Re: [PATCH v3] mtd: limit OTP NVMEM Cell parse to non Nand devices

From: Rafał Miłecki
Date: Wed Mar 27 2024 - 11:06:58 EST


On 2024-03-22 05:09, Christian Marangi wrote:
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.

Er... thank you?


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.

So as you noticed this problem was *exposed* by adding OTP support for
Macronix NAND chips.


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 detect the MTD type is Nand.

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")

It's not that commit however that introduced the problem. Introducing
"add_legacy_fixed_of_cells" just added a clean way of enabling parsing
of old cells syntax. Even before my commit NVMEM subsystem was looking
for NVMEM cells in NAND devices.

I booted kernel 6.6 which has commit e87161321a40 ("mtd: rawnand:
macronix: OTP > access for MX30LFxG18AC") but does NOT have commit
2cc3b37f5b6d ("nvmem: add explicit config option to read old syntax
fixed OF cells").

Look at this log from Broadcom Northstar (Linux 6.6):
[ 0.410107] nand: device found, Manufacturer ID: 0xc2, Chip ID: 0xdc
[ 0.416531] nand: Macronix MX30LF4G18AC
[ 0.420409] nand: 512 MiB, SLC, erase size: 128 KiB, page size: 2048, OOB size: 64
[ 0.428022] iproc_nand 18028000.nand-controller: detected 512MiB total, 128KiB blocks, 2KiB pages, 16B OOB, 8-bit, BCH-8
[ 0.438991] Scanning device for bad blocks
[ 0.873598] Bad eraseblock 738 at 0x000005c40000
[ 1.030279] random: crng init done
[ 1.854895] Bad eraseblock 2414 at 0x000012dc0000
[ 2.657354] Bad eraseblock 3783 at 0x00001d8e0000
[ 2.662967] Bad eraseblock 3785 at 0x00001d920000
[ 2.848418] nvmem user-otp1: nvmem: invalid reg on /nand-controller@18028000/nand@0
[ 2.856126] iproc_nand 18028000.nand-controller: error -EINVAL: Failed to register OTP NVMEM device

So to summary it up:
1. Problem exists since much earlier and wasn't introduced by 2cc3b37f5b6d
2. Commit 2cc3b37f5b6d just gives you a clean way of solving this issue
3. Problem was exposed by commit e87161321a40
4. We miss fix for v6.6 which doesn't have 2cc3b37f5b6d (it hit v6.7)


Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Christian Marangi <ansuelsmth@xxxxxxxxx>