Memory Layout and Configuration Guide#

This document is based on the Kconfig, linker scripts, and board-level implementation in the SDK source code, and describes the memory layout, configuration entries, and tuning methods for the K230/K230D RT-Smart.

Overall Memory Model#

In the current SDK, DDR is mainly divided into two regions:

RT-Smart system region (kernel + user space page pool + dynamic heap)
MMZ region (media-related large block memory)

The core configuration items are located in Board Configuration under Memory Layout:

CONFIG_MEM_RTSMART_BASE: RT-Smart start base address (default 0x0)
CONFIG_RTSMART_OPENSIB_MEMORY_SIZE: OpenSBI reserved region size (default 0x20000)
CONFIG_MEM_RTSMART_SIZE: RT-Smart total memory size
CONFIG_MEM_RTSMART_HEAP_SIZE: RT-Smart heap size
CONFIG_MEM_MMZ_BASE: MMZ base address
CONFIG_MEM_MMZ_SIZE: MMZ size
CONFIG_MEM_TOTAL_SIZE: DDR total size (configured in static mode)

Source code reference:

boards/Kconfig
boards/Kconfig.memory_static
boards/Kconfig.memory_auto
src/rtsmart/rtsmart/kernel/bsp/maix3/board/board.c
src/rtsmart/rtsmart/kernel/bsp/maix3/board/board.h
src/rtsmart/rtsmart/kernel/bsp/maix3/link.lds.in

Two Configuration Modes#

Static Configuration Mode#

When CONFIG_AUTO_DETECT_DDR_SIZE=n, a set of fixed values from Kconfig.memory_static is used:

MEM_TOTAL_SIZE
MEM_RTSMART_SIZE
MEM_RTSMART_HEAP_SIZE
MEM_MMZ_BASE
MEM_MMZ_SIZE

These values are directly used as compilation configuration for linking and runtime initialization.

Applicable scenarios:

Board DDR is fixed (e.g., SiP with fixed capacity)
Need precise control over the size of each region

Auto DDR Detection Mode#

When CONFIG_AUTO_DETECT_DDR_SIZE=y, the tier configurations in Kconfig.memory_auto are used:

512M tier: *_512
1024M tier: *_1024
2048M tier: *_2048

Runtime flow (see board.c):

Obtain DDR capacity via k230_atag_get_ddr_size()
If the retrieval fails, fall back to 0x20000000 (512M)
Select the corresponding tier’s RT-Smart size, heap size, MMZ base address, and MMZ size based on capacity

Note: In auto mode, board.h redirects the CONFIG_MEM_* macros to the return values of get_*() functions, implementing switching based on the actual DDR size.

Key Address Calculation Formulas#

The following formulas come from board.c and link.lds.in:

System start address

\[ RTT\_SYS\_BASE = MEM\_RTSMART\_BASE + RTSMART\_OPENSIB\_MEMORY\_SIZE \]

System available total region (aligned to KB and reserving 1KB)

\[ RTT\_SYS\_SIZE = \left(\left\lfloor\frac{MEM\_RTSMART\_SIZE - RTSMART\_OPENSIB\_MEMORY\_SIZE}{1024}\right\rfloor - 1\right) \times 1024 \]

RAM_END (reserve an additional 4KB at the end)

\[ RAM\_END = RTT\_SYS\_BASE + RTT\_SYS\_SIZE - 4096 \]

Heap region

RT_HW_HEAP_BEGIN = &__bss_end
RT_HW_HEAP_END = RT_HW_HEAP_BEGIN + CONFIG_MEM_RTSMART_HEAP_SIZE

MMZ region

MEM_MMZ_BASE = CONFIG_MEM_MMZ_BASE
MEM_MMZ_SIZE = CONFIG_MEM_MMZ_SIZE - 4096 (see board.h)

Default Tiers (Auto Mode)#

From boards/Kconfig.memory_auto:

512M tier

MEM_RTSMART_SIZE_512 = 0x10000000 (256MB)
MEM_RTSMART_HEAP_SIZE_512 = 0x02000000 (32MB)
MEM_MMZ_BASE_512 = 0x10000000
MEM_MMZ_SIZE_512 = 0x10000000 (256MB)

1024M tier

MEM_RTSMART_SIZE_1024 = 0x20000000 (512MB)
MEM_RTSMART_HEAP_SIZE_1024 = 0x04000000 (64MB)
MEM_MMZ_BASE_1024 = 0x20000000
MEM_MMZ_SIZE_1024 = 0x20000000 (512MB)

2048M tier

MEM_RTSMART_SIZE_2048 = 0x20000000 (512MB)
MEM_RTSMART_HEAP_SIZE_2048 = 0x04000000 (64MB)
MEM_MMZ_BASE_2048 = 0x20000000
MEM_MMZ_SIZE_2048 = 0x60000000 (1536MB)

Typical Configuration Entry#

Execute in the SDK root directory:

make menuconfig

Path:

Board Configuration
  -> Memory Layout

You can:

Enable/disable Auto Detect DRAM Size
Configure 512/1024/2048 tier parameters in auto mode
Directly configure MEM_TOTAL_SIZE, MEM_RTSMART_SIZE, MEM_RTSMART_HEAP_SIZE, MEM_MMZ_BASE, MEM_MMZ_SIZE in static mode

Configuration Recommendations and Constraints#

It is recommended to follow these rules:

Total size constraint

MEM_RTSMART_SIZE + MEM_MMZ_SIZE should not exceed MEM_TOTAL_SIZE

Contiguous layout constraint (recommended)

It is recommended to set MEM_MMZ_BASE = MEM_RTSMART_SIZE (under the default layout where MEM_RTSMART_BASE=0)

Heap size constraint

MEM_RTSMART_HEAP_SIZE must be smaller than the actual available space in the system region (after deducting kernel image, BSS, and page management overhead)

Media workload priority scenarios

Increase MEM_MMZ_SIZE
At the same time, confirm that the RT-Smart heap still meets application requirements

Application workload priority scenarios

Increase MEM_RTSMART_HEAP_SIZE
Avoid squeezing MMZ and causing media module allocation failures

Verification Steps After Changes#

Recompile the image:

time make log

Check whether the target items in .config take effect (e.g., CONFIG_MEM_RTSMART_SIZE, CONFIG_MEM_MMZ_SIZE)
Execute typical business scenarios (media, AI, network) on the board side and observe whether out-of-memory or allocation failures occur

K230D Example (Static Configuration)#

Visible in configs/k230d_rtos_evb_defconfig:

CONFIG_MEM_RTSMART_SIZE=0x4400000
CONFIG_MEM_RTSMART_HEAP_SIZE=0x1000000
CONFIG_MEM_MMZ_BASE=0x4400000
CONFIG_MEM_MMZ_SIZE=0x3C00000

This is a typical “system region + MMZ region” contiguous partitioning scheme.

Frequently Asked Questions#

Why does the actual behavior not match expectations under auto DDR detection?#

First confirm:

CONFIG_AUTO_DETECT_DDR_SIZE=y
Whether the corresponding tier option exists (512/1024/2048)
Whether the board side can correctly obtain the ATAG DDR size (otherwise it will fall back to the 512M tier)

After modifying the memory configuration, compilation passes but runtime crashes?#

Priority troubleshooting:

Whether MEM_RTSMART_HEAP_SIZE is too large and encroaches on subsequent page space
Whether MEM_MMZ_BASE/MEM_MMZ_SIZE is out of bounds or overlaps with the system region
Whether the actual DDR capacity of the board matches the configuration