linux-kernel - Re: [PATCH] Enable backlight when trigger is activated

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Date:   Mon, 22 Jul 2019 22:25:05 +0200
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Pavel Machek <pavel@....cz>,
        Ezequiel Garcia <ezequiel@...labora.com>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-omap@...r.kernel.org, tony@...mide.com, sre@...nel.org,
        nekit1000@...il.com, mpartap@....net, merlijn@...zup.org,
        linux-leds@...r.kernel.org, b.zolnierkie@...sung.com,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org
Subject: Re: [PATCH] Enable backlight when trigger is activated

Hi Pavel,

On 7/22/19 9:50 AM, Pavel Machek wrote:
> Hi!
> 
>>> Configuring backlight trigger from dts results in backlight off during
>>> boot. Machine looks dead upon boot, which is not good.
>>>
>>> Fix that by enabling LED on trigger activation.
> 
>>> +++ b/drivers/leds/trigger/ledtrig-backlight.c
>>> @@ -114,6 +114,8 @@ static int bl_trig_activate(struct led_classdev *led)
>>>  	n->old_status = UNBLANK;
>>>  	n->notifier.notifier_call = fb_notifier_callback;
>>>  
>>> +	led_set_brightness(led, LED_ON);
>>> +
>>
>> This looks fishy.
>>
>> Maybe you should use a default-state = "keep" instead? (and you'll have
>> to support it in the LED driver).
>>
>> That'll give you proper "don't touch the LED if it was turned on" behavior,
>> which is what you seem to want.
> 
> Actually no, that's not what I want. LED should go on if the display
> is active, as soon as trigger is activated.
> 
> Unfortunately, I have see no good way to tell if the display is
> active (and display is usually active when trigger is activated).

default-state DT property can be also set to "on"
(see Documentation/devicetree/bindings/leds/common.txt).

You could make use of LED_INIT_DEFAULT_TRIGGER flag and
parse DT property in the activate op. Similar approach has been
applied e.g. in ledtrig-pattern.c.

-- 
Best regards,
Jacek Anaszewski