Re: samsung-laptop backlight control not working

From: David Herrmann
Date: Thu May 19 2011 - 14:54:18 EST

On Thu, May 19, 2011 at 8:27 PM, Greg KH <gregkh@xxxxxxx> wrote:
> On Thu, May 19, 2011 at 08:18:16PM +0200, David Herrmann wrote:
>> Hi
>> I tried the new samsung-laptop driver with samsung backlight support.
> What kernel are you trying?  We have some recent patches to the driver
> to try to resolve some of these issues on your laptop.

I use linus' tree. 2.6.39

>> It is detected as:
>> [109282.931433] samsung_laptop: found laptop model 'N150P/N210P/N220P'
>> However, backlight support is kind of weird:
>> "echo 1 >/sys/class/backlight/samsung/brightness"
>> does not set brightness to 1 but instead reduces brightness for 1.
>> That is, if my current brightness setting is 8, I need to call "echo 1
>> >..." 7 times to get brightness value 1.
>> "cat /sys/class/backlight...."
>> returns 1 after the first try, though.
>> "echo 3 >..."
>> reduces brightness also for 1 but stops at brightness level 3 if I
>> repeat the command.
>> dmesg shows the following for every "echo" I call:
>> [110690.073783] ACPI: Failed to switch the brightness
>> Furthermore "echo 0 >..." doesn't disable backlight but instead
>> behaves like a brightness setting below 1.
> Yeah, don't echo 0 there, that doesn't work to well.  I have some
> patches here that you can try if you want, just let me know.

That would be great! I have really no idea about acpi so I can't fix
it myself. If you could give me some git tree where I can find them, I
could test them.

>> However, I know the following works on my machine:
>> setpci -s 00:02.0 F4.B=255
>> It accepts a setting between 0-255 and 255 is full brightness and 0 is
>> backlight off.
> Don't do that, the video driver will not like you as you are changing
> things without telling the bios that something is really happening.
> Of course you can continue to do this, it's just strongly not
> recommended.

Ouh, I didn't know that. It was recommended in some distro wiki so I
tried it and it worked. Anyway, I thought it might give you some
information about the N210 model.

> thanks,
> greg k-h

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