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

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Message-ID: <20230704154051.GC385243@aspen.lan>
Date:   Tue, 4 Jul 2023 16:40:51 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Måns Rullgård <mans@...sr.com>
Cc:     Lee Jones <lee@...nel.org>, Jingoo Han <jingoohan1@...il.com>,
        Helge Deller <deller@....de>, dri-devel@...ts.freedesktop.org,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] backlight: led_bl: fix initial power state

On Tue, Jul 04, 2023 at 04:31:51PM +0100, Måns Rullgård wrote:
> Daniel Thompson <daniel.thompson@...aro.org> writes:
>
> > On Tue, Jul 04, 2023 at 03:07:50PM +0100, Mans Rullgard wrote:
> >> The condition for the initial power state based on the default
> >> brightness value is reversed.  Fix it.
> >>
> >> Fixes: ae232e45acf9 ("backlight: add led-backlight driver")
> >> Signed-off-by: Mans Rullgard <mans@...sr.com>
> >> ---
> >>  drivers/video/backlight/led_bl.c | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/video/backlight/led_bl.c b/drivers/video/backlight/led_bl.c
> >> index 3259292fda76..28e83618a296 100644
> >> --- a/drivers/video/backlight/led_bl.c
> >> +++ b/drivers/video/backlight/led_bl.c
> >> @@ -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;
> >
> > Note that active_at_boot is not the same as (priv->default_brightness > 0)
> > since the value read by led_bl_get_leds() can be clobbered when we
> > parse the properties.
>
> Am I understanding correctly that the code should be using the
> default_brightness value as set by led_bl_get_leds() to determine the
> initial power state, not whatever default value the devicetree provides?

Yes.

The devicetree allows us to specify a default brightness but the DT
value cannot not be used to decide if the backlight has already been
lit up.


Daniel.