[PATCH] mtd: limit OTP NVMEM Cell parse to non Nand devices

[Christian Marangi]

MTD OTP logic is very fragile and can be problematic with some specific
kind of devices.

NVMEM across the years had various iteration on how Cells could be
declared in DT and MTD OTP probably was left behind and
add_legacy_fixed_of_cells was enabled without thinking of the consequences.

That option enables NVMEM to scan the provided of_node and treat each
child as a NVMEM Cell, this was to support legacy NVMEM implementation
and don't cause regression.

This is problematic if we have devices like Nand where the OTP is
triggered by setting a special mode in the flash. In this context real
partitions declared in the Nand node are registered as OTP Cells and
this cause probe fail with -EINVAL error.

This was never notice due to the fact that till now, no Nand supported
the OTP feature. With commit e87161321a40 ("mtd: rawnand: macronix: OTP
access for MX30LFxG18AC") this changed and coincidentally this Nand is
used on an FritzBox 7530 supported on OpenWrt.

Alternative and more robust way to declare OTP Cells are already
prossible by using the fixed-layout node or by declaring a child node
with the compatible set to "otp-user" or "otp-factory".

To fix this and limit any regression with other MTD that makes use of
declaring OTP as direct child of the dev node, disable
add_legacy_fixed_of_cells if we have a node called nand since it's the
standard property name to identify Nand devices attached to a Nand
Controller.

With the following logic, the OTP NVMEM entry is correctly created with
no Cells and the MTD Nand is correctly probed and partitions are
correctly exposed.

Fixes: 2cc3b37f5b6d ("nvmem: add explicit config option to read old syntax fixed OF cells")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Christian Marangi <ansuelsmth@xxxxxxxxx>
---
 drivers/mtd/mtdcore.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
index 5887feb347a4..6872477a5129 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -900,7 +900,7 @@ static struct nvmem_device *mtd_otp_nvmem_register(struct mtd_info *mtd,
 	config.name = compatible;
 	config.id = NVMEM_DEVID_AUTO;
 	config.owner = THIS_MODULE;
-	config.add_legacy_fixed_of_cells = true;
+	config.add_legacy_fixed_of_cells = !of_node_name_eq(mtd->dev.of_node, "nand");
 	config.type = NVMEM_TYPE_OTP;
 	config.root_only = true;
 	config.ignore_wp = true;
-- 
2.43.0