Fix save config when no audio is playing

firmware/code/configuration_manager.c

@@ -433,6 +433,12 @@ bool process_cmd(tlv_header* cmd) {
         case SAVE_CONFIGURATION: {
             if (cmd->length == 4) {
                 saveState = SaveRequested;
+                if (audio_state.interface == 0) {
+                    // The OS will configure the alternate "zero" interface when the device is not in use
+                    // in this sate we can write to flash now. Otherwise, defer the save until we get the next
+                    // usb packet.
+                    save_config();
+                }
                 result->type = OK;
                 result->length = 4;
                 return true;

firmware/code/run.c

@@ -52,6 +52,7 @@ static uint8_t *userbuf;
 audio_state_config audio_state = {
     .freq = 48000,
     .de_emphasis_frequency = 0x1, // 48khz
+    .interface = 0
 };
 
 preprocessing_config preprocessing = {
@@ -769,6 +770,7 @@ static const struct usb_transfer_type _audio_cmd_transfer_type = {
 
 static bool as_set_alternate(struct usb_interface *interface, uint alt) {
     assert(interface == &as_op_interface);
+    audio_state.interface = alt;
     switch (alt) {
         case 0: power_down_dac(); return true;
         case 1: power_up_dac(); return true;

firmware/code/run.h

@@ -76,6 +76,7 @@ typedef struct _audio_state_config {
         int16_t _target_pcm3060_registers;
     };
     int16_t pcm3060_registers;
+    int8_t interface;
 } audio_state_config;
 extern audio_state_config audio_state;