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

From: Clément Léger
Date: Thu Jun 23 2022 - 06:52:18 EST


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