[Bug 573984] Re: Screen brightness can not be adjusted in Lucid with Intel Arrandale graphics

David Boersma david.boersma at gmail.com
Mon Jun 14 11:02:46 UTC 2010


Hi all,

I think saghauer may be right, the screen brightness is probably not a
graphics card issue, as it is just the setting of the brightness of the
backlight. Still, under /proc/acpi, the brightness is listed under vga.
In particular:

cat /proc/acpi/video/VGA/LCDD/brightness 
levels:  5 11 17 23 29 35 41 47 53 59 65 72 79 86 93 100
current: 47

when get root permissions (sudo bash -l) then i can try to change the
brightness using e.g.:

echo -n 5 >  /proc/acpi/video/VGA/LCDD/brightness

that works. but strange enough, not all listed valid values work. e.g., setting it to 100 gives this error:
bash: echo: write error: Invalid argument

now when i loop over all integer values from 0 to 1024, and try setting
the brightness to each of those values, then I see that only the first
half works (5 11 17 23 29 35 41 47) and within the tested range there
are no other valid values. another weirdity is that the actual
brightness poorly correlates with value to which i have set it. This is
the list of valid values, sorted to apparent brightness (darkest to
brightest):

17 5 35 23 41 11 29 47

i do not see any difference in apparent brighness between settings 5 and
35; if there actually is a difference then they might need to be
swapped.

there is probably a deep reason for this apparent randomization (bit
inversion? big endian versus little endian?) but how exactly is not
obvious to me. no time for further investigation. i hope that this
observation at least provides some helpful clue/diagnostic to any expert
who happens to read this post.

HTH,
David

-- 
Screen brightness can not be adjusted in Lucid with Intel Arrandale graphics
https://bugs.launchpad.net/bugs/573984
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