Memory Protection Unit (MPU)

IC: RTL8721Dx RTL8720E RTL8726E RTL8730E

Introduction

The Memory Protection Unit (MPU) is a component provided by Arm and is used to provide hardware protection by software definition. The SDK provides the mpu_region_config struct to set the region memory attribute of MPU.

The following table lists the member variables of the mpu_region_config struct.

Member variable name	Type	Description
region_base	uint32_t	MPU region base address, 32 bytes aligned
region_size	uint32_t	MPU region size, 32 bytes aligned
xn	uint8_t	Execute Never attribute MPU_EXEC_ALLOW: Allows program execution in this region MPU_EXEC_NEVER: Does not allow program execution in this region
ap	uint8_t	Access permissions MPU_PRIV_RW: Read/write by privileged code only MPU_UN_PRIV_RW: Read/write by any privilege level MPU_PRIV_R: Read only by privileged code only MPU_PRIV_W: Read only by any privilege level
sh	uint8_t	Shareability of Normal memory MPU_NON_SHAREABLE: Non-shareable MPU_OUT_SHAREABLE: Outer shareable MPU_INR_SHAREABLE: Inner shareable
attr_idx	uint8_t	Memory attribute indirect index This parameter can be a value of 0 ~ 7, with detailed attributes defined in mpu_init() and customizable. The typical definition is as follows: 0: MPU_MEM_ATTR_IDX_NC, defines memory attribute of Normal memory as non-cacheable. 1: MPU_MEM_ATTR_IDX_WT_T_RA, defines memory attribute of Normal memory as write-through transient, read allocation. 2: MPU_MEM_ATTR_IDX_WB_T_RWA, defines memory attribute of Normal memory as write-back transient, read and write allocation. 3 ~ 7: MPU_MEM_ATTR_IDX_DEVICE, defines memory attribute of Device memory as non-gathering, non-recording, non-early Write Acknowledge.

MPU APIs

RTL8721DxRTL8720ERTL8726ERTL8730E

mpu_init

Items	Description
Introduction	Initialize MPU region memory attribute to typical value
Parameters	None
Return	None

mpu_set_mem_attr

Items	Description
Introduction	Change MPU region memory attribute
Parameters	attr_idx: region memory attribute index, which can be 0 ~ 7. mem_attr: region memory attributes.
Return	None

mpu_region_cfg

Items	Description
Introduction	Configure MPU region memory attribute.
Parameters	region_num: KM4_NS: 0 ~ 7 KM4_S: 0 ~ 3 pmpu_cfg: point to the mpu_region_config struct which has been configured
Return	None

mpu_entry_free

Items	Description
Introduction	Free MPU entry
Parameters	MPU entry index: KM4_NS: 0 ~ 7 KM4_S: 0 ~ 3
Return	None

mpu_entry_alloc

Items	Description
Introduction	Allocate a free MPU entry
Parameters	None
Return	MPU entry index: KM4_NS: 0 ~ 7 KM4_S: 0 ~ 3 Fail: -1

Usage

Follow these steps to set an MPU region:

1. Define new variable and struct

   • Define a variable to store MPU entry index

   • Define a struct mpu_region_config to store the region memory attribute

2. Call mpu_entry_alloc() to allocate a free MPU entry

3. Set the struct for region memory attribute

4. Call mpu_region_cfg() to configure MPU region memory attribute

Note

For KR4, a physical memory protection (PMP) unit is implemented, but the code in SDK does not use PMP to provide access privileges control. Refer to RISC-V privileged architecture proposal for the physical memory protection scheme to set PMP when access privileges control is needed.