Hi,
I am working on a new DisplayPort bridge-driver and there is a couple of
things that I do not know how to handle.
1. When should the link training happen?
a) In connector detect()?
- This would enable us to do mode filtering (in mode_valid())
based on the established link band-width (then again
mode_valid() documentation suggests that modes should only
be filtered based on "configuration-invariant hardware
constraints").
b) In check phase (this would currently mean mode_fixup)?
- This is the last point where we can reject a mode that can not
be sent over the DP-link
c) In commit phase (e.g. bridge enable())
- This is bad since we should not fail any more in the commit
phase
2. DP-link sometimes drops after a succesful link training and DP-sink
is supposed to send short HPD pulse about it. What are the
recommended ways to handle the situation?
a) Send hotplug event and let the DRM client deal with it?
- This does not work too well because even if the client tries
to restore the display by committing the same state again -
like fbdev does - the bridge does not go trough disable-enable
cycle, since display mode has not changed.
- Despite it not working so well, this is what the most drivers
appear to do.
b) Driver internally re-trains the link but send a hotplug event
always after it?
- This is what i915 does, if I read the code right.
- How to treat a training failure? Sending hotplug event does not
really help (see above).
c) Silently re-train the link if we were able to restore the link
and the display mode, and send HPD only if something went wrong?
Best regards,
Jyri
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