fickle backlight goes off permanently

[Ali Bahar <alih@internetDog.org>]

Unless if you're ineterested in the control mechanism of an ibook
backlight, this post may not interest you.


Hi,

the backlight of my ibook has become increasingly fickle over the last
10 days. It turns off with slight movements or with slight tilts of
the screen, and does so permanently: fblevel, or /sbin/backlight, do
not undo it; only a reboot does.

This problem reproduces on linux but not on osx. I suspect a hw
problem -- most likely a loose connection -- which is being handled by
osx, but not by linux.

The ibook will be taken in for repair in a few days, but I would
appreciate any pointers in how to debug this. I'm completely
unfamiliar with the source code of the display driver and related
utilities. But if anyone has a clue of how to debug this, I'd be happy
to run some tests, add any debug code you could think of, re-build and
try out different things.

I'm an embedded developer, so I ought to be of some use in this. ;-)

Thanks much,
ali





Additional info:

This is a 6-months old (dual-USB, 2001-05, v9.1) ibook. It dual-boots
osx and linux (debian woody, last upgraded in late December, I
think). This runs a main branch (ie non-benh) 2.4.10 kernel.

Some observations:

  - all was well until 10 days ago. No sw/hw changes had taken place
    for months. 

  - happens on both X and console;

  - it's independent of sleep. Disabling pmud will still produce the
    bug;

  - I do manage to (intermittently/often) reproduce this during the
    booting of osx. Closing the lid any/most time before the osx
    splash screen disappears (ie by the time osx finishes its system
    configuration), will reproduce this bug. 

      - osx will complete its init normally;
      - prior to 10 days ago, interfering with booting in this manner,
        did not cause such a problem.

    The problem never occurs under normal osx runtime;

  - other operations (in both osx/linux) are unaffected. Just the
    backlight is off;

  - /var/log/* have not shown any odd behaviour. But I concluded this
    by doing before/after comparisons, not thru knowledge of what
    ought to be there;

  - fblevel works at other times. But it does not help recover from
    this situation.

-- 
	Linux: The Most Beautiful OS in the World.


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