Re: extent brightness adjust in 2.6.18?

[timotheus <pbb...@ts...>]

Resending due to local mail issues. Sorry if this becomes a duplicate post.

>> 
>> 1. Do you see more than one entry in /sys/class/backlight?
>

For me, I have a PowerBook5,2 running kernel 2.6.18 (with unrelated
patches).

>corsac@hidalgo: ls -R /sys/class/backlight                           [~]
>/sys/class/backlight:
>total 0
>drwxr-xr-x 2 root root 0 1904-01-01 00:10 radeonbl0
>
>/sys/class/backlight/radeonbl0:
>total 0
>-r--r--r-- 1 root root 4.0K 2006-09-24 12:20 actual_brightness
>-rw-r--r-- 1 root root 4.0K 2006-09-24 12:20 brightness
>-r--r--r-- 1 root root 4.0K 2006-09-24 12:20 max_brightness
>-rw-r--r-- 1 root root 4.0K 2006-09-24 12:20 power
>lrwxrwxrwx 1 root root    0 2006-09-24 12:20 subsystem
>-> ../../../class/backlight

Same for me.

>
>> 2. If so, what are they for? 
>> 3. Has the keyboard backlight an entry in /sys/class/backlight too?
>
>No it seems not (but maybe I'm lacking some config option)
>
>>    I studied the kernel source and found no clue for that but I might
>>    have missed something.

My subjective impression from the code and recent kernel trends:

The keyboard is unrelated.  This is a kernel reworking to have a
backlight class for all LCD backlights.  Radeon is one of the first,
but not to be the only, driver with the same backlight sysfs
interface. This allows independence of the PMU driver and merging of
PC and Mac code. This information is somewhat mentioned in the driver
help (make menuconfig , help items).

I have no part in developing this driver, but from experimentation
I have determined the following.

The 'actual_brightness' file is read-only and seems to always have the
same value as 'brightness'.

The 'brightness' file is writable and immediately adjusts the
LCD. Appropriate values are 0 (full dark), 1 (min. brightness), to the
value of 'max_brightness', which is 127 on this laptop.

'max_brightness' is the maximum value allowed to be written to
'brightness', otherwise an error is displayed on the terminal:

    # echo 128 > brightness
    bash: echo: write error: Invalid argument

'power' has the value of 0, seemingly always.



Relation to current pbbuttonsd release 0.7.8:

pbb value  ,	   sysfs-brightness value

1	   ,	   8
2	   ,	   16
3	   ,	   24
4	   ,	   32
5	   ,	   40
6	   ,	   48
7	   ,	   56
8	   ,	   64
9	   ,	   72
10	   ,	   80
11	   ,	   88
12	   ,	   96
13	   ,	   104
14	   ,	   112
15	   ,	   120



The sysfs brightness value *does* smoothly adjust the LCD by each
increment from 0 to 127, as demonstrated with the command:

# (for n in `seq 1 127`; do echo $n > brightness; sleep .05s; done)


One interesting note.  Pbbuttonsd 0.7.x has a bug that sometimes
occurs by leaving the machine idle for hours and then returning to
using it.  The display remains at maximum brightness and
does not respond to F1/F2 adjustments until using these keys to adjust
to full-dark, and then increasing brightness again.

When this bug is active, the display brightness value is '127'.  But,
the maximum value pbbuttonsd sets by using F1/F2 is '120'.  I assume
that this bug will disappear when using the sysfs interface anyways,
but thought it interesting.

Best regards,
timotheus