Display-Port HPD handling, link status, and bandwidth checks

[Ilia Mirkin]

You'll probably get a more detailed reply during the week, but for now
have a look at the "link-status" property, which was made for
precisely this situation. I think basically the idea is to ignore link
training as part of the modeset, and just return the link status
depending on the success. (And you should filter out totally
infeasible modes, i.e. outside the monitor's max lanes/bandwidth
capabilities, which I believe are available via DPCD or EDID.)

See https://www.kernel.org/doc/html/latest/gpu/drm-kms.html for a bit
more info as well.

Cheers,

  -ilia

On Sun, Aug 25, 2019 at 7:12 AM Jyri Sarha <jsarha at ti.com> wrote:
>
> 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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