TARGET_NRF51822 - Override critical section enter/exit when Nordic SDK v10 is used.

This change is not located in the hal because SDK v10 is not available from this
place. It is safe to provide the implementation in BLE because the SDK V10 is
defined there. Default implementations of critical section enter/exit are safe
as long as the softdevice is not used.

features/FEATURE_BLE/targets/TARGET_NORDIC/TARGET_MCU_NRF51822/hal_patch/nordic_critical.c

@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2015-2016, ARM Limited, All Rights Reserved
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * 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 <stdint.h>                  // uint32_t, UINT32_MAX
+#include <assert.h>                  // uint32_t, UINT32_MAX
+#include "cmsis.h"
+#include "nrf_soc.h"
+#include "nrf_sdm.h"
+
+static volatile union {
+    uint32_t _PRIMASK_state;
+    uint8_t  _sd_state;
+} _state = { 0 } ;
+static volatile uint32_t _entry_count = 0;
+static volatile bool _use_softdevice_routine = false;
+
+void core_util_critical_section_enter()
+{
+    // if a critical section has already been entered, just update the counter
+    if (_entry_count) {
+        ++_entry_count;
+        return;
+    }
+
+    // in this path, a critical section has never been entered
+    uint32_t primask = __get_PRIMASK();
+
+    // if interrupts are enabled, try to use the soft device
+    uint8_t sd_enabled;
+    if ((primask == 0) && (sd_softdevice_is_enabled(&sd_enabled) == NRF_SUCCESS) && sd_enabled == 1) {
+        // if the soft device can be use, use it
+        sd_nvic_critical_region_enter(&_state._sd_state);
+        _use_softdevice_routine = true;
+    } else {
+        // if interrupts where enabled, disable them
+        if(primask == 0) {
+            __disable_irq();
+        }
+
+        // store the PRIMASK state, it will be restored at the end of the critical section
+        _state._PRIMASK_state = primask;
+        _use_softdevice_routine = false;
+    }
+
+    assert(_entry_count == 0); // entry count should always be equal to 0 at this point
+    ++_entry_count;
+}
+
+void core_util_critical_section_exit()
+{
+    assert(_entry_count > 0);
+    --_entry_count;
+
+    // If their is other segments which have entered the critical section, just leave
+    if (_entry_count) {
+        return;
+    }
+
+    // This is the last segment of the critical section, state should be restored as before entering
+    // the critical section
+    if (_use_softdevice_routine) {
+        sd_nvic_critical_region_exit(_state._sd_state);
+    } else {
+        __set_PRIMASK(_state._PRIMASK_state);
+    }
+}