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

[Daniel Vetter]

On Tue, Apr 16, 2013 at 09:53:24AM +0200, "David Müller (ELSOFT AG)" wrote:
> 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

I guess we should force bit-banging like we already do for sdvo outputs,
see the calls to intel_gmbus_force_bit in intel_sdvo.c. Can you please
write such a patch, test it on your machine (I don't have any gen2 dvo
machines) and then submit it for inclusion?

Please cc me on the patch (and please also cc the intel-gfx mailing list)
so it doesn't get lost.

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch