[Bug 105267] Screen flickering after getting hpd irq

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=105267

--- Comment #15 from Jani Nikula <jani.nikula at intel.com> ---
Thanks for testing.

The problem with the patch referenced in comment #10 is that will regress older
platforms. We can't do that.

The background is that we have tried to use optimal link parameters, and we
have tried to optimize for both fewer lanes with higher rate, and more lanes
with lower rate. All of this failed, until we learned that, uh, a certain other
OS always used the maximum link parameters reported by the display. Apparently
that was the only configuration that the panel/laptop vendors then ended up
validating. We switched to using max link rate and lane count, which presumably
correspond to the native resolution of the display anyway, and we haven't had
issues with that approach until now.

Arguably all the displays should work with all the lane counts and rates they
report, but sadly this appears not to be the case. In this bug, the display
does not work with the maximum parameters it reports.

Apparently nobody has double checked the DDI buffer translation and voltage
swing etc. parameters that I suggested in comment #5. :(

eDP 1.4 also adds two somewhat related features. Link rate select to support
more intermediate rates between what's available for DP. DSC to support stream
compression.

1) Please attach /sys/kernel/debug/dri/0/i915_vbt. The VBT is supposed to
contain the port specific maximums, but perhaps that's not being used.

2) Please see that you have CONFIG_DRM_DP_AUX_CHARDEV=y, and try to use
/dev/drm_dp_auxN node to hexdump the DPCD, and attach them.

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