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

[Doug Anderson <dianders@chromium.org>]

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"

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?

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