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

[Heikki Krogerus <heikki.krogerus@linux.intel.com>]

+Hans and Imre

On Mon, Dec 06, 2021 at 02:31:40PM -0800, Bjorn Andersson wrote:
> 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@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?

Originally I wanted an API that we could use to pass all the details
that we have in the USB Type-C drivers (that would be the
configuration and status) to the GPU drivers, but Hans was against
that because, if I remember correctly, the OOB hotplug event may need
to be delivered to the GPU drivers in some cases even when the
connector is not USB Type-C connector, and he wanted a common API.
Hans, please correct me if I got it wrong.

I think that the GPU drivers need to handle USB Type-C connectors
separately one way or the other, but maybe the notification from the
connector can continue to be generic - not USB Type-C specific.

Imre proposed that the GPU drivers should be able to query the
DisplayPort configuration and status from the USB Type-C drivers
instead of the USB Type-C drivers just dumping the information
together with the notification about some event (so connection,
disconnection or attention) like I originally proposed. Imre, please
correct me if I misunderstood you :-).

I'm fine with anything, but we do need improvement here as you guys
can see.

thanks,

-- 
heikki