dtoverlay: allow top-level phandles to optionally be preserved

[compholio]

Look for the dtoverlay,preserve-phandle flag in the overlay and, if it's present, keep and merge the phandle.

This is useful with ethernet PHYs, like so:

/ {
	enet: fragment@1 {
		overlay: __overlay__ {
			status = "okay";
		};
	};
	phy: fragment@3 {
		phy_handle: __dormant__ {
			dtoverlay,preserve-phandle;
		};
	};
};

/ {
	__overrides__ {
		enet =
			<&enet>,"target-path"
		;
		phy =
			<0>,"+3"
		,
			<&overlay>,"phy-handle:0=",<&phy_handle>
		,
			<&phy>,"target-path"
		;
	};
};

Without this patch the top-level phandle is not preserved, so the phy-handle does not match the node for the PHY.

[pelwell]

I think we can make it shorter (and more efficient):

@@ -564,6 +564,7 @@ int dtoverlay_merge_fragment(DTBLOB_T *base_dtb, int target_off,
                                     int overlay_off, int depth)
 {
     int prop_off, subnode_off;
+    int preserve_phandles;
     int err = 0;
 
     if (dtoverlay_debug_enabled)
@@ -578,6 +579,10 @@ int dtoverlay_merge_fragment(DTBLOB_T *base_dtb, int target_off,
                         overlay_path);
     }
 
+    // Check for a property flag to indicate preserving top-level phandles
+    preserve_phandles = depth > 0 ||
+        fdt_getprop(overlay_dtb->fdt, overlay_off, "dtoverlay,preserve-phandle", NULL);
+
     // Merge each property of the node
     for (prop_off = fdt_first_property_offset(overlay_dtb->fdt, overlay_off);
          (prop_off >= 0) && (err == 0);
@@ -592,10 +597,11 @@ int dtoverlay_merge_fragment(DTBLOB_T *base_dtb, int target_off,
         prop_val = fdt_getprop_by_offset(overlay_dtb->fdt, prop_off,
                                          &prop_name, &prop_len);
 
-        /* Skip these system properties (only phandles in the first level) */
+        // Skip system properties and top-level phandles unless preserved
         if ((strcmp(prop_name, "name") == 0) ||
-            ((depth == 0) && ((strcmp(prop_name, "phandle") == 0) ||
-                              (strcmp(prop_name, "linux,phandle") == 0))))
+            (strcmp(prop_name, "dtoverlay,preserve-phandle") == 0) ||
+            (!preserve_phandles && ((strcmp(prop_name, "phandle") == 0) ||
+                                    (strcmp(prop_name, "linux,phandle") == 0))))
             continue;
 
         dtoverlay_debug("  +prop(%s)", prop_name);

[compholio]

Updated with your suggestions (I like your way a lot better).

[pelwell]

Thanks!

BTW, #199 is the equivalent change to ovmerge.