pm: fix calculation of maximum of two frequencies

The old code calculated MAX() of two enum values, but CPU frequency
enum values are not ordered (2MHz goes after 240MHz). This caused
incorrect configuration to be set.

components/esp32/pm_esp32.c

@@ -148,6 +148,21 @@ pm_mode_t esp_pm_impl_get_mode(esp_pm_lock_type_t type, int arg)
     }
 }
 
+/* rtc_cpu_freq_t enum is not ordered by frequency, so convert to MHz,
+ * figure out the maximum value, then convert back to rtc_cpu_freq_t.
+ */
+static rtc_cpu_freq_t max_freq_of(rtc_cpu_freq_t f1, rtc_cpu_freq_t f2)
+{
+    int f1_hz = rtc_clk_cpu_freq_value(f1);
+    int f2_hz = rtc_clk_cpu_freq_value(f2);
+    int f_max_hz = MAX(f1_hz, f2_hz);
+    rtc_cpu_freq_t result = RTC_CPU_FREQ_XTAL;
+    if (!rtc_clk_cpu_freq_from_mhz(f_max_hz/1000000, &result)) {
+        assert(false && "unsupported frequency");
+    }
+    return result;
+}
+
 esp_err_t esp_pm_configure(const void* vconfig)
 {
 #ifndef CONFIG_PM_ENABLE
@@ -160,6 +175,11 @@ esp_err_t esp_pm_configure(const void* vconfig)
     }
     rtc_cpu_freq_t min_freq = config->min_cpu_freq;
     rtc_cpu_freq_t max_freq = config->max_cpu_freq;
+    int min_freq_mhz = rtc_clk_cpu_freq_value(min_freq);
+    int max_freq_mhz = rtc_clk_cpu_freq_value(max_freq);
+    if (min_freq_mhz > max_freq_mhz) {
+        return ESP_ERR_INVALID_ARG;
+    }
 
     rtc_cpu_freq_t apb_max_freq; /* CPU frequency in APB_MAX mode */
     if (max_freq == RTC_CPU_FREQ_240M) {
@@ -174,7 +194,7 @@ esp_err_t esp_pm_configure(const void* vconfig)
         apb_max_freq = RTC_CPU_FREQ_80M;
     }
 
-    apb_max_freq = MAX(apb_max_freq, min_freq);
+    apb_max_freq = max_freq_of(apb_max_freq, min_freq);
 
     ESP_LOGI(TAG, "Frequency switching config: "
                   "CPU_MAX: %s, APB_MAX: %s, APB_MIN: %s, Light sleep: %s",