[RFC PATCH 3/3] i2c: i2c-core-of: Handle i2c bus extensions

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

i2c bus extensions were introduced to decouple i2c busses when they are
wired to connectors. Combined with devicetree overlays, they introduce
an additional level of indirection, which is needed to decouple the
overlay (describing the hardware available on addon baord) and the base
tree (describing resources provided to the addon board).

For instance, the following devicetree fragment, available once
overlays are applied, is legit:

    i2c1: i2c@abcd0000 {
        compatible = "xyz,i2c-ctrl";
        i2c-bus-extension@0 {
            reg = <0>;
            i2c-bus = <&i2c-ctrl>;
        };
        ...
    };

    connector {
        i2c-ctrl {
            i2c-parent = <&i2c1>;
            #address-cells = <1>;
            #size-cells = <0>;

            i2c-bus-extension@0 {
                reg = <0>;
                i2c-bus = <&i2c-other-connector>;
            };

            device@10 {
                compatible = "xyz,foo";
                reg = <0x10>;
            };
        };

        devices {
            other-connector {
                i2c-at-other-connector {
                    i2c-parent = <&i2c-ctrl>;
                    #address-cells = <1>;
                    #size-cells = <0>;

                    device@20 {
                       compatible = "xyz,bar";
                       reg = <0x20>;
                    };
                };
            };
        };
    };

Current processing done when a i2c adapter is registered registers
i2c clients described at the adapter node level.

With i2c bus extensions, the process needs to look also at
extensions to register devices described in those extensions and so
connected to the adapter.

Extend of_i2c_register_children() to look recursively at those
i2c bus extensions.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
---
 drivers/i2c/i2c-core-of.c | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/drivers/i2c/i2c-core-of.c b/drivers/i2c/i2c-core-of.c
index b4c9db137f5a..406565beabbe 100644
--- a/drivers/i2c/i2c-core-of.c
+++ b/drivers/i2c/i2c-core-of.c
@@ -85,13 +85,20 @@ static struct i2c_client *of_i2c_register_device(struct i2c_adapter *adap,
 static void of_i2c_register_children(struct i2c_adapter *adap,
 				     struct device_node *bus)
 {
+	struct device_node *node, *extension;
 	struct i2c_client *client;
-	struct device_node *node;
 
 	dev_dbg(&adap->dev, "of_i2c: walking child nodes from %pOF\n", bus);
 
-	/* Register device directly attached to this bus */
+	/*
+	 * Register device directly described in this bus node before looking
+	 * at extensions.
+	 */
 	for_each_available_child_of_node(bus, node) {
+		/* Filter out extension node */
+		if (of_node_name_eq(node, "i2c-bus-extension"))
+			continue;
+
 		if (of_node_test_and_set_flag(node, OF_POPULATED))
 			continue;
 
@@ -103,6 +110,23 @@ static void of_i2c_register_children(struct i2c_adapter *adap,
 			of_node_clear_flag(node, OF_POPULATED);
 		}
 	}
+
+	/* Look at extensions */
+	for_each_available_child_of_node(bus, node) {
+		if (!of_node_name_eq(node, "i2c-bus-extension"))
+			continue;
+
+		extension = of_parse_phandle(node, "i2c-bus", 0);
+		if (!extension)
+			continue;
+
+		/*
+		 * Register children available at this extension possibly
+		 * walking other chained extensions.
+		 */
+		of_i2c_register_children(adap, extension);
+		of_node_put(extension);
+	}
 }
 
 void of_i2c_register_devices(struct i2c_adapter *adap)
-- 
2.47.1