components/esp32: add CPU frequency selection in menuconfig

Note that with WiFi stack enabled, system_init will reset frequency to 240MHz.
To make this setting useful, esp32-wifi-libs submodule needs to be updated.

components/esp32/cpu_freq.c

@@ -0,0 +1,68 @@
+// Copyright 2015-2016 Espressif Systems (Shanghai) PTE LTD
+//
+// 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>
+#include "rom/ets_sys.h"
+#include "sdkconfig.h"
+
+typedef enum{
+    XTAL_40M = 40,
+    XTAL_26M = 26,
+    XTAL_24M = 24,
+    XTAL_AUTO = 0
+} xtal_freq_t;
+
+typedef enum{
+    CPU_80M = 1,
+    CPU_160M = 2,
+    CPU_240M = 3,
+} cpu_freq_t;
+
+extern void phy_get_romfunc_addr();
+
+// TODO: these functions need to be moved from librtc to ESP-IDF
+extern void rtc_init_lite();
+extern void rtc_set_cpu_freq(xtal_freq_t xtal_freq, cpu_freq_t cpu_freq);
+
+/*
+ * This function is not exposed as an API at this point,
+ * because FreeRTOS doesn't yet support dynamic changing of
+ * CPU frequency. Also we need to implement hooks for
+ * components which want to be notified of CPU frequency
+ * changes.
+ */
+void esp_set_cpu_freq()
+{
+    uint32_t freq_mhz = CONFIG_ESP32_DEFAULT_CPU_FREQ_MHZ;
+    phy_get_romfunc_addr();
+    rtc_init_lite();
+    cpu_freq_t freq = CPU_80M;
+    switch(freq_mhz) {
+        case 240:
+            freq = CPU_240M;
+            break;
+        case 160:
+            freq = CPU_160M;
+            break;
+        default:
+            freq_mhz = 80;
+            /* no break */
+        case 80:
+            freq = CPU_80M;
+            break;
+    }
+    rtc_set_cpu_freq(XTAL_AUTO, freq);
+    ets_update_cpu_frequency(freq_mhz);
+}
+