Turn off Backlight on Shutdown

[iglesiasmarianod]

Hi guys, Is there a way to turn off the backlight before the cubieboard shuts down?
My image shows a strange fading screen on shutdown. If the backlight was off it would not show.
It is something similar to what happened at satrtup before @kedder fixed the logo sequence.

Thanks!

[kedder]

It used to work on kernel 5.2 (backlight turned off on shutdown), but after upgrading to 5.7 it is no longer happening (in fact, it backlight turns of sometimes, but not always)

[iglesiasmarianod]

Thanks again @kedder!
Do you think adding these lines to the .dts file might help?

	backlight: lcd {
		compatible = "pwm-backlight";
		pinctrl-names = "default";
		pwms = <&pwm 0 5000000 0>;
		brightness-levels = <0 8 16 32 64 128 255 384 512 768 1024>;
		default-brightness-level = <10>;
		post-pwm-on-delay-ms = <10>;
		pwm-off-delay-ms = <10>;
	};

[kedder]

Hm, I believe backlight: lcd is already on the .dts, except post-pwm-on-delay-ms and pwm-off-delay-ms. TBH, I have no idea whether they might help or not. Do you want me to try? Did you try?

[iglesiasmarianod]

Just asked perhaps you knew before trying.
I added both lines

post-pwm-on-delay-ms = <10>;
pwm-off-delay-ms = <10>;

I'm building the image. No errors so far.
I'll let you know if it works when I load it to the OpenVario.

[iglesiasmarianod]

It did not help.
I'll keep looking in case I find something useful.

[kedder]

I've discovered something funny in relation to this issue:

The backlight power can be controlled by writing 0 or 4 to /sys/devices/platform/lcd/backlight/lcd/bl_power file. 0 means "power on", 4 means "power off" (according to https://www.kernel.org/doc/Documentation/ABI/stable/sysfs-class-backlight).

So, if you do echo 4 > /sys/devices/platform/lcd/backlight/lcd/bl_power the screen is supposed to go dark, and with echo 0 > ... screen should go back bright.

However, this is not what happens on my openvario. The backlight turns off only sometimes. And the probability of it turning off seems to correlate with screen brightness itself: the higher the brightness, the less often screen goes dark when I write 4 to bl_power. When I set the screen to full brightness, it never goes dar. On 50% of the brightness it goes dark about half of the times. Interestingly, when screen does not go dark, it goes to full brightness.

I assume, when system shuts down, it effectively does the same as writing 4 to bl_power, so screen does not turn dark.

This all feels like a bug, just not clear where exactly.

[kedder]

It feels like the line, controlled by pwm stays in whatever state it was the moment backlight was turned down - either full 1 or 0. Even though the pwm backlight driver explicitly asks to set the duty cycle to 0:

https://github.com/torvalds/linux/blob/master/drivers/video/backlight/pwm_bl.c#L81-L83

It might be that pwm-sun4i module does not execute that instruction very precisely. Sigh, where can I even discuss that? :(

[iglesiasmarianod]

Great Andrey! What do you think about writing 0 brightness with the shutdown script and writing previous brightness at menu startup? Would that impact Uboot backlight settings? I know it is not the best solution. It would only be a patch.

…

On Wed, Aug 26, 2020, 7:00 PM Andrey Lebedev ***@***.***> wrote: It feels like the line, controlled by pwm stays in whatever state it was the moment backlight was turned down - either full 1 or 0. Even though the pwm backlight driver explicitly asks to set the duty cycle to 0: https://github.com/torvalds/linux/blob/master/drivers/video/backlight/pwm_bl.c#L81-L83 It might be that pwm-sun4i module does not execute that instruction very precisely. Sigh, where can I even discuss that? :( — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#103 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AKZLFH7CUBJVHBIM6Z2NC73SCWAYPANCNFSM4PYX5G2A> .

[kedder]

I'm not a huge fan of that, because one day the screen will simply not turn on (for whatever reason). I'd rather live with the screen that does not turn off on shutdown. But I still think this can be fixed or worked around somehow.

[iglesiasmarianod]

You are right. I didn't think about that.

[kedder]

Submitted the bug report to linux devs: https://www.spinics.net/lists/linux-pwm/msg13689.html

[kedder]

It looks like we have a fix for this: http://lists.infradead.org/pipermail/linux-arm-kernel/2020-September/597482.html, going to submit a PR soon