diff --git a/platform/cocoa/btstack_run_loop_cocoa.m b/platform/cocoa/btstack_run_loop_cocoa.m
index 03419f529..a9c81f170 100644
--- a/platform/cocoa/btstack_run_loop_cocoa.m
+++ b/platform/cocoa/btstack_run_loop_cocoa.m
@@ -72,7 +72,7 @@ static void socketDataCallback (
     if (callbackType == kCFSocketReadCallBack && info){
         btstack_data_source_t *dataSource = (btstack_data_source_t *) info;
         // printf("btstack_run_loop_cocoa_data_source %x - fd %u, CFSocket %x, CFRunLoopSource %x\n", (int) dataSource, dataSource->fd, (int) s, (int) dataSource->item.next);
-        dataSource->process(dataSource);
+        dataSource->process(dataSource, DATA_SOURCE_CALLBACK_READ);
     }
 }
 
diff --git a/platform/daemon/src/socket_connection.c b/platform/daemon/src/socket_connection.c
index d21b6b19f..a7a05b477 100644
--- a/platform/daemon/src/socket_connection.c
+++ b/platform/daemon/src/socket_connection.c
@@ -282,7 +282,7 @@ int  socket_connection_has_parked_connections(void){
     return parked != NULL;
 }
 
-static int socket_connection_accept(btstack_data_source_t *socket_ds) {
+static void socket_connection_accept(btstack_data_source_t *socket_ds, btstack_data_source_callback_type_t callback_type) {
     struct sockaddr_storage ss;
     socklen_t slen = sizeof(ss);
     int socket_fd = btstack_run_loop_get_data_source_fd(ds);
diff --git a/platform/embedded/btstack_run_loop_embedded.c b/platform/embedded/btstack_run_loop_embedded.c
index 8732195c8..752473a6f 100644
--- a/platform/embedded/btstack_run_loop_embedded.c
+++ b/platform/embedded/btstack_run_loop_embedded.c
@@ -187,7 +187,7 @@ void btstack_run_loop_embedded_execute_once(void) {
         btstack_timer_source_t *ts = (btstack_timer_source_t *) timers;
         if (ts->timeout > now) break;
         btstack_run_loop_remove_timer(ts);
-        ts->process(ts);
+        ts->process(ts, DATA_SOURCE_CALLBACK_POLL);
     }
 #endif
     
diff --git a/platform/embedded/hci_transport_h4_ehcill_embedded.c b/platform/embedded/hci_transport_h4_ehcill_embedded.c
index f6154fd0d..832a97ad5 100644
--- a/platform/embedded/hci_transport_h4_ehcill_embedded.c
+++ b/platform/embedded/hci_transport_h4_ehcill_embedded.c
@@ -102,7 +102,7 @@ typedef struct hci_transport_h4 {
 } hci_transport_h4_t;
 
 // prototypes
-static int h4_process(btstack_data_source_t *ds);
+static void h4_process(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type);
 static void dummy_handler(uint8_t packet_type, uint8_t *packet, uint16_t size); 
 static void h4_block_received(void);
 static void h4_block_sent(void);
@@ -387,7 +387,7 @@ static void h4_block_sent(void){
     }
 }
 
-static int h4_process(btstack_data_source_t *ds) {
+static void h4_process(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type) {
     
     // reset tx state before emitting packet sent event
     // to allow for positive can_send_now
diff --git a/platform/embedded/hci_transport_h4_embedded.c b/platform/embedded/hci_transport_h4_embedded.c
index 43207c87c..28fe3a3a3 100644
--- a/platform/embedded/hci_transport_h4_embedded.c
+++ b/platform/embedded/hci_transport_h4_embedded.c
@@ -83,7 +83,7 @@ typedef struct hci_transport_h4 {
 } hci_transport_h4_t;
 
 // prototypes
-static int h4_process(btstack_data_source_t *ds);
+static void h4_process(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type);
 static void dummy_handler(uint8_t packet_type, uint8_t *packet, uint16_t size); 
 static void h4_block_received(void);
 static void h4_block_sent(void);
@@ -243,7 +243,7 @@ static void h4_register_packet_handler(void (*handler)(uint8_t packet_type, uint
     packet_handler = handler;
 }
 
-static int h4_process(btstack_data_source_t *ds) {
+static void h4_process(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type) {
     
     // notify about packet sent
     if (tx_state == TX_DONE){
diff --git a/platform/posix/btstack_run_loop_posix.c b/platform/posix/btstack_run_loop_posix.c
index bccbb916c..ac20bd971 100644
--- a/platform/posix/btstack_run_loop_posix.c
+++ b/platform/posix/btstack_run_loop_posix.c
@@ -180,7 +180,7 @@ static void btstack_run_loop_posix_execute(void) {
             // log_info("btstack_run_loop_posix_execute: check %x with fd %u\n", (int) ds, ds->fd);
             if (FD_ISSET(ds->fd, &descriptors)) {
                 // log_info("btstack_run_loop_posix_execute: process %x with fd %u\n", (int) ds, ds->fd);
-                ds->process(ds);
+                ds->process(ds, DATA_SOURCE_CALLBACK_READ);
             }
         }
         // log_info("btstack_run_loop_posix_execute: after ds check\n");
diff --git a/platform/posix/hci_transport_h4_posix.c b/platform/posix/hci_transport_h4_posix.c
index 5f67c7905..fee656e33 100644
--- a/platform/posix/hci_transport_h4_posix.c
+++ b/platform/posix/hci_transport_h4_posix.c
@@ -65,7 +65,7 @@
 #error HCI_OUTGOING_PRE_BUFFER_SIZE not defined. Please update hci.h
 #endif
 
