[PATCH v3 06/13] nvmem: microchip-otpc: add tag-based packet lookup
Claudiu Beznea
claudiu.beznea at tuxon.dev
Tue Jul 21 12:14:19 PDT 2026
Hi, Varshini,
On 6/30/26 12:35, 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")
>
I think this:
> 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.
should fall in a different patch?
>
> 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 at microchip.com>
> ---
> drivers/nvmem/microchip-otpc.c | 143 ++++++++++++++++--
> .../nvmem/microchip,sama7g5-otpc.h | 4 +-
> 2 files changed, 136 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/nvmem/microchip-otpc.c b/drivers/nvmem/microchip-otpc.c
> index df979e8549fd..bf8589048e17 100644
> --- a/drivers/nvmem/microchip-otpc.c
> +++ b/drivers/nvmem/microchip-otpc.c
> @@ -18,16 +18,20 @@
> #define MCHP_OTPC_CR_READ BIT(6)
> #define MCHP_OTPC_MR (0x4)
> #define MCHP_OTPC_MR_ADDR GENMASK(31, 16)
> +#define MCHP_OTPC_MR_EMUL BIT(7)
> #define MCHP_OTPC_AR (0x8)
> #define MCHP_OTPC_SR (0xc)
> #define MCHP_OTPC_SR_READ BIT(6)
> #define MCHP_OTPC_HR (0x20)
> #define MCHP_OTPC_HR_SIZE GENMASK(15, 8)
> +#define MCHP_OTPC_HR_PACKET_TYPE GENMASK(2, 0)
Nit: in SAMA7D65 manual this is simply packet. Maybe rename it:
MCHP_OTPC_HR_PACKET to match the manual.
> #define MCHP_OTPC_DR (0x24)
>
> #define MCHP_OTPC_NAME "mchp-otpc"
> #define MCHP_OTPC_SIZE (11 * 1024)
>
> +#define PACKET_TYPE_REGULAR 1
I would move this close to MCHP_OTPC_HR_PACKET_TYPE and name it something like:
MCHP_OTPC_HR_PACKET_REGULAR to match the datasheet.
> +
> /**
> * struct mchp_otpc - OTPC private data structure
> * @base: base address
> @@ -47,11 +51,15 @@ struct mchp_otpc {
> * @list: list head
> * @id: packet ID
> * @offset: packet offset (in words) in OTP memory
> + * @type: type of the packet
> + * @tag: 4-byte ASCII tag of the packet
> */
> struct mchp_otpc_packet {
> struct list_head list;
> u32 id;
> u32 offset;
> + u32 type;
This can be dropped for now since it's used only in the initialization path.
> + u32 tag;
> };
>
> static struct mchp_otpc_packet *mchp_otpc_id_to_packet(struct mchp_otpc *otpc,
> @@ -70,6 +78,56 @@ static struct mchp_otpc_packet *mchp_otpc_id_to_packet(struct mchp_otpc *otpc,
> return NULL;
> }
>
> +/**
> + * mchp_otpc_tag_to_packet() - find packet by tag
> + * @otpc: OTPC private data
> + * @tag: 4-byte ASCII tag to search for
> + *
> + * Return: pointer to packet if found, NULL otherwise
> + */
I'm not sure we need this description since the function is simple enough. I
would drop it.
> +static struct mchp_otpc_packet *mchp_otpc_tag_to_packet(struct mchp_otpc *otpc,
> + u32 tag)
> +{
> + struct mchp_otpc_packet *packet;
> +
> + list_for_each_entry(packet, &otpc->packets, list) {
> + if (packet->tag == tag)
> + return packet;
> + }
> +
> + return NULL;
> +}
> +
> +/**
> + * mchp_otpc_resolve_packet() - resolve offset to packet
> + * @otpc: OTPC private data
> + * @off: NVMEM offset (legacy ID-based or TAG-based)
> + *
> + * Legacy offsets (multiples of 4 within valid ID range) are resolved
> + * through ID lookup. Other offsets are treated as 4-byte ASCII tags.
> + *
> + * Return: pointer to packet if found, NULL otherwise
> + */
Same here.
