mtd: nand: Error out if cmd_ctrl() is missing

If no user specified chip->select_chip() function is provided, code in
nand_base.c will automatically set this hook to nand_select_chip(),
which in turn depends on chip->cmd_ctrl() hook being valid. Not
providing both of those functions in NAND controller driver (for example
by mistake) will result in a bit cryptic segfault. Same is true for
chip->cmdfunc().

To avoid the above scenario add a check in nand_scan_dent and error out
if cmd_ctrl() is not provided.
Suggested-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Suggested-by: NBrian Norris <computersforpeace@gmail.com>
Signed-off-by: NAndrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: NBoris Brezillon <boris.brezillon@free-electrons.com>

drivers/mtd/nand/nand_base.c

@@ -4140,6 +4140,15 @@ int nand_scan_ident(struct mtd_info *mtd, int maxchips,
 	if (!mtd->name && mtd->dev.parent)
 		mtd->name = dev_name(mtd->dev.parent);
 
+	if ((!chip->cmdfunc || !chip->select_chip) && !chip->cmd_ctrl) {
+		/*
+		 * Default functions assigned for chip_select() and
+		 * cmdfunc() both expect cmd_ctrl() to be populated,
+		 * so we need to check that that's the case
+		 */
+		pr_err("chip.cmd_ctrl() callback is not provided");
+		return -EINVAL;
+	}
 	/* Set the default functions */
 	nand_set_defaults(chip, chip->options & NAND_BUSWIDTH_16);