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

[Miquel Raynal]

Hi Christian,

ansuelsmth at gmail.com wrote on Wed, 20 Mar 2024 17:29:25 +0100:

> 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

The beauty of backward compatibility...

> 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.

Thanks for the investigation and the fix. An implementation detail
below.

> 
> Fixes: 2cc3b37f5b6d ("nvmem: add explicit config option to read old syntax fixed OF cells")
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Christian Marangi <ansuelsmth at gmail.com>
> ---
>  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");

Could we use mtd_type_is_nand() instead?

>  	config.type = NVMEM_TYPE_OTP;
>  	config.root_only = true;
>  	config.ignore_wp = true;

Thanks,
Miquèl