[PATCH] checks: Document possible false warning for graph child addresses

The dtc graph_child_address check can't distinguish between bindings
where there can only be a single endpoint, and cases where there can be
multiple endpoints.

In cases where the bindings allow for multiple endpoints but only one is
described false warnings about unnecessary #address-cells/#size-cells
can be generated, but only if the endpoint described have an address of
0 (A), for single endpoints with a non-zero address (B) no warnings are
generated.

A)
    ports {
	#address-cells = <1>;
	#size-cells = <0>;

	port@0 {
	    #address-cells = <1>;
	    #size-cells = <0>;

	    sourceA: endpoint@0 {
		reg = <0>
	    };
	};
    };

B)
    ports {
	#address-cells = <1>;
	#size-cells = <0>;

	port@0 {
	    #address-cells = <1>;
	    #size-cells = <0>;

	    sourceB: endpoint@1 {
		reg = <1>
	    };
	};
    };

Add a comment in the check to document this.

Signed-off-by: Niklas Söderlund <[email protected]>
---
Hello,

This was previously part of a patch posted to [email protected]
[1], but as Rob's points out in that thread it should have been posted
separately to [email protected]. Sorry for not
realising that dtc changes go to thru a separate tree.

1.  https://lore.kernel.org/all/20250702085008.689727-1-niklas.soderlund%[email protected]/
---
 checks.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/checks.c b/checks.c
index 123f2eb425f4..52d09fcf8d3a 100644
--- a/checks.c
+++ b/checks.c
@@ -1913,6 +1913,11 @@ static void check_graph_child_address(struct check *c, struct dt_info *dti,
 		cnt++;
 	}
 
+	/*
+	 * This check can produce false warnings if the bindings allow for more
+	 * then one endpoint in the node but only one is present and it has a
+	 * unit address of zero.
+	 */
 	if (cnt == 1 && node->addr_cells != -1)
 		FAIL(c, dti, node, "graph node has single child node '%s', #address-cells/#size-cells are not necessary",
 		     node->children->name);
-- 
2.50.0