Add in functions to enable and disable the MPU background region. Add the calls to it in the MPU Setup function. Change the port's scheduler running variable name so it is different from the Kernels. Fix two incorrect comments SPSR -> CPSR

portable/GCC/ARM_CRx_MPU/mpu_wrappers_v2_asm.S

@@ -72,7 +72,7 @@ vPortSystemCallExit:
 .weak xPortIsPrivileged
 .type xPortIsPrivileged, %function
 xPortIsPrivileged:
-    /* Load value of SPSR into R0 */
+    /* Load value of CPSR into R0 */
     MRS     R0, CPSR
     /* Get the relevant mode bits */
     AND     R0, R0, #0x1F
@@ -82,6 +82,7 @@ xPortIsPrivileged:
     MOVEQ   R0, #0x0
     /* Otherwise set R0 to 1 for the return */
     MOVNE   R0, #0x01
+    /* Return to the caller */
     BLX      LR
 
 /*-------------------------------------------------------------------------------*/
@@ -102,7 +103,7 @@ ulPortCountLeadingZeros:
 .macro processorInUserMode
     /* Push R0 before using it */
     PUSH    {R0}
-    /* Load value of SPSR into R0 */
+    /* Load value of CPSR into R0 */
     MRS     R0, CPSR
     /* Get the relevant mode bits */
     AND     R0, R0, #0x1F

portable/GCC/ARM_CRx_MPU/port.c

@@ -71,7 +71,7 @@ PRIVILEGED_DATA volatile uint32_t ulPortInterruptNesting = 0UL;
  * @ingroup Scheduler
  * @note This variable is set to pdTRUE when the scheduler is started.
  */
-PRIVILEGED_DATA static BaseType_t xSchedulerRunning = pdFALSE;
+PRIVILEGED_DATA static BaseType_t prvPortSchedulerRunning = pdFALSE;
 
 /*---------------------------------------------------------------------------*/
 
@@ -534,6 +534,10 @@ PRIVILEGED_FUNCTION static void prvSetupMPU( void )
         portMPU_PRIV_RW_USER_NA_NOEXEC | portMPU_NORMAL_OIWTNOWA_SHARED
     );
 
+    /* Enable the MPU Background region, allows privileged operating modes access to
+     * unmapped regions of memory without generating a fault. */
+    vMPUEnableBackgroundRegion();
+
     /* After setting default regions, enable the MPU */
     vMPUEnable();
 }
@@ -611,7 +615,7 @@ PRIVILEGED_FUNCTION static BaseType_t prvTaskCanAccessRegion(
     xMPU_SETTINGS * xTaskMPUSettings = NULL;
     xMPU_REGION_REGISTERS * xTaskMPURegion = NULL;
 
-    if( pdFALSE == xSchedulerRunning )
+    if( pdFALSE == prvPortSchedulerRunning )
     {
         /* Before the scheduler starts an unknown task will be pxCurrentTCB */
         xAccessGranted = pdTRUE;
@@ -692,7 +696,7 @@ BaseType_t xPortStartScheduler( void )
     /* Configure the regions in the MPU that are common to all tasks. */
     prvSetupMPU();
 
-    xSchedulerRunning = pdTRUE;
+    prvPortSchedulerRunning = pdTRUE;
 
     /* Load the context of the first task, starting the FreeRTOS-Scheduler's control. */
     vPortStartFirstTask();
@@ -717,7 +721,7 @@ BaseType_t xPortStartScheduler( void )
     BaseType_t xAccessGranted = pdFALSE;
     const xMPU_SETTINGS * xTaskMpuSettings;
 
-    if( pdFALSE == xSchedulerRunning )
+    if( pdFALSE == prvPortSchedulerRunning )
     {
         /* Grant access to all the kernel objects before the scheduler
          * is started. It is necessary because there is no task running
@@ -831,10 +835,10 @@ void prvTaskExitError( void )
  */
 void vPortEndScheduler( void )
 {
-    xSchedulerRunning = pdFALSE;
+    prvPortSchedulerRunning = pdFALSE;
     /* Not implemented in ports where there is nothing to return to.
      * Artificially force an assert. */
-    configASSERT( xSchedulerRunning );
+    configASSERT( prvPortSchedulerRunning );
 }
 
