MPU Cstop - stm32mp157c-dk2

[IYetk]

Hello,

I am applying the second scenario of example openamp_tty_echo_wakeup.

The scenario is to wake up and make the system run again from "MCU : running and MPU :Cstop".

My question: When MPU goes in Cstop mode with the command "echo mem > /sys/power/state", linux user space is suspended. Can linux do something in the background in this situation ? For example saving the messages in a file coming from M4 (ch0).

If not, do I need to bring MPU to Cstandby mode for this porpose ? I read the power modes in wiki,other docs and couldnt see any functional difference between MPU Cstop and MPU Cstandby.

Is there any command to bring the system from MCU : running and MPU :running" to "MCU : running and MPU :Cstandby" ?

(there is no example for this scenario)

tnx

[Olivier GALLIEN]

Hi @IYetk​ 

Doing "echo mem > /sys/power/state" put the whole linux and A7 in Low Power mode.

Whatever CStop or CStandby, linux is doing nothing in background.

Most of the time sending a message from M4 will be a wakeup source for A7.

>> Is there any command to bring the system from MCU : running and MPU :running" to "MCU : running and MPU :Cstandby" ?

The targeted low power mode is defined by the wake-up source.

It's the same command to enter the mode

Refer to AN5109

https://www.st.com/content/ccc/resource/technical/document/application_note/group1/26/e6/ee/56/08/2f/4e/5c/DM00449434/files/DM00449434.pdf/jcr:content/translations/en.DM00449434.pdf

Note that we do NOT recommend this strategy. Wakeup A7 from CStandby request a MPU reset and complete Linux boot sequence.

This maybe long, and present many challenges to not impact the M4 running process.

Hope it help,

Olivier

[IYetk]

thanks. it helped