i915 4.9 regression: DP AUX CH sanitization no longer working on Asus desktops

[Manasi Navare]

On Thu, May 04, 2017 at 02:52:09PM -0600, Daniel Drake wrote:
> On Thu, May 4, 2017 at 2:37 PM, Ville Syrjälä
> <ville.syrjala at linux.intel.com> wrote:
> > Please check if commit bb1d132935c2 ("drm/i915/vbt: split out defaults
> > that are set when there is no VBT") fixes things for you.
> 
> I think this is not going to help. This would only make a difference
> when there is no VBT at all at which point we would see this message
> in the logs:
> 
>                 DRM_INFO("Failed to find VBIOS tables (VBT)\n");
> 
> but in this case we have a VBT for ports B, C and E.
> 
>  [drm:intel_bios_init [i915]] Port B VBT info: DP:1 HDMI:1 DVI:1 EDP:0 CRT:0
>  [drm:intel_bios_init [i915]] VBT HDMI level shift for port B: 8
>  [drm:intel_bios_init [i915]] Port C VBT info: DP:0 HDMI:1 DVI:1 EDP:0 CRT:0
>  [drm:intel_bios_init [i915]] VBT HDMI level shift for port C: 8
>  [drm:intel_bios_init [i915]] Port E VBT info: DP:1 HDMI:0 DVI:0 EDP:0 CRT:0
>  [drm:intel_bios_init [i915]] VBT HDMI level shift for port E: 0
> 
> Let me know if I'm missing something and we will test it anyway
>

I think now without the "Split out defaults that are set when there is no VBT" patch,
what happens is it enables DP on PORTA by default and then since there is a DP-VGA
adapter on Port E it also enables DP on Port E and since both of these ports use the
same AUX channel, it causes the AUX CH sanitization issues. Atleast that's my guess at triage.

So after the correct VBT parsing, it should actually not enable any AUX transactions
on Port A and detect that no child devices connected to Port A. So the AUX CH should
only be used for Port E and VGA output should work fine.

Manasi

 
> Thanks
> Daniel
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