Re: [RFC] Adding export-symbols to specification

[Herve Codina <herve.codina@bootlin.com>]

On Tue, 4 Feb 2025 22:54:52 +0530
Ayush Singh <ayush@beagleboard.org> wrote:

> > You have perfectly summarized the export-symbols goal and the benefit of
> > this new feature.
> >
> > I am waiting for feedback from other people. I hope we will move forward
> > on this topic and unblock several users (me included) stuck on this real
> > issue.
> >
> > Thanks a lot!
> >
> > Best regards,
> > Hervé  
> 
> While working on a Patch series to the specification itself, I realized 
> that I was missing some edge cases, so wanted to discuss those:
> 
> # Scope
> 
> Should export-symbols only be used to resolve the properties in parent, 
> or should all other children (and their decedents) use the 
> `export-symbols` for resolving phandles and path references?
> 
> For example, should the following work:
> 
> 
> parent {
> 
>      sibling {
> 
>          led = <&gpio 0 GPIO_ACTIVE_HIGH>
> 
>      };
> 
> 
>      export-symbols {
> 
>          gpio = <&my_gpio1>;
> 
>      };
> 
> };
> 
> 
> This would also mean that bottoms up lookup needs to take place for all 
> `export-symbols` that might be present in path to root, before using top 
> level `__symbols__` or `/aliases`.

I restricted the use of export-symbols node when an overlay is applied at a node
which contains an export-symbols sub-node.

In your example, If you apply an overlay at parent node, your export-symbols
is used but if you apply an overlay at sibling node, your export-symbols is
not used.

Of course if your overlay applied at parent node looks like the following:

  __overlay__ {
    sibling {
       prop = <&gpio>;
    };
  };

&gpio will be resolved using your export-symbols. __overlay__ is applied
at parent -> visible in the overlay.

If the base device-tree looks like this:
	parent {
		sibling {
 
			export-symbols {
				gpio = <&my_gpio1>;
			};
		};
	};

The same overlay applied at parent will failed.
Indeed, no export-symbols is available at parent node and so the
gpio symbols used by the overlay will not be resolved even
if an export-symbols exists in the sibling node.
To see this export-symbols from the overlay, the overlay has to be applied
at sibling node and not parent node.

> 
> 
> # Export symbols phandles
> 
> Can export symbols reference each other? For example is the following valid:
> 
> 
> parent {
> 
>      export-symbols {
> 
>          shadow_gpio = <&my_gpio1>;
> 
>          gpio = <&shadow_gpio>;
> 
>      };
> 
> };

For sure, not planned.

Also, maybe I missed something but I am not sure you can have a
phandle referencing a property.

In your example, shadow_gpio is a property, even if we add a label
to this property i.e.
  label_shadow_gpio: shadow_gpio = <&my_gpio1>;

How can we set:
  gpio = <&label_shadow_gpio>;


Best regards,
Hervé