This goes to intel-gfx and dri-devel.
I'm outraged! My computer has fell victim to witchcraft against which I find no defense! Hear my case:
I use an HP Laptop, model name Touchsmart tm-2 2010eg [1]. Some background information:
The laptop has a convertible capacitive touchscreen which also features a wacom tablet layer. Like most laptops, the FX function keys have alternative functions assigned to them, which can be accessed by holding the Fn-key (or not, so the Fn-key has to be held to access their regular FX-use, if a special BIOS option is set). F2 and F3 have the special meaninf of decreasing and increasing brightness respectively, henceforth called BINC and BDEC.
There are two graphic cards, a discrete Radeon HD 5450 [1002:68e0] and an Intel onboard [8086:0046] which runs with i915. The whole system is set up for vgaswitcheroo, the default on boot is the i915 which immediately goes into KMS (has to)
Until recently, more specifically: until I had the computer in for repairs to replace the screen for the wacom was faulty - the screen blanked on boot [2]. Though bloody annoying, the cause could at least be somewhat found: ACPI wasn't working properly [3]. Backlight could then be brought back on by pressing BINC - at least when the intel card was switcheroo'ed on. The DIS radeon wouldn't acknowledge backlight at all, once activated.
Since the computer got back from repairs and I installed a brand new system with 39.1 stable (gentoo, by the way), things got ugly:
The screen blanks upon boot as before. Booting 39.1, the BINC is rendered useless. You can press it, but contrary to before, it will not ignite the backlight. The desired backlight brightness increases, but the screen wont ignite unless closed and re-opened, when it goes to the desired brightness.
Now the witchcraft starts: When you boot into 38.8 (mind that before the pc was in for repairs, some 39-next was running) and THEN boot into 39.1 - from a complete power off or just a reboot, it doesn't matter - the BINC works!
BUT it works only for the first time to ignite the backlight. BDEC'ing it to be OFF again, and trying to re-ignite it a second time may work or may not. A pure matter of luck. You may be able to BDEC/OFF the backlight a dozen times in a row and suddenly, the next time, it stops working and wont start working unless you boot into 38.8 again (or, under rare circumstances, until you boot again).
Not crazy enough? Here comes the burner: As already pointed out in [2], xrandr plays a significant role in this drama:
Assuming a situtation in which (39.1) BINC from OFF has ceased to work, there is, besides the method of closing and re-opening the lid, another method which, so far, worked reliably:
Rotating the screen by xrandr
BY EXACTLY +-90 DEGREES
Rotating by anything but 90 degrees will not do, but rotating by exactly 90, from either inverted, left, right, or none to a 90 rotated will work with certainty. If the desired brightness, as indicated through the presses of BINC or BDEC, indicated in /sys/class/backlight/.../brightness device is still 0, the backlight goes to full power when doing so. If the desired brightness is another value, the backlight will go to the desired value.
In case of enlightenment refer to [4], which does not yet all the information provided in this email.
kind regards and thank you, Manday
[1] http://h10025.www1.hp.com/ewfrf/wc/document?docname=c02262928&tmp_task=p... [2] https://bugzilla.kernel.org/show_bug.cgi?id=34582 [3] https://bugs.freedesktop.org/show_bug.cgi?id=36823 [4] https://bugzilla.kernel.org/show_bug.cgi?id=37282