fs: add block_is_erased() to check if a block is truly erased

Makefile

@@ -23,6 +23,8 @@ obj-y += source/shell/flash.o
 
 obj-y += source/core/flash.o
 
+obj-y += source/fs/block.o
+
 obj = $(addprefix $(BUILD)/, $(obj-y))
 
 # Include the dependencies.

include/fs/block.h

@@ -0,0 +1,3 @@
+#pragma once
+
+int block_is_erased(struct flash_dev *dev, uint32_t addr, uint32_t len);

source/fs/block.c

@@ -0,0 +1,28 @@
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <flash.h>
+#include <macros.h>
+
+#include <fs/block.h>
+
+int block_is_erased(struct flash_dev *dev, uint32_t addr, uint32_t len)
+{
+	char buf[32], cmp[32];
+	size_t count;
+
+	memset(cmp, 0xff, sizeof cmp);
+
+	for (; len; addr += count, len -= count) {
+		count = min(len, sizeof buf);
+
+		if (flash_read(dev, addr, buf, count) < 0)
+			return -1;
+
+		if (memcmp(buf, cmp, count) != 0)
+			return -1;
+	}
+
+	return 0;
+}