RT-Thread RTOS
An open source embedded real-time operating system
NVMEM device model (DM)

NVMEM under <tt>RT_USING_DM</tt>

**RT_USING_NVMEM** builds **nvmem.c** only. Provider drivers register **struct rt_nvmem_device** and set **rt_ofw_data(provider_np)**. Consumers parse **nvmem-cells** phandles.

API: NVMEM subsystem. Sources: **nvmem.c**, **nvmem.h**.

Kconfig

Option Role
**RT_USING_DM** Required
**RT_USING_OFW** Required
**RT_USING_PIN** Optional **wp** GPIO
**RT_USING_NVMEM** Core; selects **RT_USING_ADT_REF**
**SOC_DM_NVMEM_DIR** BSP OTP/eFuse/EEPROM drivers

No **rt_bus** — provider **reg_read/reg_write** + OFW lookup only.

Architecture

BSP provider (efuse/otp/eeprom)
| rt_nvmem_device_register, rt_ofw_data = ndev
v
Consumer: nvmem-cells = <&provider N>;
v
rt_nvmem_get_cell_by_name → rt_nvmem_cell_read/write
v
rt_nvmem_put_cell

Provider DT

efuse: efuse@110000 {
compatible = "vendor,soc-efuse";
reg = <0x110000 0x1000>;
#address-cells = <1>;
#size-cells = <1>;
#nvmem-cell-cells = <1>;
read-only;
mac@0 { reg = <0x00 0x06>; };
};
Property Role
**#nvmem-cell-cells** Usually 1 — phandle arg = cell index
**read-only** Forces read-only at register
**wp** Write-protect GPIO (unless **ignore_wp**)

Consumer DT

gmac: ethernet@0 {
nvmem-cells = <&efuse 0>;
nvmem-cell-names = "mac-address";
};

**ofw_nvmem_get_cell** order:

  1. **rt_ofw_data(cell_np)** if cached
  2. Match **append_cell** static list by index/id
  3. Allocate dynamic cell from child **reg** (**offset**, **bytes**)

Dynamic cells are **free_able** and removed on last **put_cell**.

Provider checklist

  1. Implement **reg_read**; **reg_write** only if programmable.
  2. Set **size**, **parent.ofw_node**, callbacks.
  3. **rt_nvmem_device_register**.
  4. Optional **append_cell** for fixed index/id layouts.
  5. **remove**: all **put_cell**, then **unregister** when ref is 1.
  6. Add driver under **SOC_DM_NVMEM_DIR**.

Bit-packed cells

**bit_offset** / **nbits** on **struct rt_nvmem_cell**: core masks after byte read; write merges into existing bytes. Prefer byte-aligned **reg = <offset length>**; validate parsing in **nvmem.c** or use **append_cell**.

Pitfalls

Issue Mitigation
Provider not probed **rt_platform_ofw_request** first
**#nvmem-cell-cells mismatch** Match provider binding
Concurrent OTP Serialize in **reg_read/reg_write**
**unregister EBUSY** Release all cell refs

See also