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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <yw1xo7krzo9k.fsf@mansr.com>
Date:   Tue, 04 Jul 2023 16:31:51 +0100
From:   Måns Rullgård <mans@...sr.com>
To:     Daniel Thompson <daniel.thompson@...aro.org>
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

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?

-- 
Måns Rullgård