[Intel-gfx] [PATCH] Revert "drm/i915: Implement Link Rate fallback on Link training failure"

[Ville Syrjälä]

On Wed, Mar 01, 2017 at 07:35:15PM +0200, Jani Nikula wrote:
> On Wed, 01 Mar 2017, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > On Wed, Mar 01, 2017 at 06:17:49PM +0100, Daniel Vetter wrote:
> >> This reverts commit 233ce881dd91fb13eb6b09deefae33168e6ead4c.
> >> 
> >> I assumed it's ok, but really should have double-checked - CI caught
> >> tons of fail :(
> 
> Considering the velocity of drm-tip, I think any CI results for patches
> have a rather limited best before date. The patch should've been resent
> and gone through testing again before merging.
> 
> > For the record, the failure comes from the error message in
> > intel_dp_get_link_train_fallback_values() as take the fallback path. As
> > userspace is informed, we don't need an *ERROR* at that point.
> >
> > The really interesting question is why we are seeing link-training
> > failures in CI at all, and whether igt should be checking and reporting
> > link-status=BAD.
> 
> It's possible (I didn't check the logs) this pertains to the failure
> mode I've sometimes seen, where clock recovery fails, but as we continue
> with channel equalization anyway (without this patch), everything
> succeeds there. At worst we need to root cause and fix that issue
> first. :(

The skl case seems pretty clear. We register DP for both port A and port
E even though we should register it only for port E (I think). They
both end up both using AUX A and so we think the same sink is connected
to both, and then we try to enable port A which fail for obvious reasons.

The culprit is init_vbt_defaults() which always sets .supports_dp=true
for port A unless later overridden by the VBT. In this case the VBT has
no port A, so we leave the .supports_dp flag set. So presumably we
should just nuke this stuff from init_vbt_defaults().

IIRC this was discussed at some point between Imre and Paulo, but I
can't remember what the conclusion was, or if in fact there was one.

The ilk failure case is a lot less clear. It's one of those cases
where the sink just keeps requesting the same vswing/preemph all
the time. I've seen it sometime in the past, but I've never been
able to figure out what has caused it to happen with any specific
sink.

-- 
Ville Syrjälä
Intel OTC