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

[Rafał Miłecki]

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 at 18028000/nand at 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 at vger.kernel.org>
> Signed-off-by: Christian Marangi <ansuelsmth at gmail.com>