STM32F7 and MPU

[Anton1]

Hello everyone,

I apply STM32F746BE with externel SDRAM 32MB (MT48LC16M16A2P-75 IT:D) and externel FLASH 16MB (MX25L12845GM2I-10G), Ethernet (LAN8710AI-EZK), LCD.

Vector' table is located at 0x20000000. Firmware is runned from externel FLASH 0x90000000

How me configure correctly MPU? What parameters should be changed?

MPU_InitStruct.Enable            = MPU_REGION_ENABLE;
MPU_InitStruct.BaseAddress  = 0x20000000;
MPU_InitStruct.Size              = MPU_REGION_SIZE_8KB;
MPU_InitStruct.AccessPermission = MPU_REGION_FULL_ACCESS;
MPU_InitStruct.IsCacheable      = MPU_ACCESS_NOT_CACHEABLE;
MPU_InitStruct.IsBufferable      = MPU_ACCESS_NOT_BUFFERABLE;
MPU_InitStruct.IsShareable      = MPU_ACCESS_SHAREABLE;
MPU_InitStruct.Number            = MPU_REGION_NUMBER0;
MPU_InitStruct.TypeExtField      = MPU_TEX_LEVEL0;
MPU_InitStruct.SubRegionDisable = 0x00;
MPU_InitStruct.DisableExec      = MPU_INSTRUCTION_ACCESS_ENABLE;
 
// Descriptors for Ethernet' driver
MPU_InitStruct.Enable            = MPU_REGION_ENABLE;
MPU_InitStruct.BaseAddress      = 0x20001400;
MPU_InitStruct.Size              = MPU_REGION_SIZE_1KB;
MPU_InitStruct.AccessPermission = MPU_REGION_FULL_ACCESS;
MPU_InitStruct.IsCacheable      = MPU_ACCESS_NOT_CACHEABLE;
MPU_InitStruct.IsBufferable      = MPU_ACCESS_NOT_BUFFERABLE;
MPU_InitStruct.IsShareable      = MPU_ACCESS_SHAREABLE;
MPU_InitStruct.Number            = MPU_REGION_NUMBER1;
MPU_InitStruct.TypeExtField      = MPU_TEX_LEVEL0;
MPU_InitStruct.SubRegionDisable = 0x00;
MPU_InitStruct.DisableExec      = MPU_INSTRUCTION_ACCESS_DISABLE;
 
// Buffer RX for Ethernet' driver
MPU_InitStruct.Enable            = MPU_REGION_ENABLE;
MPU_InitStruct.BaseAddress      = 0x20001800;
MPU_InitStruct.Size              = MPU_REGION_SIZE_8KB;
MPU_InitStruct.AccessPermission = MPU_REGION_FULL_ACCESS;
MPU_InitStruct.IsCacheable      = MPU_ACCESS_NOT_CACHEABLE;
MPU_InitStruct.IsBufferable      = MPU_ACCESS_NOT_BUFFERABLE;
MPU_InitStruct.IsShareable      = MPU_ACCESS_SHAREABLE;
MPU_InitStruct.Number            = MPU_REGION_NUMBER2;
MPU_InitStruct.TypeExtField      = MPU_TEX_LEVEL1;
MPU_InitStruct.SubRegionDisable = 0x00;
MPU_InitStruct.DisableExec      = MPU_INSTRUCTION_ACCESS_DISABLE;
 
// Buffer TX for Ethernet' driver
MPU_InitStruct.Enable            = MPU_REGION_ENABLE;
MPU_InitStruct.BaseAddress      = 0x20003800;
MPU_InitStruct.Size              = MPU_REGION_SIZE_8KB;
MPU_InitStruct.AccessPermission = MPU_REGION_FULL_ACCESS;
MPU_InitStruct.IsCacheable      = MPU_ACCESS_NOT_CACHEABLE;
MPU_InitStruct.IsBufferable      = MPU_ACCESS_NOT_BUFFERABLE;
MPU_InitStruct.IsShareable      = MPU_ACCESS_SHAREABLE;
MPU_InitStruct.Number            = MPU_REGION_NUMBER3;
MPU_InitStruct.TypeExtField      = MPU_TEX_LEVEL1;
MPU_InitStruct.SubRegionDisable = 0x00;
MPU_InitStruct.DisableExec      = MPU_INSTRUCTION_ACCESS_DISABLE;
 
// LCD buffer
MPU_InitStruct.Enable            = MPU_REGION_ENABLE;
MPU_InitStruct.BaseAddress      = 0x61F00000;
MPU_InitStruct.Size              = MPU_REGION_SIZE_1MB;
MPU_InitStruct.AccessPermission = MPU_REGION_FULL_ACCESS;
MPU_InitStruct.IsCacheable      = MPU_ACCESS_NOT_CACHEABLE;
MPU_InitStruct.IsBufferable      = MPU_ACCESS_NOT_BUFFERABLE;
MPU_InitStruct.IsShareable      = MPU_ACCESS_SHAREABLE;
MPU_InitStruct.Number            = MPU_REGION_NUMBER5;
MPU_InitStruct.TypeExtField      = MPU_TEX_LEVEL1;
MPU_InitStruct.SubRegionDisable = 0x00;
MPU_InitStruct.DisableExec      = MPU_INSTRUCTION_ACCESS_DISABLE;
 
