Add Critical Section HAL API specification

- Define header functions for Critical Section HAL API
  - hal_critical_section_enter()
  - hal_critical_section_exit()

- Add weak default implementation for HAL API. The default implementation
  matches the previous behaviour stored in mbed_critical:
  - The first call to enter a critical section stores the state of interrupts
    before disabling and each successive call re-disables interrupts.
  - The last call (non-nested) will restore the IRQ state that was set on the
    enter to the critical section. Nested calls are ignored.

- Add function 'core_util_in_critical_section' to User facing API to determine
  if the program is currently in a critical section, instead of depending on
  'core_util_interrupts_enabled'.

hal/critical_section_api.h

@@ -0,0 +1,70 @@
+/** \addtogroup hal */
+/** @{*/
+/* mbed Microcontroller Library
+ * Copyright (c) 2006-2017 ARM Limited
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#ifndef MBED_CRITICAL_SECTION_API_H
+#define MBED_CRITICAL_SECTION_API_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * \defgroup hal_critical Critical Section HAL functions
+ * @{
+ */
+
+/**
+  * Mark the start of a critical section
+  *
+  * This function is called directly by core_util_critical_section_enter on
+  * first entrance to a critical section.
+  *
+  * The default behavior of this function is to save the current state of
+  * interrupts before disabling them.
+  *
+  * The function is only called once per critical section by
+  * core_util_critical_section_enter. When implementing this function for a
+  * platform you must store any state that you intend to alter within this
+  * function so it can be restored when exiting the critical section.
+  *
+  */
+void hal_critical_section_enter(void);
+
+/** Mark the end of a critical section
+  *
+  * This function is called directly by core_util_critical_section_exit on the
+  * final exit from a critical section.
+  *
+  * The default behavior of this function is to restore the state of interrupts
+  * as they were prior to entering this critical section.
+  *
+  * This function is only called once per critical section. When implemented
+  * for a specific platform it must restore any state that was saved upon
+  * entering the current critical section.
+  *
+  */
+void hal_critical_section_exit(void);
+
+/**@}*/
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // MBED_CRITICAL_SECTION_API_H
+
+/** @}*/

hal/mbed_critical_section_api.c

@@ -0,0 +1,65 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2017 ARM Limited
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include "cmsis.h"
+#include "hal/critical_section_api.h"
+#include "platform/mbed_assert.h"
+#include "platform/mbed_toolchain.h"
+
+#include <stdbool.h>
+
+static volatile bool critical_interrupts_enabled = false;
+
+static bool are_interrupts_enabled(void)
+{
+#if defined(__CORTEX_A9)
+    return ((__get_CPSR() & 0x80) == 0);
+#else
+    return ((__get_PRIMASK() & 0x1) == 0);
+#endif
+}
+
+
+MBED_WEAK void hal_critical_section_enter(void)
+{
+    critical_interrupts_enabled = are_interrupts_enabled();
+
+#ifndef FEATURE_UVISOR
+    // If we are in a nested critical section and interrupts are still enabled
+    // something has gone wrong.
+    MBED_ASSERT(!are_interrupts_enabled());
+#else
+    #warning "core_util_critical_section_enter needs fixing to work from unprivileged code"
+#endif /* FEATURE_UVISOR */
+
+    __disable_irq();
+}
+
+
+MBED_WEAK void hal_critical_section_exit()
+{
+// FIXME
+#ifndef FEATURE_UVISOR
+    // Interrupts must be disabled on invoking an exit from a critical section
+    MBED_ASSERT(!are_interrupts_enabled());
+#else
+    #warning "core_util_critical_section_exit needs fixing to work from unprivileged code"
+#endif /* FEATURE_UVISOR */
+
+    // Restore the IRQs to their state prior to entering the critical section
+    if (critical_interrupts_enabled == true) {
+      __enable_irq();
+    }
+}

platform/mbed_critical.c

@@ -17,14 +17,14 @@
 
 /* Declare __STDC_LIMIT_MACROS so stdint.h defines UINT32_MAX when using C++ */
 #define __STDC_LIMIT_MACROS
-#include "platform/mbed_critical.h"
+#include "hal/critical_section_api.h"
 
