[RFC] drm/msm/dp: Allow attaching a drm_panel

[Bjorn Andersson]

On Thu 07 Oct 03:17 PDT 2021, Heikki Krogerus wrote:
> On Wed, Oct 06, 2021 at 01:26:35PM -0700, Prashant Malani wrote:
> > (CC+ Heikki)
[..]
> > On Wed, Oct 6, 2021 at 8:19 AM Doug Anderson <dianders at chromium.org> wrote:
[..]
>         void drm_connector_oob_hotplug_event(struct fwnode_handle *connector_fwnode);
> 
> If your USB Type-C controller/port driver does not yet register the DP
> alt mode, the it's responsible of handling HPD separately by calling
> drm_connector_oob_hotplug_event() on its own.
> 

Finally found my way back to this topic and it doesn't look like I can
reuse the existing altmode code with the firmware interface provided by
Qualcomm, so  I just hacked something up that invokes
drm_connector_oob_hotplug_event().

But I'm not able to make sense of what the expected usage is. Reading
altmode/displayport.c, it seems that I should only invoke
drm_connector_oob_hotplug_event() as HPD state toggles.

I made a trial implementation of this, where my firmware interface
driver calls drm_connector_oob_hotplug_event() every time HPD state
changes and then in my oob_hotplug_event callback I flip the DP
controller between on and off.

Unfortunately when I then connect my HDMI dongle, I get HPD state HIGH,
call the oob_hotplug_event, the DP driver powers up and concludes that
there's nothing connected to the dongle and goes to idle. I then connect
the HDMI cable to the dongle, the firmware sends me another message with
HPD irq and state HIGH, which I ignore because it's not a change in
state.

In the end I hacked up drm_connector_oob_hotplug_event() to allow me to
pass the HPD state and this solves my problem. I can now distinguish
between connect, disconnect and attention.

Can you please help shed some light on what I might be missing?

Thanks,
Bjorn