Comment # 1 on bug 91871 from Jani Nikula

To be fair, the same limitation was there before atomic.

Part of the problem is our modeset sequence, which we can't easily backtrack if
the modeset fails at link training phase. Another part is that even if our
modeset sequence supported that, we couldn't just reduce the mode to
accommodate the lower link rate behind userspace's back. (We already pick the
lowest rate that can support the mode; there's no margin to reduce.) We'd have
to involve userspace somehow.

Drivers are free to use the drm dp helpers or use their own hooks for modeset.
I think we should look at this from drm/i915 perspective, at least at first, so
resassigning to DRM/Intel. If we come up with a way to negotiate this with
userspace, that should be a generic solution.