Cortex-M7 MPU and default regions

Kernel
1

thomask wrote on Friday, February 08, 2019:

Hi!

I would like to use the MPU on a STM32H743. As I saw in the app notes, the MPU has some default regions, that are active if the MPU is actually turned off.

For example, see page 5:
https://www.st.com/content/ccc/resource/technical/document/application_note/group0/08/dd/25/9c/4d/83/43/12/DM00272913/files/DM00272913.pdf/jcr:content/translations/en.DM00272913.pdf

If I use FreeRTOS MPU port, do I need to re-create these default settings using explicit MPU regions?
Or will this set of defaults always be “in the background”?

best regards
Thomas

2

rtel wrote on Saturday, February 09, 2019:

I don’t know about any chip specifics, but FreeRTOS always sets up the MPU in the same way, as described in the attached.

3

thomask wrote on Saturday, February 09, 2019:

It’s not really chip specific…

The default addres map of the STM32H7 is defined in the ARMv7M Architecture Reference Manual:

https://static.docs.arm.com/ddi0403/eb/DDI0403E_B_armv7m_arm.pdf

Chapter B3.1, page 648.

If I’m reading the document correctly, the default map is only active when the MPU is off.

So if I enable the MPU, I suspect I have to explicitly program the RAM cache regions, device memory attributes, etc. ?

The MPU document you have linked above seems to set only the read/write attributes, but not the cache bits (of course it’s written for a Cortex-M3, so that’s ok).

So what about Cortex-M7 chips with more complex MPUs?

4

waveringradiant wrote on Saturday, February 09, 2019:

I’m not sure if this addresses / answers your concern/question, but I believe that:

  • When the MPU is enabled, if MPU_CTRL[PRIVDEFENA] is set to 1, any access to an address that is not in a defined MPU region acts as though it is going through the default map.

  • When the MPU is enabled and MPU_CTRL[PRIVDEFENA] is set to 0, any access to an address that is not in a defined MPU region results in a fault (processor exception)

  • If the MPU is not enabled, of course the default map is what’s used.

I don’t think I can explain it better than that.

5

thomask wrote on Monday, February 11, 2019:

Thank you!
That was the missing piece I was looking for!