 #include "cmsis.h"
 #include "platform/mbed_assert.h"
+#include "platform/mbed_critical.h"
 #include "platform/mbed_toolchain.h"
 
-static volatile uint32_t interrupt_enable_counter = 0;
-static volatile bool critical_interrupts_disabled = false;
+static volatile uint32_t critical_section_reentrancy_counter = 0;
 
 bool core_util_are_interrupts_enabled(void)
 {
@@ -51,53 +51,34 @@ bool core_util_is_isr_active(void)
 #endif
 }
 
-MBED_WEAK void core_util_critical_section_enter(void)
+bool core_util_in_critical_section(void)
 {
-    bool interrupts_disabled = !core_util_are_interrupts_enabled();
-    __disable_irq();
-
-    /* Save the interrupt disabled state as it was prior to any nested critical section lock use */
-    if (!interrupt_enable_counter) {
-        critical_interrupts_disabled = interrupts_disabled;
-    }
-
-    /* If the interrupt_enable_counter overflows or we are in a nested critical section and interrupts
-       are enabled, then something has gone badly wrong thus assert an error.
-    */
-    MBED_ASSERT(interrupt_enable_counter < UINT32_MAX); 
-// FIXME
-#ifndef   FEATURE_UVISOR
-    if (interrupt_enable_counter > 0) {
-        MBED_ASSERT(interrupts_disabled);
-    }
-#else
-#warning "core_util_critical_section_enter needs fixing to work from unprivileged code"
-#endif /* FEATURE_UVISOR */
-    interrupt_enable_counter++;
+    return (critical_section_reentrancy_counter != 0);
 }
 
-MBED_WEAK void core_util_critical_section_exit(void)
+void core_util_critical_section_enter(void)
 {
-    /* If critical_section_enter has not previously been called, do nothing */
-    if (interrupt_enable_counter) {
+    // If the reentrancy counter overflows something has gone badly wrong.
+    MBED_ASSERT(critical_section_reentrancy_counter < UINT32_MAX);
 
-// FIXME
-#ifndef   FEATURE_UVISOR
-        bool interrupts_disabled = !core_util_are_interrupts_enabled(); /* get the current interrupt disabled state */
+    if (critical_section_reentrancy_counter == 0) {
+        hal_critical_section_enter();
+    }
 
-        MBED_ASSERT(interrupts_disabled); /* Interrupts must be disabled on invoking an exit from a critical section */
-#else
-#warning "core_util_critical_section_exit needs fixing to work from unprivileged code"
-#endif /* FEATURE_UVISOR */
+    critical_section_reentrancy_counter++;
+}
 
-        interrupt_enable_counter--;
+void core_util_critical_section_exit(void)
+{
+    // If critical_section_enter has not previously been called, do nothing
+    if (critical_section_reentrancy_counter == 0) {
+        return;
+    }
 
-        /* Only re-enable interrupts if we are exiting the last of the nested critical sections and
-           interrupts were enabled on entry to the first critical section.
-        */
-        if (!interrupt_enable_counter && !critical_interrupts_disabled) {
-            __enable_irq();
-        }
+    critical_section_reentrancy_counter--;
+
+    if (critical_section_reentrancy_counter == 0) {
+        hal_critical_section_exit();
     }
 }
 

platform/mbed_critical.h

@@ -82,6 +82,13 @@ void core_util_critical_section_enter(void);
   */
 void core_util_critical_section_exit(void);
 
+/**
+ * Determine if we are currently in a critical section
+ *
+ * @return true if in a critical section, false otherwise.
+ */
+bool core_util_in_critical_section(void);
+
 /**
  * Atomic compare and set. It compares the contents of a memory location to a
  * given value and, only if they are the same, modifies the contents of that