// QSPI at memory-mapped mode
MPU_InitStruct.Enable            = MPU_REGION_ENABLE;
MPU_InitStruct.BaseAddress      = 0x90000000;
MPU_InitStruct.Size              = MPU_REGION_SIZE_16MB;
MPU_InitStruct.AccessPermission = MPU_REGION_PRIV_RO_URO;
MPU_InitStruct.IsCacheable      = MPU_ACCESS_CACHEABLE;
MPU_InitStruct.IsBufferable      = MPU_ACCESS_BUFFERABLE;
MPU_InitStruct.IsShareable      = MPU_ACCESS_SHAREABLE;
MPU_InitStruct.Number            = MPU_REGION_NUMBER6;
MPU_InitStruct.TypeExtField      = MPU_TEX_LEVEL0;
MPU_InitStruct.SubRegionDisable = 0x00;
MPU_InitStruct.DisableExec      = MPU_INSTRUCTION_ACCESS_ENABLE;
 

Thank you,

Anton

[Pavel A.]

Please read this: http://infocenter.arm.com/help/topic/com.arm.doc.dui0646c/BIHJJABA.html

And then, read that piece of code again...,

-- pa

[Anton1]

Hello, Pavel A.

I am familiar with http://infocenter.arm.com/help/topic/com.arm.doc.dui0646c/BIHJJABA.html and https://www.st.com/resource/en/application_note/dm00272912-managing-memory-protection-unit-in-stm32-mcus-stmicroelectronics.pdf

But I have doubts about the correct MPU settings in my case. Can you recommend a specific configuration option?

[Anton1]

Message deleted

[TDK]

The code you posted doesn't actually do anything except write/overwrite a bunch of values to memory. You have to actually call HAL_MPU_ConfigRegion and HAL_MPU_Enable.

> But I have doubts about the correct MPU settings in my case.

There is no correct setting. It's up to you to decide what is right for you application.

> Can you recommend a specific configuration option?

I would recommend you don't use the MPU unless you have a good reason to.

[ADior.1]

Hello TDK,

Anton is my colleague. Thank you for your answer.

Anton ask if there are any wrong settings for MPU?

Your recommendation to not to use MPU seem to me very strange. It's like:"You have a car, do not use it, it's too complicated". MPU is protecting memory for wrong accesses and handles cached and not cached memory accesses.

Regards, Andrey

[TDK]

> Your recommendation to not to use MPU seem to me very strange. It's like:"You have a car, do not use it, it's too complicated".

I don't think that straw man argument accurately reflects what I'm saying. The MCU will work perfectly fine without engaging the MPU. And since the OP lists no other reasons for using it other than:

> How me configure correctly MPU? What parameters should be changed?

My recommendation is to not bother with it until you need to.

[Pavel A.]

It is a good question. I too could not easily understand which MPU settings are best for external SRAM or flash.

Settings for ethernet descriptors for DMA have been already discussed here.

Simple attributes like "no code execution" or privileged vs nonprivileged are easy to understand.

Bufferable and shareable maybe are palatable as well.

But then there are various TEX levels that change meaning of the shareable/bufferable/cacheable flags ...

and what are the defaults for the "background region" ?

-- pa

[Piranha]

AN4667, AN4838, AN4839.

And here are an especially ignored topics by ST...

https://developer.arm.com/ja/docs/ddi0489/latest/memory-system/speculative-accesses

https://developer.arm.com/docs/ddi0489/latest/memory-system/speculative-accesses/considerations-for-system-design

My consolidated report on practical usage for Ethernet, but very useful for everything:

https://community.st.com/s/question/0D50X0000C4Nk4GSQS/bug-missing-compiler-and-cpu-memory-barriers

By the way it seems that compiler barriers, memory barriers, speculative accesses and instruction reordering seems to be the topics ignored not only by ST's brainless code-monkeys, but by almost everyone...

P.S. For problems with ST's implementation of Ethernet/lwIP look at my other topic:

https://community.st.com/s/question/0D50X0000BOtfhnSQB/how-to-make-ethernet-and-lwip-working-on-stm32

[Pavel A.]

> By the way it seems that compiler barriers, memory barriers, speculative accesses and instruction reordering seems to be the topics ignored [snip] by almost everyone...

Except of hackers looking for vulnerabilities ;)

So are these "speculative access" topics applicable to STM32? to which families?

-- pa

[world04]

If you want to use the MPU, you should have a specific reason for it, because it is only necessary in very rare cases to use it. But it doesn't mean that you shouldn't even bother with it. The recommended document about the use and functioning of the MPU is IMHO, essential to acquire the necessary knowledge. The question would not even arise once you have understood this documentation. I think that Parvel.A wanted to suggest this with his reference.

Nobody except the TO can decide whether or not to use the MPU. So I find the comments about whether it makes sense to use the MPU or not, not very helpful.

However, I also see it the same way as the helpful commentators - if you want to use the MPU, you have to understand how it works. If you do not have this, you simply omit the use.

To go into the comparison with the car. The fastest car won't move if the driver can't drive. A car is of no use without a road.