Linux System Memory Usage

By default, the DDR range accessible to the Linux system is:

Start address: 0x8400_0000

End address: 0x8000_0000 + DDR capacity

The Linux Kernel device tree reserves a portion of memory for dedicated use by certain modules. The table below lists all memory reserved in the device tree:

Memory Partition	Memory Purpose	Memory Region	Memory Size
kernel_ram	Stores kernel code and data	0x8000_0000..0x87FFF_FFFF	128MB (0x2000000)
uboot_log_reserved	Retrieves boot logs within the kernel	0x87FFC000..0x87FFFFFF	16KB (0x4000)
cma_reserved	Used for CMA memory allocation	0x88000000..0x9FDFFFFF	382MB (0x17E00000)
firewall_ddr_default	Address automatically directed when a firewall access violation occurs	0x9FE7F000..0x9FE7FFFF	4KB (0x1000)
adsp_ddr	Memory space used by ADSP firmware	0x9FE80000..0xA207FFFF	34MB (0x2200000)
ramoops_reserved	Used for ramoops temporary memory allocation	0xA4080000..0xA40BFFFF	256KB (0x40000)
ion_reserved	Used for ION memory allocation	0xA4100000..0xD40FFFFF	768MB (0x2E000000)

• The reserved address space is defined in the arch/arm64/boot/dts/hobot/x5-memory.dtsi file under the kernel directory.
• You can view the system memory allocation status using the cat /proc/iomem command.

By default, the reserved memory occupies approximately 1150MB of memory space.

The available memory displayed by the free command has already excluded this reserved portion, reflecting only the memory capacity actually usable by the kernel.

ION Memory

Overview

The ION system is a memory management subsystem within the Linux kernel used to manage memory sharing between devices.

In the RDK X5 system, ION memory is primarily used in the following four scenarios:

• BPU subsystem
• HIFI subsystem (DSP)
• ION memory allocated by developers via the hbmem interface
• Multimedia system: Includes hardware acceleration units such as VIN, ISP, VSE, GDC, VPU, OSD, GPU, and Display

Three main ION regions are used in the RDK X5: cma_reserved, carveout, and ion_cma.

• cma_reserved primarily provides memory for the multimedia system.
• carveout primarily provides memory for the BPU subsystem.
• ion_cma serves as a backup region. When the above two regions have no available space, the ION management subsystem automatically allocates memory from the ion_cma region.

How to Adjust ION Region Size

The RDK X5 uses the srpi-config tool to adjust the size of ION regions. For specific operations, please refer to the ION memory section in srpi-config.