linux-kernel - Re: [PATCH] backlight: led_bl: fix initial power state

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Date:   Tue, 4 Jul 2023 19:07:31 +0200
From:   Sam Ravnborg <sam@...nborg.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Mans Rullgard <mans@...sr.com>, linux-fbdev@...r.kernel.org,
        Jingoo Han <jingoohan1@...il.com>,
        Helge Deller <deller@....de>, Lee Jones <lee@...nel.org>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Tomi Valkeinen <tomi.valkeinen@...com>
Subject: Re: [PATCH] backlight: led_bl: fix initial power state

Hi Daniel,

> > @@ -200,8 +200,8 @@ static int led_bl_probe(struct platform_device *pdev)
> >  	props.type = BACKLIGHT_RAW;
> >  	props.max_brightness = priv->max_brightness;
> >  	props.brightness = priv->default_brightness;
> > -	props.power = (priv->default_brightness > 0) ? FB_BLANK_POWERDOWN :
> > -		      FB_BLANK_UNBLANK;
> > +	props.power = (priv->default_brightness > 0) ? FB_BLANK_UNBLANK :
> > +		      FB_BLANK_POWERDOWN;
> 
> The logic was wrong before but I think will still be wrong after the
> change too (e.g. the bogus logic is probably avoiding backlight flicker
> in some use cases).
> 
> The logic here needs to be similar to what pwm_bl.c implements in
> pwm_backlight_initial_power_state(). Whilst it might be better
> to implement this in led_bl_get_leds() let me show what I mean
> in code that fits in the current line:
> 
> 	/*
> 	 * Activate the backlight if the LEDs are already lit *or*
> 	 * there is no phandle link (meaning the backlight power
> 	 * state cannot be synced with the display state).
> 	 */
> 	props.power = (active_at_boot || !dev->node->phandle) ?
> 			FB_BLANK_UNBLANK : FB_BLANK_POWERDOWN;
> 
The following code does the same using helpers:

	if (active_at_boot || !dev->node->phandle))
		backlight_enable(bd);
	else
		backlight_disable(bd);

The code needs to execute after backlight_device_register() so maybe not
so great an idea?!?

	Sam