On 25/08/2019 23:23, Ilia Mirkin wrote:

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.

It looks like only i915 is really implementing the "link-status" property (e.g. setting it to something else than "GOOD"). And it only sets it in delayed work right after a failed link-training.

But it looks like setting "link-status" bad and calling drm_kms_helper_hotplug_event() indeed triggers another modeset at least from fbdev console.

I guess the correct way would be checking if the link is still up after receiving an HPD short pulse and setting link-status bad and calling drm_kms_helper_hotplug_event() if the link is down.

I just wonder if the real user space clients like Weston or X work the same way as fbdev does.

Maybe my first question is now answered, but I am still wondering about this:

...

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

Thanks, Jyri

Cheers,

-ilia

On Sun, Aug 25, 2019 at 7:12 AM Jyri Sarha jsarha@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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