Configuring the MPU on STM32H747

[TonyFocusLCDs]

Hello,

I am wondering how to configure the MPU in CubeMX (6.12.1) so that I can enable the D-Cache and the I-Cache. I know the instructions that enable the caches (SCB_Enable_x_Cache(); replace _x_ with I or D) but was wondering on the configuration CubeMX generates.

I am using the STM32H747I-Discovery board and CubeMX produced:

MPU_InitStruct.Enable = MPU_REGION_ENABLE;
MPU_InitStruct.Number = MPU_REGION_NUMBER0;
MPU_InitStruct.BaseAddress = 0x0;
MPU_InitStruct.Size = MPU_REGION_SIZE_4GB;
MPU_InitStruct.SubRegionDisable = 0x87;
MPU_InitStruct.TypeExtField = MPU_TEX_LEVEL0;
MPU_InitStruct.AccessPermission = MPU_REGION_NO_ACCESS;
MPU_InitStruct.DisableExec = MPU_INSTRUCTION_ACCESS_DISABLE;
MPU_InitStruct.IsShareable = MPU_ACCESS_SHAREABLE;
MPU_InitStruct.IsCacheable = MPU_ACCESS_NOT_CACHEABLE;
MPU_InitStruct.IsBufferable = MPU_ACCESS_NOT_BUFFERABLE;

HAL_MPU_ConfigRegion(&MPU_InitStruct);
/* Enables the MPU */
HAL_MPU_Enable(MPU_PRIVILEGED_DEFAULT);

Does this seem correct? My questions arise from the lines:

MPU_InitStruct.AccessPermission = MPU_REGION_NO_ACCESS;
MPU_InitStruct.DisableExec = MPU_INSTRUCTION_ACCESS_DISABLE;

HAL_MPU_Enable(MPU_PRIVILEGED_DEFAULT);

It sounds like it is disabling access to the memory. I read through the reference manual and looked through the code (core_cm7.h, stm32h7xx_hal_cortex.h) but could not find a clear answer to how I would want the MPU configured.

I would like to have the caches enabled as I want the highest performance from the M7 core.

Thank you for your time and assistance with this question

[TDK · Accepted solution]

To enable data and instruction cache, enable them in the CORTEX_M7 tab, just above the MPU. No additional MPU configuration is needed as the default will allow them to be cached.

Lots of information in Level 1 cache on STM32F7 Series and STM32H7 Series.

In your code, the "MPU_InitStruct.SubRegionDisable = 0x87;" line prevents the MPU settings you have from affecting the RAM, which is where data is stored. Nicely explained here by @SofLit:

Solved: Information about MPU settings for the STM32H743 - STMicroelectronics Community

[TonyFocusLCDs]

Hi @TDK,

Thank you for the additional information. I had enabled the MPU but did not understand the setting in CubeMX. 

I did not make any changes in CubeMX for the MPU (except enable it) when it generated the code. The "MPU_InitStruct.SubRegionDisable = 0x87" was what CubeMX generated, I did not even know what exactly it meant.

The links you provided were helpful.

Thank you again for your assistance!