RT-Thread RTOS
An open source embedded real-time operating system
OFW boot and FDT

FDT bring-up (<tt>fdt.c</tt>)

Raw DTB → memblock → unflattened **rt_ofw_node** tree. After unflatten, use **rt_ofw_*** (OFW nodes and properties), not **rt_fdt_***, unless still touching the blob.

Header: **drivers/ofw_fdt.h**. Source: **components/drivers/ofw/fdt.c**.

Boot sequence

rt_fdt_prefetch(fdt)
→ rt_fdt_scan_root()
→ rt_fdt_scan_memory() /* memblock */
→ rt_fdt_scan_initrd() /* optional */
→ rt_fdt_unflatten() /* rt_ofw_node tree */
→ INIT_PLATFORM_EXPORT(platform_ofw_device_probe)

Global nodes (**ofw_internal.h**): **ofw_node_root**, **ofw_node_cpus**, **ofw_node_chosen**, **ofw_node_aliases**, **ofw_node_reserved_memory**.

API (blob phase)

API Role
**rt_fdt_prefetch** Store **_fdt**, validate header, **rt_fdt_scan_root**
**rt_fdt_scan_root** Root **#address-cells / #size-cells**
**rt_fdt_scan_memory** **device_type = "memory"** → **rt_memblock_add_memory**
**rt_fdt_scan_initrd** Reserve initrd range
**rt_fdt_unflatten** Build in-memory tree; **ofw_alias_scan**; phandle hash
**rt_fdt_unflatten_single** Unflatten with explicit **fdt** pointer
**rt_fdt_device_is_available** **status** on blob node (pre-unflatten)

Chosen / bootargs

API Role
**rt_fdt_scan_chosen_stdout** **stdout-path**
**rt_fdt_bootargs_select** Parse **bootargs** token
**rt_ofw_bootargs_select** Same on unflattened tree

Early console

API / macro Role
**RT_FDT_EARLYCON_EXPORT** Register **rt_fdt_earlycon_id** in linker section
**rt_fdt_earlycon_output** Print before UART driver probe
**rt_fdt_earlycon_kick** Hand off to real console

libfdt helpers

API Role
**rt_fdt_read_number**, **rt_fdt_next_cell** Cell decoding
**rt_fdt_translate_address** Address on raw blob

Memory and reserved regions

Source Effect
DTB **/memreserve** **rt_memblock_reserve_memory**
**/reserved-memory** nodes Validated **reg** (+ **ranges**)
**rt_fdt_scan_initrd** Initrd carve-out

Unflatten marks system nodes with **RT_OFW_F_SYSTEM | RT_OFW_F_READLY** so they are not platform devices.

Builtin DTB

**RT_USING_BUILTIN_FDT** embeds **RT_BUILTIN_FDT_PATH**. Build flow: Devicetree Compiler.

Pitfalls

Issue Mitigation
**rt_fdt_* after unflatten** Prefer **rt_ofw_*** for properties
Missing **/cpus** **RT_ASSERT** in unflatten — BSP must supply
Memory scan skipped Call **scan_memory** before allocator use

See also