[i915][855GME][GMBUS] Detection of DVO transmitter unreliable

["David Müller (ELSOFT AG)"]

Hello

I have a 855GME based system with a TFP410 DVO transmitter and running a
vanilla Linux-3.8 kernel.

With the activation of the hardware assisted GMBUS feature (somewhere
near kernel 3.5), the detection of the TFP410 DVO transmitter has become
flacky, which results in an unusable DVO port.

As a first test, i reduced the I2C frequency from 100kHz to 50kHz and
the problem vanished. I used a scope to check the signal quality on the
I2C lines to the DVO transmitter, but everything seems to be within the
specs.

I also checked the whole DVO transmitter initialisation sequence defined
in "intel_dvo.c" and there i noticed that the I2C communication is
completely messed up for 1 or 2 I2C transaction after the receipt of the
first NAK (in my case, this will be for the CH7xxx chip at addr 0x76).

In the "intel_i2c.c" file, there is a remark that one has to be careful
when clearing the NAK condition, otherwise the next I2C transaction may
fail. I played around with this piece of code by adding additional
delays but to no avail so far.

In the same file, there is also a line which disables the hardware
assisted GMBUS functionality for i830 based system.
Does anybody know if the 855GME may be struck by the same problem as the
i830?


Currenty i have figured out two possible work-arounds:
 - add "IS_I85X()" to the list of broken systems
 - reduce the clock rate from 100kHz to 50kHz


Does anybody have an idea what this could be and how to fix it?


Dave