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

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Message-ID: <20230705140731.GB6265@aspen.lan>
Date:   Wed, 5 Jul 2023 15:07:31 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Sam Ravnborg <sam@...nborg.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

On Tue, Jul 04, 2023 at 07:07:31PM +0200, Sam Ravnborg wrote:
> 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?!?

It would introduce a need for a bunch of new locks since userspace
could get in between device creation and the call to the helpers.

Additionally, it is even correct for a driver to forcefully set
fb_blank to powerdown during the probe? All current drivers set
fb_blank to unblank during the probe.


Daniel.