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

[Andy Shevchenko]

On Tue, Jun 30, 2026 at 03:05:56PM +0530, Varshini Rajendran 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.

...

> +static struct mchp_otpc_packet *mchp_otpc_tag_to_packet(struct mchp_otpc *otpc,
> +							u32 tag)

Despite the length I would place all in a single line.

> +{
> +	struct mchp_otpc_packet *packet;
> +
> +	list_for_each_entry(packet, &otpc->packets, list) {
> +		if (packet->tag == tag)
> +			return packet;
> +	}
> +
> +	return NULL;
> +}

...

> +static struct mchp_otpc_packet *mchp_otpc_resolve_packet(struct mchp_otpc *otpc,
> +							 u32 off)

Ditto.

> +{
> +	/*
> +	 * Legacy id based packet access: offset = id * 4
> +	 * Inside the driver we use continuous unsigned integer numbers
> +	 * for packet id, thus divide off by 4 before passing it to
> +	 * mchp_otpc_id_to_packet().
> +	 */
> +	u32 id = off / 4;

Make a temporary variable for off % 4. In such a case it might be compiled into
one assembly instruction (yes, it may be not achievable IRL currently, but it's
just a better style in case one will use this piece of code to copy'n'paste
somewhere where it will make more sense).

> +	if (!(off % 4) && id < otpc->npackets)
> +		return mchp_otpc_id_to_packet(otpc, id);
> +
> +	/*
> +	 * TAG-based packet access: offset is a 4-byte ASCII tag
> +	 */
> +	return mchp_otpc_tag_to_packet(otpc, off);
> +}


-- 
With Best Regards,
Andy Shevchenko