How to link setterm -powerdown to backlight?

[Malvineous]

Thanks so much for this excellent project, and for taking the time to get it merged into the kernel!

I have a Linux framebuffer console (i.e. no X11) using fb_ili9341 and everything is working fine.

I can switch the backlight off using echo 1 > /sys/class/backlight/fb_ili9341/bl_power (and echo 0 to turn it back on again).

However even if I run setterm -powerdown 1 to tell the console to switch off after one minute, nothing happens. I can run setterm -blank force and the screen will immediately blank (go black) but the backlight remains on (which is normal for -blank but shows the setterm command is working correctly).

Is there a way to link in the backlight control with the console's existing power saving functionality?

[notro]

I have chased down the call chain:

setterm -powerdown does this: printf("\033[14;%d]", ctl->opt_pd_min);

In the kernel:

The setterm_command() for 14 does this: vesa_off_interval = min(vc->vc_par[1], 60U) * 60 * HZ;

vesa_off_interval is used in do_blank_screen where it calls the con_blank callback.

For fbdev this callback is fbcon_blank() which calls fb_blank() which in turn calls the fb_blank callback fbtft_fb_blank.
This function mimicks the fbdev core behaviour and returns -EINVAL if there's no fbtft blank callback. fb_ili9341 has no such callback since it has no direct display controller control of the backlight. fb_blank() thus get -EINVAL from the callback and this line is not executed: fb_notifier_call_chain(FB_EVENT_BLANK, &event);

So the backlight subsystem is not told to turn off the backlight, which would have happened in fb_notifier_callback().

I think the solution to this problem is to have fbtft_fb_blank() return zero even if the blank callback is not set:

 static int fbtft_fb_blank(int blank, struct fb_info *info)
 {
 	struct fbtft_par *par = info->par;
- 	int ret = -EINVAL;
+ 	int ret = 0;
 
 	dev_dbg(info->dev, "%s(blank=%d)\n",
		__func__, blank);

Ufortunately I haven't got the time to test this nor to send a patch to fix it.

[Malvineous]

Wow that's really informative, thanks so much for the detail! I will see if I can do some testing.

When you say fb_ili9341 has no direct control of the backlight, what code is controlling the backlight in response to /sys/class/backlight/fb_ili9341/bl_power? Is this a separate module or a different part of the code that fbtft_fb_blank() cannot see?

[notro]

fbtft registers a backlight device if the 'led' gpio is set. This happens in fbtft_register_backlight. Userspace controls this device by writing to the file you mentioned.

Call chain: bl_power_store() -> backlight_update_status() -> fbtft_backlight_update_status()

fbdev controls the backlight subsystem using the FB_EVENT_BLANK notifier event.