Re: Brightness and Contrast controls on iMacs

[Segher Boessenkool <segher@koffie.nl>]

Benjamin Herrenschmidt wrote:
> 
> On Fri, 2003-01-10 at 11:14, Scott McMahan wrote:
> > I'm looking for a way to adjust the screen settings on my iMac.  I've
> > search the list/newsgroups and the answers I've found don't work on CRTs
> > (fblevel) or don't really address the right problem (xgamma).  Is there
> > a utility available to handle this problem?  If not, is anyone working
> > on one?  If not, can anyone help point me in the right direction so I
> > can start working on one?
> 
> My understanding is that the screen (brightness, contrast, power
> management and geometry) on iMacs is handled by tapping an i2c
> device. On some iMacs, it's on one of the PMU i2c busses, I'm not
> too sure about older iMacs, it's either on Cuda or on the video
> card's i2c bus.
> 
> There is no real support for PMU i2c currently in the kernel,
> though you can eventually try to feed PMU i2c commands via
> /dev/adb, though I'm not completely sure you'll get the replies
> properly, that's something I have to investigate.

You can send a request and then poll for a reply.  Not sure if it works
via /dev/adb, but I think it will (i2c isn't very timing sensitive :) )

Bit 0x08 of byte 0x0e of the system config says if you have such a crt
controller (all UMA iMacs and eMacs do).

The controller is at i2c address 8c, with an eeprom at a6; the eMac has
an additional thingy at 98.  All at pmu i2c bus 2.  The commands you send
it are two bytes: register, value.  That's about all I can remember
offhand.  So you send the pmu something like

	9a 09 02 00 00 8c 00 00 02 reg val

and read back two bytes for status.

> Most of the informations about the device itself can be found
> in Darwin's source AppleOnboardDisplay module.

Thanks!  I'll go read that.


Cheers,

Segher