Re: [PATCH 1/4] dt-bindings: chosen: document panel-id binding

[Rob Clark <robdclark@gmail.com>]

On Mon, Jul 1, 2019 at 7:03 AM Rob Herring <robh+dt@kernel.org> wrote:
>
> On Sun, Jun 30, 2019 at 2:36 PM Rob Clark <robdclark@gmail.com> wrote:
> >
> > From: Rob Clark <robdclark@chromium.org>
> >
> > The panel-id property in chosen can be used to communicate which panel,
> > of multiple possibilities, is installed.
> >
> > Signed-off-by: Rob Clark <robdclark@chromium.org>
> > ---
> >  Documentation/devicetree/bindings/chosen.txt | 69 ++++++++++++++++++++
> >  1 file changed, 69 insertions(+)
>
> I need to update this file to say it's moved to the schema repository...
>
> But I don't think that will matter...
>
> > diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
> > index 45e79172a646..d502e6489b8b 100644
> > --- a/Documentation/devicetree/bindings/chosen.txt
> > +++ b/Documentation/devicetree/bindings/chosen.txt
> > @@ -68,6 +68,75 @@ on PowerPC "stdout" if "stdout-path" is not found.  However, the
> >  "linux,stdout-path" and "stdout" properties are deprecated. New platforms
> >  should only use the "stdout-path" property.
> >
> > +panel-id
> > +--------
> > +
> > +For devices that have multiple possible display panels (multi-sourcing the
> > +display panels is common on laptops, phones, tablets), this allows the
> > +bootloader to communicate which panel is installed, e.g.
>
> How does the bootloader figure out which panel? Why can't the kernel
> do the same thing?

see jhugo's response, he has I guess more access to the bootloader than I

> > +
> > +/ {
> > +       chosen {
> > +               panel-id = <0xc4>;
> > +       };
> > +
> > +       ivo_panel {
> > +               compatible = "ivo,m133nwf4-r0";
> > +               power-supply = <&vlcm_3v3>;
> > +               no-hpd;
> > +
> > +               ports {
> > +                       port {
> > +                               ivo_panel_in_edp: endpoint {
> > +                                       remote-endpoint = <&sn65dsi86_out_ivo>;
> > +                               };
> > +                       };
> > +               };
> > +       };
> > +
> > +       boe_panel {
> > +               compatible = "boe,nv133fhm-n61";
>
> Both panels are going to probe. So the bootloader needs to disable the
> not populated panel setting 'status' (or delete the node). If you do
> that, do you even need 'panel-id'?

I don't think we can rely on bootloader knowing a thing about DT on
these devices..

OTOH I don't really think it is a big problem for both panels to
probe.  But I suppose it might be possible to have something somewhere
in the kernel that realizes and disables the unused panels.

> > +               power-supply = <&vlcm_3v3>;
> > +               no-hpd;
> > +
> > +               ports {
> > +                       port {
> > +                               boe_panel_in_edp: endpoint {
> > +                                       remote-endpoint = <&sn65dsi86_out_boe>;
> > +                               };
> > +                       };
> > +               };
> > +       };
> > +
> > +       display_or_bridge_device {
> > +
> > +               ports {
> > +                       #address-cells = <1>;
> > +                       #size-cells = <0>;
> > +
> > +                       ...
> > +
> > +                       port@0 {
> > +                               #address-cells = <1>;
> > +                               #size-cells = <0>;
> > +                               reg = <0>;
> > +
> > +                               endpoint@c4 {
> > +                                       reg = <0xc4>;
>
> What does this number represent? It is supposed to be defined by the
> display_or_bridge_device, not a specific panel.

it matches /chosen/panel-id.. in this case I'm not sure how the
panel-id's are assigned, but for our purposes all that matters is that
they are assigned.

> We also need to consider how the DSI case with panels as children of
> the DSI controller would work and how this would work with multiple
> displays each having multiple panel options.

In the non-bridge case (panel hooked directly to dsi controller), the
dsi controller could use the same ports {} mechanism.

For multiple displays, we could extend, I suppose, /chosen/panel-id to
be an array of id's indexed by display.  I think this is the type of
extension we could do later when the use-case comes up.  Just having
this solved upstream for single display would already be a huge
advancement.  (You don't want to look at how this is solved downstream
for android phones.)

Btw, if you are curious how this works on windows/ACPI, the ACPI
tables have entries for each of the panels.  The kernel is expected to
take the panel-id from that EFI variable that jhugo mentioned, and
pass it to a _ROM method which returns the appropriate panel table.
(Not entirely sure how the orchestrate reading the EFI variable early,
since it does not appear to be available after ExitBootServices)

BR,
-R