Re: [PATCH v3 06/13] nvmem: microchip-otpc: add tag-based packet lookup

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 30 Jun 2026 15:05:56 +0530
Varshini Rajendran <varshini.rajendran@microchip.com> wrote:

> Add support for accessing OTP packets by their 4-byte ASCII tag while
> preserving backward compatibility with the existing ID-based lookup.
> 
> The OTP memory layout can vary across devices and may change over time,
> making the packet ID approach unreliable when the memory map is not
> known in advance. The packet tag provides a reliable way to identify
> and access packets without prior knowledge of the OTP memory layout.
> 
> Two offset encoding are now supported:
>   1. Legacy ID-based: offset = OTP_PKT(id) = id * 4
>      Used in DT as: reg = <OTP_PKT(1) 76>;
>   2. TAG-based: offset = 4-byte ASCII packet tag
>      Used in DT as: reg = <0x41435354 0x4c>; (tag "ACST")
> 
> The driver resolves offsets matching valid legacy selectors (multiples
> of 4 within the packet count) through ID lookup, falling back to tag
> lookup for other values. This ensures existing device trees continue
> to work while enabling new tag-based access.
> 
> During probe, packet meta data including the tag is read and cached.
> The driver also validates OTP memory accessibility and emulation mode
> status. When the boot packet is not configured, emulation mode allows
> access to the other packets. When both are not available an
> informational message is logged.
> 
> The stride of the nvmem memory is set to 1 in order to support tag based
> offsets, comment in the header file is updated accordingly.
> 
> Signed-off-by: Varshini Rajendran <varshini.rajendran@microchip.com>
A couple of trivial things inline

Thanks,

Jonathan

>  static int mchp_otpc_prepare_read(struct mchp_otpc *otpc,
>  				  unsigned int offset)
>  {
> @@ -140,8 +198,29 @@ static int mchp_otpc_prepare_read(struct mchp_otpc *otpc,
>   * offset returned by hardware.
>   *
>   * For this, the read function will return the first requested bytes in the
> - * packet. The user will have to be aware of the memory footprint before doing
> - * the read request.
> + * packet.
> + *
> + * Two offset encoding are supported:

"encodings" as there are two of them.

> + *
> + * 1. Legacy ID-based: offset = OTP_PKT(id) = id * 4
> + *    Used in DT as: reg = <OTP_PKT(1) 76>;
> + * 2. TAG-based: offset = 4-byte ASCII packet tag
> + *    Used in DT as: reg = <0x41435354 0x4c>; (tag "ACST")
> + *
> + * To use the legacy ID based packet lookup the user will have to be aware of
> + * the memory footprint before doing the read request.
> + *
> + * But by using the TAG based packet lookup, the user won't have to be aware
> + * of the memory footprint before doing the read request since this driver has
> + * it abstracted and taken care of.
> + *
> + * Practically, there is no way of knowing the mapping of the OTP memory table
> + * in advance for every device. But by using the packet tag - the identifier
> + * ASCII value, the packets can be recognized without being aware of the
> + * flashed OTP memory map table and the payload can be acquired reliably.
> + *
> + * While the legacy ID based lookup is still supported, TAG based approach is
> + * recommended.
>   */
  
> @@ -244,7 +356,8 @@ static int mchp_otpc_probe(struct platform_device *pdev)
>  {
>  	struct nvmem_device *nvmem;
>  	struct mchp_otpc *otpc;
> -	u32 size;
> +	bool emul_enable;
> +	u32 size, tmp;

If possible give tmp a more meaningful name.  reg_val or better yet
something about which register.

>  	int ret;
>  
>  	otpc = devm_kzalloc(&pdev->dev, sizeof(*otpc), GFP_KERNEL);
> @@ -256,10 +369,22 @@ static int mchp_otpc_probe(struct platform_device *pdev)
>  		return PTR_ERR(otpc->base);
>  
>  	otpc->dev = &pdev->dev;
> +
> +	tmp = readl_relaxed(otpc->base + MCHP_OTPC_MR);
> +	emul_enable = tmp & MCHP_OTPC_MR_EMUL;
> +	if (emul_enable)
> +		dev_info(otpc->dev, "Emulation mode enabled\n");
> +
>  	ret = mchp_otpc_init_packets_list(otpc, &size);
>  	if (ret)
>  		return ret;
>  
> +	if (!size) {
> +		dev_warn(otpc->dev, "Cannot access OTP memory\n");
> +		if (!emul_enable)
> +			dev_info(otpc->dev, "Boot packet not programmed and emulation mode disabled\n");
> +	}
> +