linux-kernel - Re: [PATCH] backlight: gpio-backlight: Set power state instead of brightness at probe

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Date:   Tue, 18 Jun 2019 14:58:47 +0200
From:   Paul Kocialkowski <paul.kocialkowski@...tlin.com>
To:     dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Lee Jones <lee.jones@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH] backlight: gpio-backlight: Set power state instead of
 brightness at probe

Hi,

On Fri, 2019-05-17 at 17:05 +0200, Paul Kocialkowski wrote:
> On a trivial gpio-backlight setup with a panel using the backlight but
> no boot software to enable it beforehand, we fall in a case where the
> backlight is disabled (not just blanked) and thus remains disabled when
> the panel gets enabled.
> 
> Setting gbl->def_value via the device-tree prop allows enabling the
> backlight in this situation, but it will be unblanked straight away,
> in compliance with the binding. This does not work well when there was no
> boot software to display something before, since we really need to unblank
> by the time the panel is enabled, not before.
> 
> Resolve the situation by setting the brightness to 1 at probe and
> managing the power state accordingly, a bit like it's done in
> pwm-backlight.

Any feedback on this? I was under the impression that it could be quite
controversial, as it implies that the backlight can no longer be
enabled without a bound panel (which IMO makes good sense but could be
a matter to debate).

Cheers,

Paul

> Fixes: 8b770e3c9824 ("backlight: Add GPIO-based backlight driver")
> Signed-off-by: Paul Kocialkowski <paul.kocialkowski@...tlin.com>
> ---
>  drivers/video/backlight/gpio_backlight.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/video/backlight/gpio_backlight.c b/drivers/video/backlight/gpio_backlight.c
> index e470da95d806..c9cb97fa13d0 100644
> --- a/drivers/video/backlight/gpio_backlight.c
> +++ b/drivers/video/backlight/gpio_backlight.c
> @@ -57,6 +57,21 @@ static const struct backlight_ops gpio_backlight_ops = {
>  	.check_fb	= gpio_backlight_check_fb,
>  };
>  
> +static int gpio_backlight_initial_power_state(struct gpio_backlight *gbl)
> +{
> +	struct device_node *node = gbl->dev->of_node;
> +
> +	/* If we absolutely want the backlight enabled at boot. */
> +	if (gbl->def_value)
> +		return FB_BLANK_UNBLANK;
> +
> +	/* If there's no panel to unblank the backlight later. */
> +	if (!node || !node->phandle)
> +		return FB_BLANK_UNBLANK;
> +
> +	return FB_BLANK_POWERDOWN;
> +}
> +
>  static int gpio_backlight_probe_dt(struct platform_device *pdev,
>  				   struct gpio_backlight *gbl)
>  {
> @@ -142,7 +157,9 @@ static int gpio_backlight_probe(struct platform_device *pdev)
>  		return PTR_ERR(bl);
>  	}
>  
> -	bl->props.brightness = gbl->def_value;
> +	bl->props.brightness = 1;
> +	bl->props.power = gpio_backlight_initial_power_state(gbl);
> +
>  	backlight_update_status(bl);
>  
>  	platform_set_drvdata(pdev, bl);
-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com