of: fdt: Add generic support for handling usable memory range property

Add support for handling the "linux,usable-memory-range" property in the
"/chosen" node to the FDT core code.  This can co-exist safely with the
architecture-specific handling, until the latter has been removed.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/3bd69bada93ee59b7d23c38b3527fc1654e19343.1628670468.git.geert+renesas@glider.be

Documentation/devicetree/bindings/chosen.txt

@@ -79,9 +79,9 @@ a different secondary CPU release mechanism)
 linux,usable-memory-range
 -------------------------
 
-This property (arm64 only) holds a base address and size, describing a
-limited region in which memory may be considered available for use by
-the kernel. Memory outside of this range is not available for use.
+This property holds a base address and size, describing a limited region in
+which memory may be considered available for use by the kernel. Memory outside
+of this range is not available for use.
 
 This property describes a limitation: memory within this range is only
 valid when also described through another mechanism that the kernel

drivers/of/fdt.c

@@ -972,6 +972,32 @@ static void __init early_init_dt_check_for_elfcorehdr(unsigned long node)
 		 elfcorehdr_addr, elfcorehdr_size);
 }
 
+static phys_addr_t cap_mem_addr;
+static phys_addr_t cap_mem_size;
+
+/**
+ * early_init_dt_check_for_usable_mem_range - Decode usable memory range
+ * location from flat tree
+ * @node: reference to node containing usable memory range location ('chosen')
+ */
+static void __init early_init_dt_check_for_usable_mem_range(unsigned long node)
+{
+	const __be32 *prop;
+	int len;
+
+	pr_debug("Looking for usable-memory-range property... ");
+
+	prop = of_get_flat_dt_prop(node, "linux,usable-memory-range", &len);
+	if (!prop || (len < (dt_root_addr_cells + dt_root_size_cells)))
+		return;
+
+	cap_mem_addr = dt_mem_next_cell(dt_root_addr_cells, &prop);
+	cap_mem_size = dt_mem_next_cell(dt_root_size_cells, &prop);
+
+	pr_debug("cap_mem_start=%pa cap_mem_size=%pa\n", &cap_mem_addr,
+		 &cap_mem_size);
+}
+
 #ifdef CONFIG_SERIAL_EARLYCON
 
 int __init early_init_dt_scan_chosen_stdout(void)
@@ -1120,6 +1146,7 @@ int __init early_init_dt_scan_chosen(unsigned long node, const char *uname,
 
 	early_init_dt_check_for_initrd(node);
 	early_init_dt_check_for_elfcorehdr(node);
+	early_init_dt_check_for_usable_mem_range(node);
 
 	/* Retrieve command line */
 	p = of_get_flat_dt_prop(node, "bootargs", &l);
@@ -1253,6 +1280,9 @@ void __init early_init_dt_scan_nodes(void)
 
 	/* Setup memory, calling early_init_dt_add_memory_arch */
 	of_scan_flat_dt(early_init_dt_scan_memory, NULL);
+
+	/* Handle linux,usable-memory-range property */
+	memblock_cap_memory_range(cap_mem_addr, cap_mem_size);
 }
 
 bool __init early_init_dt_scan(void *params)