Re: [PATCH v2 6/6] doc: dt: Document the target root overlay method

[Rob Herring <robherring2@gmail.com>]

On Mon, May 16, 2016 at 3:18 PM, Pantelis Antoniou
<pantelis.antoniou@konsulko.com> wrote:
> Add a description of the target root overlay method to the overlay
> documention file.
>
> Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
> ---
>  Documentation/devicetree/overlay-notes.txt | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/Documentation/devicetree/overlay-notes.txt b/Documentation/devicetree/overlay-notes.txt
> index 6995fc1..3e8df30 100644
> --- a/Documentation/devicetree/overlay-notes.txt
> +++ b/Documentation/devicetree/overlay-notes.txt
> @@ -104,6 +104,10 @@ If your board has multiple slots/places where a single overlay can work
>  and each slot is defined by a node, you can use the
>  of_overlay_create_target_index() method to select the target.
>
> +For overlays on probeable busses, use the of_overlay_create_target_root() method
> +in which you supply a device node as a target root, and which all target
> +references in the overlay are performed relative to that node.

This needs a better explanation and an example. "Applying overlays to
multiple places for probeable busses" is not sufficient. Describe what
is the problem/flow, and then how does the implementation work. I
don't think this problem is limited to probeable busses either. Not
knowing the target to apply the overlay to is the same problem for
connectors with non-probeable signals.

I find a couple of things problematic with the implementation. There's
no way to validate that an overlay should apply to a base node. If
users just always make the target "/", then any overlay can apply to
any location the nodes could apply anywhere. It requires a kernel
change for every location you want to apply the overlay to. Maybe I
don't understand the usecase.

I feel like we are creating too many syntax's to apply overlays. I get
that there are different usecases, but that doesn't necessarily mean
the syntax needs to be different.

> +
>  Overlay DTS Format
>  ------------------
>
> @@ -141,3 +145,7 @@ contains the information required to map from a phandle to a tree location.
>  Using a target index requires the use of a selector target on the call to
>  of_overlay_create_target_index(). I.e. passing an index of 0 will select the
>  target in the foo node, an index of 1 the bar node, etc.
> +
> +Note that when using the target root create method all target references must
> +lie under the target root node. I.e. the overlay is not allowed to 'break' out
> +of the root.

That is not enforceable though, right?

Rob