[PATCH 1/2] of: create of_root if no dtb provided

[frowand . list]

From: Frank Rowand <frank.rowand@xxxxxxxx>

 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.

 If firmware provides a flattened device tree (FDT) then the FDT is
 unflattened via setup_arch().  setup_of() is called immediately
 after setup_arch(), and will create the default root node if it
 does not exist.

Signed-off-by: Frank Rowand <frank.rowand@xxxxxxxx>
---

This patch modifies init/main.c.  It was not clear to me whether
to add Linus to the distribution, or if the change should flow
through Rob.  (I did not add Linus.)

 drivers/of/Makefile    |  2 +-
 drivers/of/fdt.c       | 19 ++++++++++++++++++-
 include/linux/of_fdt.h |  2 ++
 init/main.c            |  2 ++
 4 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/drivers/of/Makefile b/drivers/of/Makefile
index e0360a44306e..cbae92c5ed02 100644
--- a/drivers/of/Makefile
+++ b/drivers/of/Makefile
@@ -2,7 +2,7 @@
 obj-y = base.o device.o platform.o property.o
 obj-$(CONFIG_OF_KOBJ) += kobj.o
 obj-$(CONFIG_OF_DYNAMIC) += dynamic.o
-obj-$(CONFIG_OF_FLATTREE) += fdt.o
+obj-$(CONFIG_OF_FLATTREE) += fdt.o empty_root.dtb.o
 obj-$(CONFIG_OF_EARLY_FLATTREE) += fdt_address.o
 obj-$(CONFIG_OF_PROMTREE) += pdt.o
 obj-$(CONFIG_OF_ADDRESS)  += address.o
diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index a8f5b6532165..d439ded3b6c6 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c
@@ -33,6 +33,12 @@
 
 #include "of_private.h"
 
+/*
+ * __dtb_empty_root_begin[] magically created by cmd_dt_S_dtb in
+ * scripts/Makefile.lib
+ */
+extern void *__dtb_empty_root_begin;
+
 /*
  * of_fdt_limit_memory - limit the number of regions in the /memory node
  * @limit: maximum entries
@@ -1332,8 +1338,13 @@ bool __init early_init_dt_scan(void *params)
  */
 void __init unflatten_device_tree(void)
 {
-	__unflatten_device_tree(initial_boot_params, NULL, &of_root,
+	if (!initial_boot_params) {
+		initial_boot_params = (void *) __dtb_empty_root_begin;
+		unflatten_and_copy_device_tree();
+	} else {
+		__unflatten_device_tree(initial_boot_params, NULL, &of_root,
 				early_init_dt_alloc_memory_arch, false);
+	}
 
 	/* Get pointer to "/chosen" and "/aliases" nodes for use everywhere */
 	of_alias_scan(early_init_dt_alloc_memory_arch);
@@ -1373,6 +1384,12 @@ void __init unflatten_and_copy_device_tree(void)
 	unflatten_device_tree();
 }
 
+void __init setup_of(void)
+{
+	if (!of_root)
+		unflatten_device_tree();
+}
+
 #ifdef CONFIG_SYSFS
 static ssize_t of_fdt_raw_read(struct file *filp, struct kobject *kobj,
 			       struct bin_attribute *bin_attr,
diff --git a/include/linux/of_fdt.h b/include/linux/of_fdt.h
index d69ad5bb1eb1..4566876db351 100644
--- a/include/linux/of_fdt.h
+++ b/include/linux/of_fdt.h
@@ -81,6 +81,7 @@ extern const void *of_flat_dt_match_machine(const void *default_match,
 /* Other Prototypes */
 extern void unflatten_device_tree(void);
 extern void unflatten_and_copy_device_tree(void);
+extern void setup_of(void);
 extern void early_init_devtree(void *);
 extern void early_get_first_memblock_info(void *, phys_addr_t *);
 #else /* CONFIG_OF_EARLY_FLATTREE */
@@ -91,6 +92,7 @@ static inline void early_init_fdt_reserve_self(void) {}
 static inline const char *of_flat_dt_get_machine_name(void) { return NULL; }
 static inline void unflatten_device_tree(void) {}
 static inline void unflatten_and_copy_device_tree(void) {}
+static inline void of_setup(void) {}
 #endif /* CONFIG_OF_EARLY_FLATTREE */
 
 #endif /* __ASSEMBLY__ */
diff --git a/init/main.c b/init/main.c
index 0ee39cdcfcac..8b3a60d14bea 100644
--- a/init/main.c
+++ b/init/main.c
@@ -99,6 +99,7 @@
 #include <linux/kcsan.h>
 #include <linux/init_syscalls.h>
 #include <linux/stackdepot.h>
+#include <linux/of_fdt.h>
 #include <net/net_namespace.h>
 
 #include <asm/io.h>
@@ -949,6 +950,7 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
 	pr_notice("%s", linux_banner);
 	early_security_init();
 	setup_arch(&command_line);
+	setup_of();
 	setup_boot_config();
 	setup_command_line(command_line);
 	setup_nr_cpu_ids();
-- 
Frank Rowand <frank.rowand@xxxxxxxx>