:
> > > The enable gpio of simple-panel may be used by a simplefb or other
> > > driver on the panel's display before the KMS driver get load.
> > >
> > > Get the GPIO as-is, so the panel won't be disabled, and the simplefb
> > > can work.
> > >
> > > Signed-off-by: Icenowy Zheng <icenowy@...c.xyz>
> > > ---
> > >  drivers/gpu/drm/panel/panel-simple.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> > > index 113db3c..ccee4c1 100644
> > > --- a/drivers/gpu/drm/panel/panel-simple.c
> > > +++ b/drivers/gpu/drm/panel/panel-simple.c
> > > @@ -312,7 +312,7 @@ static int panel_simple_probe(struct device *dev, const struct panel_desc *desc)
> > >                 return PTR_ERR(panel->supply);
> > >
> > >         panel->enable_gpio = devm_gpiod_get_optional(dev, "enable",
> > > -                                                    GPIOD_OUT_LOW);
> > > +                                                    GPIOD_ASIS);
> > 
> > The GPIO requested as-is might be in input mode. You should change the
> > gpiod_set_value calls to gpiod_direction_output calls. The later also
> > allows you to give an initial value. Not sure if it checks for cansleep
> > like the set_value calls though.
> 
> I'd prefer not to add gpiod_direction_output() calls outside of
> ->probe(). Instead, could we make this patch be smart about taking over
> from an earlier user? Could it read the current direction and value of
> the GPIO and not disable it if it had previously been enabled?

Seconded, the PWM backlight driver in drivers/video/backlight/pwm_bl.c
already does a similar thing.

> And even if we go this extra mile there's a possibility that the GPIO
> was just left dangling by earlier software (or hardware) and leaving it
> on would actually be worse than turning the panel off.

Is this something the encoder driver should communicate to the panel?
That one will know whether its atomic_reset state is enabled or
disabled.

regards
Philipp