Comment 20 for bug 573984

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