of: Create of_root if no dtb provided by firmware

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, the call to
unflatten(_and_copy)?_device_tree() will create an empty root node.

We make of_have_populated_dt() return true only if the DTB was loaded by
firmware so that existing callers don't change behavior after this
patch. The call in the of platform code is removed because it prevents
overlays from creating platform devices when the empty root node is
used.

[sboyd@kernel.org: Update of_have_populated_dt() to treat this empty dtb
as not populated. Drop setup_of() initcall]

Signed-off-by: Frank Rowand <frowand.list@gmail.com>
Link: https://lore.kernel.org/r/20230317053415.2254616-2-frowand.list@gmail.com
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20240217010557.2381548-3-sboyd@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>

drivers/of/Kconfig

@@ -14,9 +14,8 @@ if OF
 
 config OF_UNITTEST
 	bool "Device Tree runtime unit tests"
-	depends on !SPARC
+	depends on OF_EARLY_FLATTREE
 	select IRQ_DOMAIN
-	select OF_EARLY_FLATTREE
 	select OF_RESOLVE
 	help
 	  This option builds in test cases for the device tree infrastructure
@@ -54,7 +53,7 @@ config OF_FLATTREE
 	select CRC32
 
 config OF_EARLY_FLATTREE
-	bool
+	def_bool OF && !(SPARC || ALPHA || HEXAGON || M68K || PARISC || S390)
 	select DMA_DECLARE_COHERENT if HAS_DMA && HAS_IOMEM
 	select OF_FLATTREE
 

drivers/of/Makefile

@@ -2,7 +2,7 @@
 obj-y = base.o cpu.o device.o module.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

drivers/of/empty_root.dts

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/dts-v1/;
+
+/ {
+
+};

drivers/of/fdt.c

@@ -8,6 +8,7 @@
 
 #define pr_fmt(fmt)	"OF: fdt: " fmt
 
+#include <linux/acpi.h>
 #include <linux/crash_dump.h>
 #include <linux/crc32.h>
 #include <linux/kernel.h>
@@ -32,6 +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
@@ -1343,7 +1351,29 @@ static void *__init copy_device_tree(void *fdt)
  */
 void __init unflatten_device_tree(void)
 {
-	__unflatten_device_tree(initial_boot_params, NULL, &of_root,
+	void *fdt = initial_boot_params;
+
+	/* Don't use the bootloader provided DTB if ACPI is enabled */
+	if (!acpi_disabled)
+		fdt = NULL;
+
+	/*
+	 * Populate an empty root node when ACPI is enabled or bootloader
+	 * doesn't provide one.
+	 */
+	if (!fdt) {
+		fdt = (void *) __dtb_empty_root_begin;
+		/* fdt_totalsize() will be used for copy size */
+		if (fdt_totalsize(fdt) >
+		    __dtb_empty_root_end - __dtb_empty_root_begin) {
+			pr_err("invalid size in dtb_empty_root\n");
+			return;
+		}
+		of_fdt_crc32 = crc32_be(~0, fdt, fdt_totalsize(fdt));
+		fdt = copy_device_tree(fdt);
+	}
+
+	__unflatten_device_tree(fdt, NULL, &of_root,
 				early_init_dt_alloc_memory_arch, false);
 
 	/* Get pointer to "/chosen" and "/aliases" nodes for use everywhere */

drivers/of/platform.c

@@ -512,9 +512,6 @@ static int __init of_platform_default_populate_init(void)
 
 	device_links_supplier_sync_state_pause();
 
-	if (!of_have_populated_dt())
-		return -ENODEV;
-
 	if (IS_ENABLED(CONFIG_PPC)) {
 		struct device_node *boot_display = NULL;
 		struct platform_device *dev;

include/linux/of.h

@@ -180,11 +180,6 @@ static inline bool is_of_node(const struct fwnode_handle *fwnode)
 			&__of_fwnode_handle_node->fwnode : NULL;	\
 	})
 
-static inline bool of_have_populated_dt(void)
-{
-	return of_root != NULL;
-}
-
 static inline bool of_node_is_root(const struct device_node *node)
 {
 	return node && (node->parent == NULL);
@@ -546,11 +541,6 @@ static inline struct device_node *of_find_node_with_property(
 
 #define of_fwnode_handle(node) NULL
 
-static inline bool of_have_populated_dt(void)
-{
-	return false;
-}
-
 static inline struct device_node *of_get_compatible_child(const struct device_node *parent,
 					const char *compatible)
 {
@@ -1634,6 +1624,21 @@ static inline bool of_device_is_system_power_controller(const struct device_node
 	return of_property_read_bool(np, "system-power-controller");
 }
 
+/**
+ * of_have_populated_dt() - Has DT been populated by bootloader
+ *
+ * Return: True if a DTB has been populated by the bootloader and it isn't the
+ * empty builtin one. False otherwise.
+ */
+static inline bool of_have_populated_dt(void)
+{
+#ifdef CONFIG_OF
+	return of_property_present(of_root, "compatible");
+#else
+	return false;
+#endif
+}
+
 /*
  * Overlay support
  */