linux-kernel - Re: [PATCH] drm/panel: simple: Support reset GPIOs

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Date:   Mon, 16 Dec 2019 14:06:15 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Sam Ravnborg <sam@...nborg.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        Maxime Chevallier <maxime.chevallier@...tlin.com>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH] drm/panel: simple: Support reset GPIOs

Hi,

On Fri, Dec 13, 2019 at 07:13:25PM +0100, Miquel Raynal wrote:
> The panel common bindings provide a gpios-reset property which is
> active low by default. Let's support it in the simple driver.
>
> De-asserting the reset pin implies a physical high, which in turns is
> a logic low.
>
> Signed-off-by: Miquel Raynal <miquel.raynal@...tlin.com>

The GPIOd API asks for a logical state, so it doesn't really matter
what the polarity of the GPIO, OUT_LOW will always mean that the reset
is deasserted (and thus, it will work even if the reset pin is active
high).

> ---
>  drivers/gpu/drm/panel/panel-simple.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> index 5d487686d25c..15dd495c347d 100644
> --- a/drivers/gpu/drm/panel/panel-simple.c
> +++ b/drivers/gpu/drm/panel/panel-simple.c
> @@ -110,6 +110,7 @@ struct panel_simple {
>  	struct i2c_adapter *ddc;
>
>  	struct gpio_desc *enable_gpio;
> +	struct gpio_desc *reset_gpio;
>
>  	struct drm_display_mode override_mode;
>  };
> @@ -433,12 +434,21 @@ static int panel_simple_probe(struct device *dev, const struct panel_desc *desc)
>  	if (IS_ERR(panel->supply))
>  		return PTR_ERR(panel->supply);
>
> +	panel->reset_gpio = devm_gpiod_get_optional(dev, "reset",
> +						    GPIOD_OUT_LOW);
> +	if (IS_ERR(panel->reset_gpio)) {
> +		err = PTR_ERR(panel->reset_gpio);
> +		if (err != -EPROBE_DEFER)
> +			dev_err(dev, "failed to request reset pin: %d\n", err);
> +		return err;
> +	}
> +

However, I'm wondering if it wouldn't be better to just have the
device maintained in reset at probe (so OUT_HIGH) and moved out of
reset during either the prepare or enable callbacks.

This is pretty much what is happening with the enable-gpios already.

Also, panels usually need to wait for a minimum time after you
deassert the reset line. How is that dealt with?

I guess a good way to do that would be to add that duration to the
panel description, since this is pretty much device specific.

Thanks!
Maxime

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