of/fdt: Rework early_init_dt_scan_root() to call directly

Use of the of_scan_flat_dt() function predates libfdt and is discouraged
as libfdt provides a nicer set of APIs. Rework early_init_dt_scan_root()
to be called directly and use libfdt.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Link: https://lore.kernel.org/r/20211118181213.1433346-3-robh@kernel.org

arch/powerpc/kernel/prom.c

@@ -748,7 +748,7 @@ void __init early_init_devtree(void *params)
 	of_scan_flat_dt(early_init_dt_scan_chosen_ppc, boot_command_line);
 
 	/* Scan memory nodes and rebuild MEMBLOCKs */
-	of_scan_flat_dt(early_init_dt_scan_root, NULL);
+	early_init_dt_scan_root();
 	of_scan_flat_dt(early_init_dt_scan_memory_ppc, NULL);
 
 	parse_early_param();
@@ -857,7 +857,7 @@ void __init early_get_first_memblock_info(void *params, phys_addr_t *size)
 	 * mess the memblock.
 	 */
 	add_mem_to_memblock = 0;
-	of_scan_flat_dt(early_init_dt_scan_root, NULL);
+	early_init_dt_scan_root();
 	of_scan_flat_dt(early_init_dt_scan_memory_ppc, NULL);
 	add_mem_to_memblock = 1;
 

drivers/of/fdt.c

@@ -1042,13 +1042,14 @@ int __init early_init_dt_scan_chosen_stdout(void)
 /*
  * early_init_dt_scan_root - fetch the top level address and size cells
  */
-int __init early_init_dt_scan_root(unsigned long node, const char *uname,
-				   int depth, void *data)
+int __init early_init_dt_scan_root(void)
 {
 	const __be32 *prop;
+	const void *fdt = initial_boot_params;
+	int node = fdt_path_offset(fdt, "/");
 
-	if (depth != 0)
-		return 0;
+	if (node < 0)
+		return -ENODEV;
 
 	dt_root_size_cells = OF_ROOT_NODE_SIZE_CELLS_DEFAULT;
 	dt_root_addr_cells = OF_ROOT_NODE_ADDR_CELLS_DEFAULT;
@@ -1063,8 +1064,7 @@ int __init early_init_dt_scan_root(unsigned long node, const char *uname,
 		dt_root_addr_cells = be32_to_cpup(prop);
 	pr_debug("dt_root_addr_cells = %x\n", dt_root_addr_cells);
 
-	/* break now */
-	return 1;
+	return 0;
 }
 
 u64 __init dt_mem_next_cell(int s, const __be32 **cellp)
@@ -1263,7 +1263,7 @@ void __init early_init_dt_scan_nodes(void)
 	int rc;
 
 	/* Initialize {size,address}-cells info */
-	of_scan_flat_dt(early_init_dt_scan_root, NULL);
+	early_init_dt_scan_root();
 
 	/* Retrieve various information from the /chosen node */
 	rc = early_init_dt_scan_chosen(boot_command_line);

include/linux/of_fdt.h

@@ -68,8 +68,7 @@ extern void early_init_dt_add_memory_arch(u64 base, u64 size);
 extern u64 dt_mem_next_cell(int s, const __be32 **cellp);
 
 /* Early flat tree scan hooks */
-extern int early_init_dt_scan_root(unsigned long node, const char *uname,
-				   int depth, void *data);
+extern int early_init_dt_scan_root(void);
 
 extern bool early_init_dt_scan(void *params);
 extern bool early_init_dt_verify(void *params);