AMP Memory Partitioning

Memory resource partitioning

Memory conflicts are among the most common AMP problems and can cause intermittent crashes. A remote core's private memory region must be represented consistently in at least three places:

1. The ITS load and compile/size properties.
2. The RT-Thread/HAL linker script or build script firmware base and size.
3. Linux DTS reserved-memory.

7.1 Reserve remote-core memory in Linux

The following example reserves the remote-core runtime region 0x01800000 through 0x01ffffff:

/ {
    reserved-memory {
        #address-cells = <2>;
        #size-cells = <2>;
        ranges;

        amp_reserved: amp@1800000 {
            reg = <0x0 0x01800000 0x0 0x00800000>;
            no-map;
        };
    };
};

Linux does not add no-map memory to the normal page allocator, preventing it from overwriting remote-core code and data.

7.2 Reserve RPMsg shared memory

Rockchip examples commonly assign 5 MiB to RPMsg: 4 MiB for vrings and 1 MiB for the vdev buffer/DMA pool. For example:

reserved-memory {
    rpmsg_reserved: rpmsg@7c00000 {
        reg = <0x0 0x07c00000 0x0 0x00400000>;
        no-map;
    };

    rpmsg_dma_reserved: rpmsg-dma@8000000 {
        compatible = "shared-dma-pool";
        reg = <0x0 0x08000000 0x0 0x00100000>;
        no-map;
    };
};

The current Rockchip Linux RPMsg solution uses uncached shared memory by default. If it is changed to cacheable memory, both sides must implement correct cache clean/invalidate operations and memory barriers. Otherwise, lost messages, stale data, or vring corruption can occur.

7.3 AP and MCU address differences

Addresses in AP linker scripts are normally physical DDR addresses. An MCU often sees its load address as local address 0x0, so MCU-visible addresses have a fixed offset from physical addresses.

For example, if the MCU is loaded at 0x08200000 and its linker script places shared memory at offset 0x00100000, the physical address is:

0x08200000 + 0x00100000 = 0x08300000

When changing the MCU load address or RPMsg shared memory, check all of the following:

• The load property in amp_mcu.its.
• The MCU linker script.
• MCU memory mapping and the uncached region in U-Boot.
• Physical/virtual address conversion in rpmsg_platform.c.
• Linux DTS reserved-memory.