[PATCH v3 1/2] of: create of_root if no dtb provided

[Frank Rowand]

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().  Otherwise, setup_of() which is called
immediately after setup_arch(), and will create the default root node
if it does not exist.

Signed-off-by: Frank Rowand <frowand.list@xxxxxxxxx>
---

Please wait for test results from Clement before accepting.

Changes since version 2:
  - change of __dtb_empty_root_* from "void *" to "uint8_t []"

Changes since version 1:
  - refresh for 6.2-rc1
  - update Signed-off-by
  - fix typo in of_fdt.h: s/of_setup/setup_of
  - unflatten_device_tree(): validate size in header field dtb_empty_root
    that will be used to copy dtb_empty_root
  - add Kconfig option to manually select CONFIG_OF_EARLY_FLATTREE

 drivers/of/Kconfig        |  7 ++++++-
 drivers/of/Makefile       |  2 +-
 drivers/of/empty_root.dts |  6 ++++++
 drivers/of/fdt.c          | 27 ++++++++++++++++++++++++++-
 include/linux/of_fdt.h    |  2 ++
 init/main.c               |  2 ++
 6 files changed, 43 insertions(+), 3 deletions(-)
 create mode 100644 drivers/of/empty_root.dts

diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
index 80b5fd44ab1c..591cfe386727 100644
--- a/drivers/of/Kconfig
+++ b/drivers/of/Kconfig
@@ -42,9 +42,14 @@ config OF_FLATTREE
 	select CRC32
 
 config OF_EARLY_FLATTREE
-	bool
+	bool "Functions for accessing Flat Devicetree (FDT) early in boot"
 	select DMA_DECLARE_COHERENT if HAS_DMA
 	select OF_FLATTREE
+	help
+	  Normally selected by platforms that process an FDT that has been
+	  passed to the kernel by the bootloader.  If the bootloader does not
+	  pass an FDT to the kernel and you need an empty devicetree that
+	  contains only a root node to exist, then say Y here.
 
 config OF_PROMTREE
 	bool
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/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/;
+
+/ {
+
+};
diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index b2272bccf85c..eccc2d2107af 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c
@@ -33,6 +33,13 @@
 
 #include "of_private.h"
 
+/*
+ * __dtb_empty_root_begin[] and __dtb_empty_root_end[] magically created by
+ * cmd_dt_S_dtb in scripts/Makefile.lib
+ */
+extern uint8_t __dtb_empty_root_begin[];
+extern uint8_t __dtb_empty_root_end[];
+
 /*
  * of_fdt_limit_memory - limit the number of regions in the /memory node
  * @limit: maximum entries
@@ -1326,8 +1333,19 @@ 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;
+		/* fdt_totalsize() will be used for copy size */
+		if (fdt_totalsize(initial_boot_params) >
+		    __dtb_empty_root_end - __dtb_empty_root_begin) {
+			pr_err("invalid size in dtb_empty_root\n");
+			return;
+		}
+		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);
@@ -1367,6 +1385,13 @@ void __init unflatten_and_copy_device_tree(void)
 	unflatten_device_tree();
 }
 
+void __init setup_of(void)
+{
+	/* if architecture did not unflatten devicetree, do it now */
+	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..f0dc46d576da 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 setup_of(void) {}
 #endif /* CONFIG_OF_EARLY_FLATTREE */
 
 #endif /* __ASSEMBLY__ */
diff --git a/init/main.c b/init/main.c
index e1c3911d7c70..31e0931b5134 100644
--- a/init/main.c
+++ b/init/main.c
@@ -101,6 +101,7 @@
 #include <linux/init_syscalls.h>
 #include <linux/stackdepot.h>
 #include <linux/randomize_kstack.h>
+#include <linux/of_fdt.h>
 #include <net/net_namespace.h>
 
 #include <asm/io.h>
@@ -961,6 +962,7 @@ 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 <frowand.list@xxxxxxxxx>