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Date:   Mon, 4 Oct 2021 21:11:57 -0500
From:   Bjorn Andersson <bjorn.andersson@linaro.org>
To:     Stephen Boyd <swboyd@chromium.org>
Cc:     Doug Anderson <dianders@chromium.org>,
        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>,
        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 20:50 CDT 2021, Stephen Boyd wrote:

> Quoting Bjorn Andersson (2021-10-04 18:11:11)
> > 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.
> 
> Where is this DT? Is it in the kernel tree?
> 

Still missing a bunch of driver pieces, so I haven't yet pushed any of
this upstream.

But if you're interested you can find some work-in-progress here:
https://github.com/andersson/kernel/commits/wip/sc8180x-next-20210819

> >
> > > 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.
> 
> That's "good" right?
> 

Well, it's expected that the connectors aren't panels...

> >
> > 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.
> >
> 
> I'd like to create a bridge, not panel, for USB connectors, so that we
> can push sideband HPD signaling through to the DP driver. But either way
> this should work, right? If drm_of_find_panel_or_bridge() returns
> -EPROBE_DEFER, then assume the connector is DP. Otherwise if there's a
> valid pointer then treat it as eDP. We can't go too crazy though because
> once we attach a bridge we're assuming eDP which may not actually be
> true.
> 

How will I be able to distinguish this from "the eDP panel is not yet
probed"? Unless we first implement the rest of this suggestion to make
sure drm_of_find_panel_or_bridge() has something to find in both cases.

> If we make a bridge for type-C USB connectors then we'll be able to use
> the drm_bridge_connector code to automatically figure out the connector
> type (eDP vs. DP vs. whatever else is chained onto the end of the DP
> connector). That would require updating the bridge connector code to
> treat DP as a connector type though. And then the eDP path would need to
> be handled when there's no bridge really involved, like in your case
> where the eDP hardware is directly connected to the eDP panel.
> 
> In this case I think we're supposed to make a bridge in this DP driver
> itself that does pretty basic stuff and assumes the connector is eDP or
> DP based on the hardware type it is. Then if we wire a type-c connector
> up to the eDP hardware the eDP bridge we make in this driver will see a
> type-c connector that makes a bridge saying "I'm a DP connector" and the
> drm_bridge_connector code will look at the last bridge in the chain to
> see that it's actually a DP connector.

This is rather far from how I do handle USB, and its HPD interrupts
today. But perhaps I'm missing something there...

Let me get that patch on the list as well then.

Regards,
Bjorn