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

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Mon, Jan 21, 2019 at 10:07:11AM -0600, Rob Herring wrote:
> 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.

How does that work (as far as the Linux implementation) ?  I can't see
a way that could work, while allowing the flexibility that Armada DRM
allows (two completely independent LCD controllers as two separate DRM
devices vs one DRM device containing both LCD controllers.)

> > 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.

That doesn't work - it would appear in the wrong part of the driver.

> > 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).

That seems completely upside down and rediculous to me - are you
really suggesting that we should have some kind of virtual device
in DT, and omit the _real_ physical devices for that, having the
driver create the device with all the appropriate SoC resources?

> 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.

This driver has been in-tree with this setup for some time, although
the documentation has been missing (we actually have a _lot_ of
instances of that.)  However, we have no in-tree DT using it.

I don't really see how to satisfy your comments without totally
restructuring the driver, which is going to be quite a big chunk
of work.  I'm not sure I have the motivation to do that right now.

-- 
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