Non-volatile memory (NVMEM)

Header: **components/drivers/include/drivers/nvmem.h**. Core: **components/drivers/nvmem/nvmem.c**.

NVMEM models OTP, eFuse, EEPROM slices, PMIC trim, MAC/calibration blobs as a provider device plus cells (named or indexed byte/bit ranges). Consumers resolve cells from device tree phandles—see NVMEM device model (DM).

There is no in-tree OTP driver under **components/drivers/nvmem/**; SoC providers live under **SOC_DM_NVMEM_DIR** in the BSP.

When to use NVMEM

Use *`rt_nvmem_`** when…	Use a dedicated driver when…
Multiple consumers need slices of one OTP/eFuse controller.	Only one driver reads a fixed offset once at boot.
DT uses `nvmem-cells` / `nvmem-cell-names` (Linux-style).	MAC/calibration is hard-coded in BSP C sources.
You want refcounted `rt_nvmem_cell` handles.	Direct MMIO in a single platform driver is enough.

Data structures

struct rt_nvmem_device {
    struct rt_device parent;
    int cells_nr;
    rt_list_t cell_nodes;
    rt_ssize_t (*reg_read)(struct rt_nvmem_device *, int offset, void *val, rt_size_t bytes);
    rt_ssize_t (*reg_write)(struct rt_nvmem_device *, int offset, void *val, rt_size_t bytes);
    rt_ssize_t size;
    int word_size;
    int stride;
    rt_bool_t read_only;
    rt_bool_t ignore_wp;
    rt_base_t wp_pin;
    rt_uint8_t wp_pin_active;
    struct rt_ref ref;
    struct rt_spinlock spinlock;
    void *priv;
};

Field	Role
`reg_read` / `reg_write`	Byte access at `offset`
`read_only`	DT `read-only`, no `reg_write`, or preset flag
`wp_pin`	Optional `wp` GPIO for EEPROM

Provider API

rt_err_t rt_nvmem_device_register(struct rt_nvmem_device *ndev);
rt_err_t rt_nvmem_device_unregister(struct rt_nvmem_device *ndev);
rt_err_t rt_nvmem_device_append_cell(struct rt_nvmem_device *ndev, struct rt_nvmem_cell *cell);

**register**: optional **wp** pin, **read_only** from DT / missing **reg_write**, **rt_ofw_data(np)=ndev**.

**unregister**: only if **rt_ref_read(&ndev->ref)==1**.

**append_cell**: static cells (**free_able false**) known at probe time.

Consumer API

struct rt_nvmem_cell *rt_nvmem_get_cell_by_index(struct rt_device *dev, int index);
struct rt_nvmem_cell *rt_nvmem_get_cell_by_name(struct rt_device *dev, const char *id);
void rt_nvmem_put_cell(struct rt_nvmem_cell *cell);
rt_ssize_t rt_nvmem_cell_read(struct rt_nvmem_cell *cell, void *buffer, rt_size_t len);
rt_ssize_t rt_nvmem_cell_write(struct rt_nvmem_cell *cell, void *buffer, rt_size_t len);

**len <= cell->bytes** and **<= nvmem->size**
Bit fields: **bit_offset**, **nbits** — read shifts/masks; write read-modify-write
**wp_pin** deasserted during **reg_write**
Typed helpers: **rt_nvmem_cell_read_u8/u16/u32/u64**

Example

cell = rt_nvmem_get_cell_by_name(&pdev->parent, "mac-address");
if (!cell || rt_is_err_ptr(cell)) return -RT_ERROR;
rt_nvmem_cell_read(cell, mac, 6);
rt_nvmem_put_cell(cell);

Pitfalls

Issue	Mitigation
Forgot `put_cell`	Blocks unregister
OTP write	Omit `reg_write`
No BSP provider	`SOC_DM_NVMEM_DIR`