mtdblock: Warn if added for a NAND device

There is a surprisingly large number of tutorials
that suggest using mtdblock to mount SquashFS filesystems
on flash devices, including NAND devices.

This approach is suboptimal than using UBI. If the flash device
is NAND, this is specially true, due to wear leveling, bit-flips and
badblocks. In this case UBI is strongly preferred, so be nice to users
and print a warning suggesting to consider UBI block, if mtdblock
is added for a NAND device.
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210801234509.18774-8-ezequiel@collabora.com

drivers/mtd/mtdblock.c

@@ -322,6 +322,10 @@ static void mtdblock_add_mtd(struct mtd_blktrans_ops *tr, struct mtd_info *mtd)
 	if (!(mtd->flags & MTD_WRITEABLE))
 		dev->mbd.readonly = 1;
 
+	if (mtd_type_is_nand(mtd))
+		pr_warn("%s: MTD device '%s' is NAND, please consider using UBI block devices instead.\n",
+			tr->name, mtd->name);
+
 	if (add_mtd_blktrans_dev(&dev->mbd))
 		kfree(dev);
 }

drivers/mtd/mtdblock_ro.c

@@ -46,6 +46,10 @@ static void mtdblock_add_mtd(struct mtd_blktrans_ops *tr, struct mtd_info *mtd)
 	dev->tr = tr;
 	dev->readonly = 1;
 
+	if (mtd_type_is_nand(mtd))
+		pr_warn("%s: MTD device '%s' is NAND, please consider using UBI block devices instead.\n",
+			tr->name, mtd->name);
+
 	if (add_mtd_blktrans_dev(dev))
 		kfree(dev);
 }