[PATCH v1 1/2] of: base: populate of_root node if not set

[Clément Léger <clement.leger@xxxxxxxxxxx>]

When enabling CONFIG_OF on a platform where of_root is not populated by
firmware, we end up without a root node. In order to apply overlays and
create subnodes of the root node, we need one. Create this root node
by unflattening an empty builtin dtb with of_fdt_unflatten().

Co-developed-by: Rob Herring <robh@xxxxxxxxxx>
Signed-off-by: Rob Herring <robh@xxxxxxxxxx>
Signed-off-by: Clément Léger <clement.leger@xxxxxxxxxxx>
---
 drivers/of/Makefile       |  2 +-
 drivers/of/base.c         | 18 ++++++++++++++++--
 drivers/of/empty_root.dts |  6 ++++++
 3 files changed, 23 insertions(+), 3 deletions(-)
 create mode 100644 drivers/of/empty_root.dts

diff --git a/drivers/of/Makefile b/drivers/of/Makefile
index e0360a44306e..ce56c8b95c83 100644
--- a/drivers/of/Makefile
+++ b/drivers/of/Makefile
@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0
-obj-y = base.o device.o platform.o property.o
+obj-y = base.o empty_root.dtb.o device.o platform.o property.o
 obj-$(CONFIG_OF_KOBJ) += kobj.o
 obj-$(CONFIG_OF_DYNAMIC) += dynamic.o
 obj-$(CONFIG_OF_FLATTREE) += fdt.o
diff --git a/drivers/of/base.c b/drivers/of/base.c
index d4f98c8469ed..43e0f027a49c 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c
@@ -22,6 +22,7 @@
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/of_device.h>
+#include <linux/of_fdt.h>
 #include <linux/of_graph.h>
 #include <linux/spinlock.h>
 #include <linux/slab.h>
@@ -163,6 +164,8 @@ void __of_phandle_cache_inv_entry(phandle handle)
 		phandle_cache[handle_hash] = NULL;
 }
 
+extern const char __dtb_empty_root_begin[];
+
 void __init of_core_init(void)
 {
 	struct device_node *np;
@@ -176,6 +179,18 @@ void __init of_core_init(void)
 		pr_err("failed to register existing nodes\n");
 		return;
 	}
+
+	if (!of_root) {
+		void *dt;
+		const unsigned long *fdt = (const unsigned long *)
+							__dtb_empty_root_begin;
+		dt = of_fdt_unflatten_tree(fdt, NULL, &of_root);
+		if (!dt) {
+			pr_err("Failed to setup empty root dt\n");
+			return;
+		}
+	}
+
 	for_each_of_allnodes(np) {
 		__of_attach_node_sysfs(np);
 		if (np->phandle && !phandle_cache[of_phandle_cache_hash(np->phandle)])
@@ -184,8 +199,7 @@ void __init of_core_init(void)
 	mutex_unlock(&of_mutex);
 
 	/* Symlink in /proc as required by userspace ABI */
-	if (of_root)
-		proc_symlink("device-tree", NULL, "/sys/firmware/devicetree/base");
+	proc_symlink("device-tree", NULL, "/sys/firmware/devicetree/base");
 }
 
 static struct property *__of_find_property(const struct device_node *np,
diff --git a/drivers/of/empty_root.dts b/drivers/of/empty_root.dts
new file mode 100644
index 000000000000..cf9e97a60f48
--- /dev/null
+++ b/drivers/of/empty_root.dts
@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/dts-v1/;
+
+/ {
+
+};
-- 
2.36.1