Re: [PATCH 4/6] dt-bindings: display: armada: Add display subsystem binding

[Rob Herring <robh+dt@kernel.org>]

On Mon, Jan 21, 2019 at 9:46 AM Lubomir Rintel <lkundrak@v3.sk> wrote:
>
> On Mon, 2019-01-21 at 09:35 -0600, Rob Herring wrote:
> > On Sun, Jan 20, 2019 at 11:26 AM Lubomir Rintel <lkundrak@v3.sk> wrote:
> > > The Marvell Armada DRM master device is a virtual device needed to list all
> > > nodes that comprise the graphics subsystem.
> > >
> > > Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
> > > ---
> > >  .../display/armada/marvell-armada-drm.txt     | 24 +++++++++++++++++++
> > >  1 file changed, 24 insertions(+)
> > >
> > > diff --git a/Documentation/devicetree/bindings/display/armada/marvell-armada-drm.txt b/Documentation/devicetree/bindings/display/armada/marvell-armada-drm.txt
> > > index de4cca9432c8..3dbfa8047f0b 100644
> > > --- a/Documentation/devicetree/bindings/display/armada/marvell-armada-drm.txt
> > > +++ b/Documentation/devicetree/bindings/display/armada/marvell-armada-drm.txt
> > > @@ -1,3 +1,27 @@
> > > +Marvell Armada DRM master device
> > > +================================
> > > +
> > > +The Marvell Armada DRM master device is a virtual device needed to list all
> > > +nodes that comprise the graphics subsystem.
> > > +
> > > +Required properties:
> > > +
> > > + - compatible: value should be "marvell,dove-display-subsystem",
> > > +   "marvell,armada-display-subsystem"
> > > + - ports: a list of phandles pointing to display interface ports of CRTC
> > > +   devices
> > > + - memory-region: phandle to a node describing memory to be used for the
> > > +   framebuffer
> > > +
> > > +Example:
> > > +
> > > +       display-subsystem {
> > > +               compatible = "marvell,dove-display-subsystem",
> > > +                            "marvell,armada-display-subsystem";
> > > +               memory-region = <&display_reserved>;
> > > +               ports = <&lcd0_port>;
> >
> > If there is only one device, you don't need this virtual node.
>
> By "one device" you mean one LCD controller (CRTC)?

Yes.

> I suppose in the (single CRTC) example case, the display-subsystem node
> used to associate it with the memory region reserved for allocating the
> frame buffers from. Could that be done differently?

Move memory-region to the LCD controller node.

> Also, if the node is indeed made optional, then it's going to
> complicate things on the DRM side. Currently the driver that binds to
> the node creates the DRM device once it sees all the components
> connected to the ports appear. If we loose it, then the LCD controller
> driver would somehow need to find out that it's alone and create the
> DRM device itself.

DT is not the only way to create devices. The DRM driver can bind to
the LCDC node and then create a child CRTC device (or even multiple
ones for h/w with multiple pipelines).

You'll also notice that there are only 3 cases of this virtual node in
the tree: STi, i.MX IPU, and Rockchip. That's because we've deprecated
doing these virtual nodes for some time now. IOW, there are several
examples of how to do this without a virtual node.

Rob