zephyr 0.52 - Portenta H7 maybe others - <err> mpu: Failed to allocate new MPU region 27

[KurtE]

I have been seeing this with some of my own Zephyr test apps, since I switched over to using
USB_next based USB/CDC-ACM.

As far as I can tell, it only happens when you first start up the board or maybe when there is no
Serial connected up to . If I reboot the processor it usually does not happen again.

In this case I wanted to test my Arduino test sketch that displays how large I can do a malloc...
https://forum.arduino.cc/t/giga-portenta-h7-difficulties-with-malloc-on-zephyr-0-52/1415377/2?u=kurte

I plugged in Portenta H7.

I double clicked on it, selected some programmer and then the flash the bootloader.
I then double clicked again and downloaded the sketch:

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  while (!Serial && millis() < 5000) {}
  delay(250);
  Serial.println("Check max malloc");
  Serial.print("Enter max KB to start: ");
  pinMode(LED_BUILTIN, OUTPUT);

  //printk("KB: %u Buffer: %p\n", buffer);
}

void loop() {
  digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
  if (Serial.available()) {
    int kb_max = Serial.parseInt();
    int kb;
    void *buffer;
    Serial.println(kb_max);
    for (kb = kb_max; kb > 0; kb--) {
      buffer = malloc(kb * 1024);
      if (buffer) break;
    }
    Serial.print(kb);
    Serial.print(": ");
    Serial.print((uint32_t)buffer, HEX);
    if (buffer) {
      Serial.print(" - ");
      Serial.print((uint32_t)buffer + kb * 1024, HEX);
    }
    Serial.print(" Stack var: ");
    Serial.println((uint32_t)&buffer, HEX);
    free(buffer);    
    Serial.print("Enter max KB to start: ");
    while (Serial.read() != -1) {}
  }
  delay(500);
}

When it came up I waited a long time before the prompt to enter max KB, which took a long time.
typed 512 took a long time...

So I then started up Serial monitor for the FTDI USB to serial adapter hooked up to the Serial1 port and
was getting an endless stream of these messages

[00:01:32.781,000] <err> mpu: Failed to allocate new MPU region 27

[00:01:32.792,000] <err> mpu: Failed to allocate new MPU region 27

[00:01:32.804,000] <err> mpu: Failed to allocate new MPU region 27

[00:01:32.915,000] <err> mpu: Failed to allocate new MPU region 27

[00:01:32.927,000] <err> mpu: Failed to allocate new MPU region 27

Took forever to print the 45K result.

I reset the H7, and then it ran as normal.