of/fdt: Fix the len check in early_init_dt_check_for_usable_mem_range()

The len value is in bytes, while `dt_root_addr_cells + dt_root_size_cells`
is in cells (4 bytes per cell). Modulo calculation between them is
incorrect, the units must be converted first.

Use helper functions to simplify the code and fix this issue.

Fixes: fb319e7 ("of: fdt: Add memory for devices by DT property "linux,usable-memory-range"")
Fixes: 2af2b50 ("of: fdt: Add generic support for handling usable memory range property")
Fixes: 8f579b1 ("arm64: limit memory regions based on DT property, usable-memory-range")
Signed-off-by: Yuntao Wang <yuntao.wang@linux.dev>
Link: https://patch.msgid.link/20251115134753.179931-4-yuntao.wang@linux.dev
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Author: ytcoode

drivers/of/fdt.c

@@ -884,23 +884,27 @@ static unsigned long chosen_node_offset = -FDT_ERR_NOTFOUND;
 void __init early_init_dt_check_for_usable_mem_range(void)
 {
 	struct memblock_region rgn[MAX_USABLE_RANGES] = {0};
-	const __be32 *prop, *endp;
+	const __be32 *prop;
 	int len, i;
+	u64 base, size;
 	unsigned long node = chosen_node_offset;
 
 	if ((long)node < 0)
 		return;
 
 	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)))
+	prop = of_flat_dt_get_addr_size_prop(node, "linux,usable-memory-range",
+					     &len);
+	if (!prop)
 		return;
 
-	endp = prop + (len / sizeof(__be32));
-	for (i = 0; i < MAX_USABLE_RANGES && prop < endp; i++) {
-		rgn[i].base = dt_mem_next_cell(dt_root_addr_cells, &prop);
-		rgn[i].size = dt_mem_next_cell(dt_root_size_cells, &prop);
+	len = min(len, MAX_USABLE_RANGES);
+
+	for (i = 0; i < len; i++) {
+		of_flat_dt_read_addr_size(prop, i, &base, &size);
+		rgn[i].base = base;
+		rgn[i].size = size;
 
 		pr_debug("cap_mem_regions[%d]: base=%pa, size=%pa\n",
 			 i, &rgn[i].base, &rgn[i].size);