Re: Best practice device tree design for display subsystems/DRM

[Sascha Hauer <s.hauer-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>]

On Thu, Jul 04, 2013 at 11:10:35AM +0200, Sebastian Hesselbarth wrote:
> On 07/04/13 10:53, Sascha Hauer wrote:
> >On Thu, Jul 04, 2013 at 10:45:40AM +0200, Sebastian Hesselbarth wrote:
> >>On 07/04/13 10:33, Sascha Hauer wrote:
> >>>
> >>>A componentized device never completes and it doesn't have to. A
> >>>componentized device can start once there is a path from an input
> >>>(crtc, i2s unit) to an output (connector, speaker).
> >>>
> >>>Consider what happens with a supernode approach. Your board provides a
> >>>devicetree which has a supernode with hdmi and lvds referenced. Now you
> >>>build a kernel with the hdmi driver disabled. You would still expect the
> >>>lvds port to be working without having the kernel wait for the supernode
> >>>being complete.
> >>>
> >>>Without supernode you can just start once you have everything between
> >>>the crtc and lvds nodes. If later a hdmi device joins in then you can
> >>>either notify the users (provided the DRM/KMS API supports it) or just
> >>>ignore it until the DRM device gets reopened.
> >>
> >>Sascha,
> >>
> >>that is what it is all about. You assume you a priori know what devices
> >>will be required for the componentized device to successfully output
> >>a video stream.
> >>
> >>We have shown setups where you don't know what is required. Cubox
> >>_needs_ lcd0 and hdmi-transmitter,
> >
> >Then your Cubox devicetree has a link (that's how they call it in v4l2,
> >a link doesn't necessarily is a direct connection but can have multiple
> >devices in it) between lcd0 and hdmi.
> 
> I haven't looked up v4l2 "link" yet. But (a) if it is a separate node
> how is that different from the "super-node" we are talking about or (b)
> if it is a property, where do you put it?

Sorry, I should have explained this. The basic idea the v4l2 guys are
following is that they describe their hardware pipelines in the devicetree.

Each device can have ports which are connected via links. In the
devicetree a link basically becomes a phandle (a remote device will have
a phandle pointing back to the original device). For an overview have a
look at

Documentation/devicetree/bindings/media/video-interfaces.txt

With this you can describe the whole graph of devices you have in the
devicetree. The examples in this file have a path from a camera sensor
via a MIPI converter to a capture interface.

The difference to a supernode is that this approach describes the data
flow in the devicetree so that we can iterate over it to find links
between source and sink rather than relying on a list of subdevices to
be completed.

Sascha

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