Re: [PATCH v3] drm: of: Lookup if child node has panel or bridge

[Maxime Ripard <maxime@cerno.tech>]

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On Wed, Jan 12, 2022 at 03:44:00PM +0530, Jagan Teki wrote:
> On Wed, Jan 12, 2022 at 3:33 PM Maxime Ripard <maxime@cerno.tech> wrote:
> >
> > On Wed, Jan 12, 2022 at 12:01:52AM +0530, Jagan Teki wrote:
> > > Some OF graphs don't require 'port' or 'ports' to represent the
> > > downstream panel or bridge; instead it simply adds a child node
> > > on a given parent node.
> >
> > All bindings using OF graph nodes require either port or ports.
> >
> > DSI Host however don't have to use the OF graph, and that's what you're
> > talking about.
> 
> Yes, right now I can see DSI but this change is generic to any OF graph.

Not really? Generic to all users of drm_of_find_panel_or_bridge sure,
but DSI is the only use case where we could have bridges or panels that
would be child of the user.

> > > drm_of_find_panel_or_bridge can lookup panel or bridge for a given
> > > node based on the OF graph port and endpoint and it fails to use
> > > if the given node has a child panel or bridge.
> > >
> > > This patch add support to lookup that given node has child panel
> > > or bridge however that child node is neither a 'port' nor a 'ports'
> > > node.
> > >
> > > Example OF graph representation of DSI host, which has 'port'
> > > but not has 'ports' and has child panel node.
> > >
> > > dsi {
> > >       compatible = "allwinner,sun6i-a31-mipi-dsi";
> > >       #address-cells = <1>;
> > >       #size-cells = <0>;
> > >
> > >       port {
> > >               dsi_in_tcon0: endpoint {
> > >                       remote-endpoint = <tcon0_out_dsi>;
> > >       };
> > >
> > >       panel@0 {
> > >               reg = <0>;
> > >       };
> > > };
> > >
> > > Example OF graph representation of DSI host, which has 'ports'
> > > but not has 'port' and has child panel node.
> > >
> > > dsi {
> > >         compatible = "samsung,exynos5433-mipi-dsi";
> > >         #address-cells = <1>;
> > >         #size-cells = <0>;
> > >
> > >       ports {
> > >               #address-cells = <1>;
> > >               #size-cells = <0>;
> > >
> > >               port@0 {
> > >                       reg = <0>;
> > >
> > >                       dsi_to_mic: endpoint {
> > >                               remote-endpoint = <&mic_to_dsi>;
> > >                       };
> > >                 };
> > >         };
> > >
> > >         panel@0 {
> > >                 reg = <0>;
> > >         };
> > > };
> >
> > I can't see how that one makes sense. The endpoint seems to have a
> > single output, yet you also have a panel under it which is also an
> > output? You should have at least the virtual channel of that endpoint
> > somewhere to differentiate data between the panel and whatever is
> > connected on the other side of that endpoint.
> 
> Same that I understood so far (based on v2 change), However we have
> exynos5433 has this pipeline and Andrzej mentioned it is valid
> pipeline on other thread.
> 
> May be Andrzej, can give more conclusive evidence for it.
> 
> >
> > > Example OF graph representation of DSI host, which has neither
> > > a 'port' nor a 'ports' but has child panel node.
> > >
> > > dsi0 {
> > >       compatible = "ste,mcde-dsi";
> > >       #address-cells = <1>;
> > >       #size-cells = <0>;
> > >
> > >       panel@0 {
> > >               reg = <0>;
> > >       };
> > > };
> > >
> > > Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
> > > ---
> > > Changes for v3:
> > > - updated based on other usecase where 'ports' used along with child
> > > Changes for v2:
> > > - drop of helper
> > > https://patchwork.kernel.org/project/dri-devel/cover/20211207054747.461029-1-jagan@amarulasolutions.com/
> > > - support 'port' alone OF graph
> > > - updated comments
> > > - added simple code
> > >
> > >  drivers/gpu/drm/drm_of.c | 18 ++++++++++++++++++
> > >  1 file changed, 18 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/drm_of.c b/drivers/gpu/drm/drm_of.c
> > > index 59d368ea006b..aeddd39b8df6 100644
> > > --- a/drivers/gpu/drm/drm_of.c
> > > +++ b/drivers/gpu/drm/drm_of.c
> > > @@ -249,6 +249,22 @@ int drm_of_find_panel_or_bridge(const struct device_node *np,
> > >       if (panel)
> > >               *panel = NULL;
> > >
> > > +     /**
> > > +      * Some OF graphs don't require 'port' or 'ports' to represent the
> > > +      * downstream panel or bridge; instead it simply adds a child node
> > > +      * on a given parent node.
> >
> > All OF graphs require either port or ports. DSI hosts can just have a
> > child node.
> 
> As commented above, it can be DSI or any host which has child nodes
> and are looking to find panel/bridge.

DSI is the only case in this situation, but ok. It still has nothing to
do with OF graph. All bindings using OF graph will require either port
or ports, and the DSI binding doesn't mandate to use OF graph. So the
comment above is misleading at best.

Maxime

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