[PATCH v6 09/17] nvmem: microchip-otpc: nvmem: microchip-otpc: add tag-based packet lookup

[Andy Shevchenko]

On Thu, Aug 06, 2026 at 01:10:16PM +0530, Varshini Rajendran wrote:
> Add support for accessing OTP packets by their tag which is a FourCC
> 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 encodings 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 (FourCC)
>      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 valid 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 stride of the nvmem memory is set to 1 in order to support tag based
> offsets, comment in the header file is updated accordingly.

...

> +static bool mchp_otpc_is_valid_fourcc(u32 tag)
> +{
> +	int i;
> +	u8 c;
> +
> +	for (i = 0; i < 4; i++) {

	for (unsigned int i = 0; i < sizeof(tag); i++) {

> +		c = (tag >> (i * 8)) & 0xff;
> +		if (c < 0x20 || c > 0x7e)
> +			return false;
> +	}
> +
> +	return true;
> +}

-- 
With Best Regards,
Andy Shevchenko