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Date:   Mon, 4 Oct 2021 18:11:11 -0700
From:   Bjorn Andersson <bjorn.andersson@linaro.org>
To:     Doug Anderson <dianders@chromium.org>
Cc:     Rob Clark <robdclark@gmail.com>, Sean Paul <sean@poorly.run>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Abhinav Kumar <abhinavk@codeaurora.org>,
        Stephen Boyd <swboyd@chromium.org>,
        Kuogee Hsieh <khsieh@codeaurora.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Vara Reddy <varar@codeaurora.org>,
        freedreno <freedreno@lists.freedesktop.org>,
        Chandan Uddaraju <chandanu@codeaurora.org>
Subject: Re: [RFC] drm/msm/dp: Allow attaching a drm_panel
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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.

Hope that clarifies my conundrum.

Regards,
Bjorn