> +static struct mchp_otpc_packet *mchp_otpc_resolve_packet(struct mchp_otpc *otpc,
> + u32 off)
> +{
> + /*
> + * 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;
> +
> + if (!(off % 4) && id < otpc->npackets)
The tag can be anything, no? Can't the tag satisfy this condition and the
execution to wrongly use mchp_otpc_id_to_packet() ?
> + 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);
> +}
> +
> 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:
> + *
> + * 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.
> */
> static int mchp_otpc_read(void *priv, unsigned int off, void *val,
> size_t bytes)
> @@ -154,12 +233,11 @@ static int mchp_otpc_read(void *priv, unsigned int off, void *val,
> int ret, payload_size;
>
> /*
> - * We reach this point with off being multiple of stride = 4 to
> - * be able to cross the subsystem. 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().
> + * From this point the offset has to be translated into the actual
> + * packet. For this we traverse the table of contents stored in a list
> + * "packet" based on the access type - packet id or tag.
> */
> - packet = mchp_otpc_id_to_packet(otpc, off / 4);
> + packet = mchp_otpc_resolve_packet(otpc, off);
> if (!packet)
> return -EINVAL;
> offset = packet->offset;
> @@ -190,6 +268,29 @@ static int mchp_otpc_read(void *priv, unsigned int off, void *val,
> return 0;
> }
>
> +/**
> + * mchp_otpc_read_packet_tag() - read tag from packet payload
> + * @otpc: OTPC private data
> + * @offset: packet offset in OTP memory
> + * @val: pointer to store the tag value
> + *
> + * Return: 0 on success, negative errno on failure
> + */
Same here regarding the description.
> +static int mchp_otpc_read_packet_tag(struct mchp_otpc *otpc, unsigned int offset,
> + unsigned int *val)
I think would be better to rename val to tag.
> +{
> + int ret;
> +
> + ret = mchp_otpc_prepare_read(otpc, offset);
> + if (ret)
> + return ret;
> +
> + writel_relaxed(0, otpc->base + MCHP_OTPC_AR);
> + *val = readl_relaxed(otpc->base + MCHP_OTPC_DR);
> +
> + return 0;
> +}
> +
> static int mchp_otpc_init_packets_list(struct mchp_otpc *otpc, u32 *size)
> {
> struct mchp_otpc_packet *packet;
> @@ -215,6 +316,17 @@ static int mchp_otpc_init_packets_list(struct mchp_otpc *otpc, u32 *size)
>
> packet->id = id++;
> packet->offset = word_pos;
> + packet->type = FIELD_GET(MCHP_OTPC_HR_PACKET_TYPE, word);
> +
> + if (packet->type == PACKET_TYPE_REGULAR) {
> + ret = mchp_otpc_read_packet_tag(otpc, packet->offset,
> + &packet->tag);
> + if (ret)
> + return ret;
> + } else {
> + packet->tag = 0;
> + }
> +
> INIT_LIST_HEAD(&packet->list);
> list_add_tail(&packet->list, &otpc->packets);
>
> @@ -236,7 +348,7 @@ static struct nvmem_config mchp_nvmem_config = {
> .type = NVMEM_TYPE_OTP,
> .read_only = true,
> .word_size = 4,
> - .stride = 4,
> + .stride = 1,
> .reg_read = mchp_otpc_read,
> };
>
> @@ -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;
> 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");
> +
Looks to me that this section ^
> 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");
> + }
> +
And this one ^ could be squashed in a different patch?
Thank you,
Claudiu
> mchp_nvmem_config.dev = otpc->dev;
> mchp_nvmem_config.add_legacy_fixed_of_cells = true;
> mchp_nvmem_config.size = size;
> diff --git a/include/dt-bindings/nvmem/microchip,sama7g5-otpc.h b/include/dt-bindings/nvmem/microchip,sama7g5-otpc.h
> index f570b23165a2..5f72e75ad091 100644
> --- a/include/dt-bindings/nvmem/microchip,sama7g5-otpc.h
> +++ b/include/dt-bindings/nvmem/microchip,sama7g5-otpc.h
> @@ -4,8 +4,8 @@
> #define _DT_BINDINGS_NVMEM_MICROCHIP_OTPC_H
>
> /*
> - * Need to have it as a multiple of 4 as NVMEM memory is registered with
> - * stride = 4.
> + * Need to have it as a multiple of 4 for the legacy id based packet
> + * access.
> */
> #define OTP_PKT(id) ((id) * 4)
>
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