 /*---------------------------------------------------------------------------*/

portable/GCC/ARM_CRx_MPU/portASM.S

@@ -121,7 +121,7 @@
     /* Load the first per-task MPU region into R5 */
     MOV     R5, #portFIRST_CONFIGURABLE_REGION
     /* When creating a loop label in a macro it has to be a numeric label.
-     * For (R5 = portFIRST_CONFIGURABLE_REGION ; R5 <= portSTACK_REGION ; R5++ ) */
+     * For (R5 = portFIRST_CONFIGURABLE_REGION ; R5 <= portNUM_CONFIGURABLE_REGIONS ; R5++ ) */
     123:
         /* Load values of struct MPU_REGION_REGISTERS into R2-R4 */
         LDMIA   R1!, { R2-R4 }
@@ -142,7 +142,7 @@
         /* R5++ */
         ADD     R5, R5, #1
         /* R5 <= R6 */
-        CMP     R5, #portSTACK_REGION
+        CMP     R5, #portNUM_CONFIGURABLE_REGIONS
         /* R5 <= R6, loop again */
         BLE     123b
     /* R5 > portSTACK_REGION, all MPU regions have been restored */
@@ -496,6 +496,45 @@ vMPUDisable:
     /* Return to caller */
     BX      LR
 
+/* ----------------------------------------------------------------------------------- */
+/* Enable the MPU Background Region */
+.align 4
+.global vMPUEnableBackgroundRegion
+.type vMPUEnableBackgroundRegion, %function
+vMPUEnableBackgroundRegion:
+    /* Save value in R0 */
+    PUSH    { R0 }
+    /* Read CP15 System Control Register into R0 */
+    MRC     p15, 0, R0, c1, c0, 0
+    /* Set bit 17 so that privileged modes won't trigger unmapped MPU region faults */
+    ORR     R0, R0, #0x20000
+    /* Write the value back out */
+    MCR     p15, 0, R0, c1, c0, 0
+    /* Restore the used register */
+    POP     { R0 }
+    /* Return to the caller */
+    BX      LR
+
+/* ----------------------------------------------------------------------------------- */
+/* Disable the MPU Background Region */
+.align 4
+.global vMPUDisableBackgroundRegion
+.type vMPUDisableBackgroundRegion, %function
+vMPUDisableBackgroundRegion:
+    /* Save value in R0 */
+    PUSH    { R0 }
+    /* Read CP15 System Control Register into R0 */
+    MRC     p15, 0, R0, c1, c0, 0
+    /* Clear bit 17 so that privileged modes won't trigger unmapped MPU region faults */
+    BIC     R0, R0, #0x20000
+    /* Write the value back out */
+    MCR     p15, 0, R0, c1, c0, 0
+    /* Restore the used register */
+    POP     { R0 }
+    /* Return to the caller */
+    BX      LR
+
+/* ----------------------------------------------------------------------------------- */
 .align 4
 .global FreeRTOS_IRQ_Handler
 .type FreeRTOS_IRQ_Handler, %function

portable/GCC/ARM_CRx_MPU/portmacro.h

@@ -69,7 +69,6 @@ extern "C" {
 #endif /* configPROTECTED_KERNEL_OBJECT_POOL_SIZE */
 
 
-
 /**
  * @brief The size in Bytes that the Privileged System Call Stack should be.
  *
@@ -397,6 +396,23 @@ void vMPUEnable( void );
  */
 void vMPUDisable( void );
 
+/**
+ * @brief Enable the MPU Background Region.
+ *
+ * @ingroup MPU Control
+ *
+ * @return void
+ */
+void vMPUEnableBackgroundRegion( void );
+
+/**
+ * @brief Disable the MPU Background Region
+ *
+ * @ingroup MPU Control
+ *
+ * @return void
+ */
+void vMPUDisableBackgroundRegion( void );
 /**
  * @brief Assembly routine to set permissions for an MPU Region.
  *

portable/GCC/ARM_CRx_MPU/portmacro_asm.h

@@ -133,12 +133,14 @@ extern "C" {
 /**
  * @brief SVC Number to use when requesting a context swap.
  * @ingroup Scheduler
+ * @note This value must not be in use in mpu_syscall_numbers.h
  */
 #define portSVC_YIELD                    0x0100
 
 /**
  * @brief SVC Number to use when exiting a FreeRTOS System Call.
  * @ingroup MPU Control
+ * @note This value must not be in use in mpu_syscall_numbers.h
  */
 #define portSVC_SYSTEM_CALL_EXIT         0x0104