DisplayPort: handling of HPD events / link training

[Dmitry Baryshkov]

Hello,

We are currently looking at checking and/or possibly redesigning the
way the MSM DRM driver handles the HPD events and link training.

After a quick glance at the drivers implementing DP support, I noticed
following main approaches:
- Perform link training at the atomic_enable time, don't report
failures (mtk, analogix, zynqmp, tegra, nouveau)
- Perform link training at the atomic_enable time, report errors using
link_status property (i915, mhdp8546)
- Perform link training on the plug event (msm, it8605).
- Perform link training from the DPMS handler, also calling it from
the enable callback (AMDGPU, radeon).

It looks like the majority wins and we should move HPD to
atomic_enable time. Is that assumption correct?

Also two related questions:
- Is there a plan to actually make use of the link_status property?
Intel presented it at FOSDEM 2018, but since that time it was not
picked up by other drivers.

- Is there any plan to create generic DP link training helpers? After
glancing through the DP drivers there is a lot of similar code in the
link training functions, with minor differences here and there. And
it's those minor differences that bug me. It means that drivers might
respond differently to similar devices. Or that there might be minor
bugs here and there.

-- 
With best wishes
Dmitry