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

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Tue 05 Oct 08:39 PDT 2021, Doug Anderson wrote:

> Hi,
> 
> On Mon, Oct 4, 2021 at 6:09 PM Bjorn Andersson
> <bjorn.andersson@linaro.org> wrote:
> >
> > On Mon 04 Oct 17:36 PDT 2021, Doug Anderson wrote:
> >
> > > Hi,
> > >
> > > On Fri, Oct 1, 2021 at 2:00 PM Bjorn Andersson
> > > <bjorn.andersson@linaro.org> wrote:
> > > >
> > > > On Fri 27 Aug 13:52 PDT 2021, Doug Anderson wrote:
> > > >
> > > > > Hi,
> > > > >
> > > > > On Mon, Jul 26, 2021 at 4:15 PM Bjorn Andersson
> > > > > <bjorn.andersson@linaro.org> wrote:
> > > > > >
> > > > > > +static int dp_parser_find_panel(struct dp_parser *parser)
> > > > > > +{
> > > > > > +       struct device_node *np = parser->pdev->dev.of_node;
> > > > > > +       int rc;
> > > > > > +
> > > > > > +       rc = drm_of_find_panel_or_bridge(np, 2, 0, &parser->drm_panel, NULL);
> > > > >
> > > > > Why port 2? Shouldn't this just be port 1 always? The yaml says that
> > > > > port 1 is "Output endpoint of the controller". We should just use port
> > > > > 1 here, right?
> > > > >
> > > >
> > > > Finally got back to this, changed it to 1 and figured out why I left it
> > > > at 2.
> > > >
> > > > drm_of_find_panel_or_bridge() on a DP controller will find the of_graph
> > > > reference to the USB-C controller, scan through the registered panels
> > > > and conclude that the of_node of the USB-C controller isn't a registered
> > > > panel and return -EPROBE_DEFER.
> > >
> > > I'm confused, but maybe it would help if I could see something
> > > concrete. Is there a specific board this was happening on?
> > >
> >
> > Right, let's make this more concrete with a snippet from the actual
> > SC8180x DT.
> >
> > > Under the DP node in the device tree I expect:
> > >
> > > ports {
> > >   port@1 {
> > >     reg = <1>;
> > >     edp_out: endpoint {
> > >       remote-endpoint = <&edp_panel_in>;
> > >     };
> > >   };
> > > };
> > >
> >
> > /* We got a panel */
> > panel {
> >     ...
> >     ports {
> >         port {
> >             auo_b133han05_in: endpoint {
> >                 remote-endpoint = <&mdss_edp_out>;
> >             };
> >         };
> >     };
> > };
> >
> > /* And a 2-port USB-C controller */
> > type-c-controller {
> >     ...
> >     connector@0 {
> >         ports {
> >             port@0 {
> >                 reg = <0>;
> >                 ucsi_port_0_dp: endpoint {
> >                     remote-endpoint = <&dp0_mode>;
> >                 };
> >             };
> >
> >             port@1 {
> >                 reg = <1>;
> >                 ucsi_port_0_switch: endpoint {
> >                     remote-endpoint = <&primary_qmp_phy>;
> >                 };
> >             };
> >         };
> >     };
> >
> >         connector@1 {
> >         ports {
> >             port@0 {
> >                 reg = <0>;
> >                 ucsi_port_1_dp: endpoint {
> >                     remote-endpoint = <&dp1_mode>;
> >                 };
> >             };
> >
> >             port@1 {
> >                 reg = <1>;
> >                 ucsi_port_1_switch: endpoint {
> >                     remote-endpoint = <&second_qmp_phy>;
> >                 };
> >             };
> >         };
> >         };
> > };
> >
> > /* And then our 2 DP and single eDP controllers */
> > &mdss_dp0 {
> >     ports {
> >         port@1 {
> >             reg = <1>;
> >             dp0_mode: endpoint {
> >                 remote-endpoint = <&ucsi_port_0_dp>;
> >             };
> >         };
> >     };
> > };
> >
> > &mdss_dp1 {
> >     ports {
> >         port@1 {
> >             reg = <1>;
> >             dp1_mode: endpoint {
> >                 remote-endpoint = <&ucsi_port_1_dp>;
> >             };
> >         };
> >     };
> > };
> >
> > &mdss_edp {
> >     ports {
> >         port@1 {
> >             reg = <1>;
> >             mdss_edp_out: endpoint {
> >                 remote-endpoint = <&auo_b133han05_in>;
> >             };
> >         };
> >     };
> > };
> >
> > > If you have "port@1" pointing to a USB-C controller but this instance
> > > of the DP controller is actually hooked up straight to a panel then
> > > you should simply delete the "port@1" that points to the typeC and
> > > replace it with one that points to a panel, right?
> > >
> >
> > As you can see, port 1 on &mdss_dp0 and &mdss_dp1 points to the two UCSI
> > connectors and the eDP points to the panel, exactly like we agreed.
> >
> > So now I call:
> >     drm_of_find_panel_or_bridge(dev->of_node, 1, 0, &panel, NULL);
> >
> > which for the two DP nodes will pass respective UCSI connector to
> > drm_find_panel() and get EPROBE_DEFER back - because they are not on
> > panel_list.
> >
> > There's nothing indicating in the of_graph that the USB connectors
> > aren't panels (or bridges), so I don't see a way to distinguish the two
> > types remotes.
> 
> As far as I can tell the way this would be solved would be to actually
> pass &bridge in and then make sure that a bridge would be in place for
> the DP connector. In the full DP case you'll get an -EPROBE_DEFER if
> the connector hasn't been probed but once it's probed then it should
> register as a bridge and thus give you the info you need (AKA that
> this isn't a panel).
> 
> I haven't done the digging to see how all this works, but according to
> Laurent [1]: "Physical connectors are already handled as bridges, see
> drivers/gpu/drm/bridge/display-connector.c"
> 

All this seems to make sense for both eDP and "native" DP.

> So basically I think this is solvable in code and there's no reason to
> mess with the devicetree bindings to solve this problem. Does that
> sound right?
> 

But I don't have a DisplayPort connector.

I have a USB-C connector, that upon determining that it's time to play
DisplayPort will use the typec_mux abstraction to tell someone on the
other side of the of_graph about DisplayPort events (HPD).

So where would I put this drm_bridge in the USB-C case?

I don't see that it fits in the Type-C side of things and putting it on
the DP side would leave us with exactly the problem we have here. So we
would have to put a fake "DP connector" inbetween the DP node and the
Type-C controller?


For reference, this is how I thought one is supposed to tie the Type-C
controller to the display driver:
https://lore.kernel.org/all/20211005022451.2037405-1-bjorn.andersson@linaro.org/


I'm afraid I must be missing something in Laurent and yours proposal
(although I think Laurent is talking about the native DP case?).

Regards,
Bjorn

> [1] https://lore.kernel.org/r/YUvMv+Y8tFcWPEHd@pendragon.ideasonboard.com/