Addressing the intel VCH on the i2c bus / R31 dithering

From: Thomas Richter <richter@tik.uni-stuttgart.de>
To: intel-gfx <intel-gfx@lists.freedesktop.org>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Ville Syrjälä" <ville.syrjala@linux.intel.com>
Subject: Addressing the intel VCH on the i2c bus / R31 dithering
Date: Fri, 27 Mar 2015 09:03:35 +0100	[thread overview]
Message-ID: <55150ED7.7050809@tik.uni-stuttgart.de> (raw)

Hi folks, hi Daniel, hi Ville,

thanks again for getting my old Laptops (the R31 and the S6010) back to
life. It's been quite a way, but everything looks fine now.

There is one interesting observation I made, though, on my IBM R31
laptop: The i830GM graphics supports 24bpp pipes, thought the panel is
only a 6-bit panel, i.e. 18bpp. As you say, the i830GM does not support
dithering, but interestingly, as I found out by accident, the iVCH DVO
chip in the laptop does. If you press FN+F8 (usually to control the
scaling of the display per bios), the image quality improves
dramatically, apparently by reconfiguring the settings of the iVCH
bypassing the kernel. This is handled by the Bios.

Now, I would like to find the settings the Bios leaves in the iVCH.
Unfortunately, there is a catch here: According to the kernel sources,
the iVCH is connected to the i2c-bus, but at address #2. My knowledge of
i2c is limited, but from what I know, this is actually not a valid
address and is rather used for some control mechanism of i2c itself.
Hence, the usual i2c tools do not allow me to reach out for the chip and
read its registers.

Question:

How can possibly read out the settings of the i2c chip with system
tools? I know the kernel does a register dump of what it installs during
DVO initialization, and have these values, but what I don't have are the
values the bios leaves when I press FN+F8.

Thanks for any ideas,
	Thomas

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