Radeon drm: dpms backlight problem / question / bug?

[Prof. Dr. Klaus Kusche]

On my Dell Precision M6500, backlight power switching does not work,
neither by using bl_power in /sys/backlight/... nor by using DPMS:
* Any changes of bl_power are simply and silently ignored
   (no effect, except that brightness is set to minimum),
   although brightness tuning works fine
   (in other words: Both the generic ACPI BIOS driver and the in-kernel
   Dell laptop backlight driver can control backlight brightness,
   but not backlight power).
* If DPMS becomes active, the screen contents on the internal display
   slowly becomes garbled or fades away (so video output drivers
   seem to be off), but the backlight remains on
   (which is very bad for battery power).
   The external screen (on display port) behaves even worse:
   It enters energy saving mode (backlight off) and immediately wakes up
   again, continuously cycling the backlight power every few seconds.

Another problem which optically looks the same:
Zapping the X server (Xorg 1.9.1, radeon 6.13.2) with Ctrl-Alt-Bksp
leaves the system in an undefined state:
* Similar to DPMS, the display fades away or gets garbled
   (video output off), but the backlight remains on.
* It does not switch to the text console automatically,
   nor is it possible to switch manually with Ctrl-Alt-Fn.

Configuration:
* Kernel 2.6.35.7-grsec
* Radeon DRM with KMS
* Radeon framebuffer with backlight control enabled
* The Radeon chip is a JUNIPER 0x1002:0x68A0,
   the kernel should contain all the microcode needed for it.
* The displays are LCD1: INTERNAL_UNIPHY and DFP1: INTERNAL_UNIPHY1

Questions:
* Is this configuration expected to work,
i.e. is DPMS expected to be able to control the backlight power?
* If it is, what should I check or try next,
what information do you need?
* If it is known not to work, is there any chance that the situation
changes in the forseeable future?

Many thanks in advance for your help!

-- 
Prof. Dr. Klaus Kusche
Private address: Rainstraße 9/1, 88316 Isny, Germany
+49 7562 6211377 Klaus.Kusche at computerix.info http://www.computerix.info
Office address: NTA Isny gGmbH, Seidenstraße 12-35, 88316 Isny, Germany
+49 7562 9707 36 kusche at nta-isny.de http://www.nta-isny.de