[Intel-gfx] Addressing the intel VCH on the i2c bus / R31 dithering

[Daniel Vetter]

On Fri, Mar 27, 2015 at 09:03:35AM +0100, Thomas Richter wrote:
> 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.

i2c uses 7 bits for address and bit 8 for read/write indication. There's a
bit confusion between tools and kernel about whether to include the
read/write bit or not, so maybe try with 1 instead?

Plan b would be to extend the ivch_dump_regs and wire that up into a
debugfs file. Then you could dump the registers through the kernel
before/after pressing the magic key.

but tbh I don't have much i2c clue either.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch