Save ROM by specifying initial MPU state

We can omit the need for the "change MPU state" calls from simple images
by specifying the initial state at init.

Author: kjbracey

hal/mpu/mbed_mpu_v8m.c

@@ -78,7 +78,8 @@ void mbed_mpu_init()
                 (3 << MPU_RBAR_AP_Pos) |             // RO allowed by all privilege levels
                 (0 << MPU_RBAR_XN_Pos);              // Execute Never disabled
     MPU->RLAR = (0x1FFFFFFF & MPU_RLAR_LIMIT_Msk) |     // Last address is 0x1FFFFFFF
-                (region << MPU_RLAR_AttrIndx_Pos);   // Attribute index - configured to be the same as the region number
+                (region << MPU_RLAR_AttrIndx_Pos) |  // Attribute index - configured to be the same as the region number
+                (1 << MPU_RLAR_EN_Pos);              // Region enabled
 
     region = 1;
     MPU->RNR = region;
@@ -90,7 +91,8 @@ void mbed_mpu_init()
                 (1 << MPU_RBAR_AP_Pos) |             // RW allowed by all privilege levels
                 (1 << MPU_RBAR_XN_Pos);              // Execute Never enabled
     MPU->RLAR = (0x3FFFFFFF & MPU_RLAR_LIMIT_Msk) |     // Last address is 0x3FFFFFFF
-                (region << MPU_RLAR_AttrIndx_Pos);   // Attribute index - configured to be the same as the region number
+                (region << MPU_RLAR_AttrIndx_Pos) |  // Attribute index - configured to be the same as the region number
+                (1 << MPU_RLAR_EN_Pos);              // Region enabled
 
     region = 2;
     MPU->RNR = region;
@@ -102,7 +104,8 @@ void mbed_mpu_init()
                 (1 << MPU_RBAR_AP_Pos) |             // RW allowed by all privilege levels
                 (1 << MPU_RBAR_XN_Pos);              // Execute Never enabled
     MPU->RLAR = (0x7FFFFFFF & MPU_RLAR_LIMIT_Msk) |     // Last address is 0x7FFFFFFF
-                (region << MPU_RLAR_AttrIndx_Pos);   // Attribute index - configured to be the same as the region number
+                (region << MPU_RLAR_AttrIndx_Pos) |  // Attribute index - configured to be the same as the region number
+                (1 << MPU_RLAR_EN_Pos);              // Region enabled
 
     region = 3;
     MPU->RNR = region;
@@ -114,7 +117,8 @@ void mbed_mpu_init()
                 (1 << MPU_RBAR_AP_Pos) |             // RW allowed by all privilege levels
                 (1 << MPU_RBAR_XN_Pos);              // Execute Never enabled
     MPU->RLAR = (0x9FFFFFFF & MPU_RLAR_LIMIT_Msk) |     // Last address is 0x9FFFFFFF
-                (region << MPU_RLAR_AttrIndx_Pos);   // Attribute index - configured to be the same as the region number
+                (region << MPU_RLAR_AttrIndx_Pos) |  // Attribute index - configured to be the same as the region number
+                (1 << MPU_RLAR_EN_Pos);              // Region enabled
 
     // Enable the MPU
     MPU->CTRL =

hal/mpu_api.h

@@ -64,8 +64,8 @@ extern "C" {
  * Initialize the MPU
  *
  * Initialize or re-initialize the memory protection unit.
- * It is implementation defined what region are protected
- * by the MPU after initialization.
+ * After initialization or re-initialization, ROM and RAM protection
+ * are both enabled.
  */
 void mbed_mpu_init(void);
 
@@ -75,7 +75,9 @@ void mbed_mpu_init(void);
  * This function is used to mark all of ROM as read and execute only.
  * When enabled writes to ROM cause a fault.
  *
- * @param enable true to disable execution in ram, false otherwise
+ * By default writes to ROM are disabled.
+ *
+ * @param enable true to disable writes to ROM, false otherwise
  */
 void mbed_mpu_enable_rom_wn(bool enable);
 
@@ -85,7 +87,9 @@ void mbed_mpu_enable_rom_wn(bool enable);
  * This function is used to mark all of RAM as execute never.
  * When enabled code is only allowed to execute from flash.
  *
- * @param enable true to disable execution in ram, false otherwise
+ * By default execution from RAM is disabled.
+ *
+ * @param enable true to disable execution from RAM, false otherwise
  */
 void mbed_mpu_enable_ram_xn(bool enable);
 

rtos/TARGET_CORTEX/mbed_boot.c

@@ -88,8 +88,6 @@ uint32_t mbed_stack_isr_size = 0;
 void mbed_init(void)
 {
     mbed_mpu_init();
-    mbed_mpu_enable_ram_xn(true);
-    mbed_mpu_enable_rom_wn(true);
     mbed_cpy_nvic();
     mbed_sdk_init();
     mbed_rtos_init();