Comment 10 for bug 872652

> You should be able to run this sequence from a terminal window on your external monitor (or while logged in to the laptop
> from another machine) -- it will reflect and affect the laptop display brightness. When the laptop screen is stuck in its dark
> state after resume, does the 'brightness' value below display as zero? Does that value seem to change when you press the
> brightness up/down keys?:
>
> $ cd /sys/class/backlight/intel_backlight
> $ cat max_brightness
> {note the max brightness value (it won't ever change)}
> $ cat brightness
> {note the current brightness value; it *should* reflect the laptop screen backlight brightness, from 0 (off) to the max
> value}
>
> You can change the brightness value manually as follows. Does changing it actually affect the screen brightness (once
> you're in the stuck dark state) or not?
>
> $ cd /sys/class/backlight/intel_backlight
> $ echo {somevalue} | sudo tee brightness
> Replace "{somevalue}" with the max brightness value.

I am back in 3.0.0-12-generic with acpi_backlight=vendor.

Here is a sequence that you might find interesting:

1. xset s activate # causes display to "hang"
2. log in from another machine, see that /sys/class/backlight/intel_backlight/brightness is in fact 0
3. changing the brightness level from this file does NOT bring back display
4. close laptop lid, wait for suspend, open lid display turns back on after resuming
5. on remote machine, manipulating brightness file DOES affect display brightness level and I am able to take it all the way to 0 and then back up again WITHOUT causing a display hang.

It appears that something about the screensaver activation (maybe dpms activation) is preventing the display from resuming, not just bringing the brightness all the way down to 0.