-static int h4_process(btstack_data_source_t *ds);
+static void h4_process(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type);
 static void dummy_handler(uint8_t packet_type, uint8_t *packet, uint16_t size); 
 
 typedef enum {
@@ -342,8 +342,8 @@ static void h4_statemachine(void){
     }
 }
 
-static int h4_process(btstack_data_source_t *ds) {
-    if (hci_transport_h4->uart_fd == 0) return -1;
+static void h4_process(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type) {
+    if (hci_transport_h4->uart_fd == 0) return;
 
     int read_now = bytes_to_read;
 
@@ -352,9 +352,7 @@ static int h4_process(btstack_data_source_t *ds) {
     // read up to bytes_to_read data in
     ssize_t bytes_read = read(hci_transport_h4->uart_fd, &hci_packet[read_pos], read_now);
     // log_info("h4_process: bytes read %u", bytes_read);
-    if (bytes_read < 0) {
-        return bytes_read;
-    }
+    if (bytes_read < 0) return;
 
     uint32_t end = btstack_run_loop_get_time_ms();
     if (end - start > 10){
@@ -363,12 +361,9 @@ static int h4_process(btstack_data_source_t *ds) {
     
     bytes_to_read -= bytes_read;
     read_pos      += bytes_read;
-    if (bytes_to_read > 0) {
-        return 0;
-    }
+    if (bytes_to_read > 0) return;
     
     h4_statemachine();
-    return 0;
 }
 
 static void dummy_handler(uint8_t packet_type, uint8_t *packet, uint16_t size){
diff --git a/port/ios/src/hci_transport_h4_iphone.c b/port/ios/src/hci_transport_h4_iphone.c
index ecbeaf8ca..62ae8867e 100644
--- a/port/ios/src/hci_transport_h4_iphone.c
+++ b/port/ios/src/hci_transport_h4_iphone.c
@@ -83,7 +83,7 @@
 #include "hci.h"
 #include "hci_transport.h"
 
-static int h4_process(btstack_data_source_t *ds);
+static void h4_process(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type);
 static void dummy_handler(uint8_t packet_type, uint8_t *packet, uint16_t size); 
 static      hci_transport_config_uart_t *hci_transport_config_uart;
 
@@ -336,7 +336,7 @@ static void h4_statemachine(void){
     }
 }
 
-static int h4_process(btstack_data_source_t *ds) {
+static void h4_process(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type) {
     if (hci_transport_h4->uart_fd == 0) return -1;
 
     int read_now = bytes_to_read;
diff --git a/port/libusb/hci_transport_h2_libusb.c b/port/libusb/hci_transport_h2_libusb.c
index cfdfb8101..d5bf285ff 100644
--- a/port/libusb/hci_transport_h2_libusb.c
+++ b/port/libusb/hci_transport_h2_libusb.c
@@ -400,7 +400,7 @@ static void handle_completed_transfer(struct libusb_transfer *transfer){
     }   
 }
 
-static int usb_process_ds(btstack_data_source_t *ds) {
+static void usb_process_ds(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type) {
     if (libusb_state != LIB_USB_TRANSFERS_ALLOCATED) return -1;
 
     // log_info("begin usb_process_ds");
diff --git a/port/mtk/hci_transport_h4_mtk.c b/port/mtk/hci_transport_h4_mtk.c
index 1facd2041..7c8cd73ad 100644
--- a/port/mtk/hci_transport_h4_mtk.c
+++ b/port/mtk/hci_transport_h4_mtk.c
@@ -54,7 +54,7 @@
 #include "hci.h"
 #include "hci_transport.h"
 
-static int h4_process(btstack_data_source_t *ds);
+static void h4_process(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type);
 static void dummy_handler(uint8_t packet_type, uint8_t *packet, uint16_t size); 
 
 typedef struct hci_transport_h4 {
@@ -122,7 +122,7 @@ static void   h4_register_packet_handler(void (*handler)(uint8_t packet_type, ui
     packet_handler = handler;
 }
 
-static int h4_process(btstack_data_source_t *ds) {
+static void h4_process(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type) {
     if (hci_transport_h4->ds->fd == 0) return -1;
 
     // read up to bytes_to_read data in
diff --git a/port/nrf5x/main.c b/port/nrf5x/main.c
index 978b893d0..2245ebffc 100755
--- a/port/nrf5x/main.c
+++ b/port/nrf5x/main.c
@@ -368,7 +368,7 @@ uint8_t ll_set_scan_enable(uint8_t le_scan_enable, uint8_t filter_duplicates){
     }
 }
 
-static int transport_run(btstack_data_source_t * ds){
+static void transport_run(btstack_data_source_t *ds, btstack_data_source_callback_type_t callback_type){
     // deliver hci packet on main thread
     if (hci_outgoing_event_ready){
         hci_outgoing_event_ready = 0;
diff --git a/src/btstack_run_loop.h b/src/btstack_run_loop.h
index 51d1fe4be..370eb59a2 100644
--- a/src/btstack_run_loop.h
+++ b/src/btstack_run_loop.h
@@ -58,13 +58,23 @@
 extern "C" {
 #endif
 	
+
+/**
+ * Callback types for run loop data sources
+ */
+typedef enum {
+	DATA_SOURCE_CALLBACK_POLL  = 1 << 0,
+	DATA_SOURCE_CALLBACK_READ  = 1 << 1,
+	DATA_SOURCE_CALLBACK_WRITE = 1 << 2,
+} btstack_data_source_callback_type_t;
+
 typedef struct btstack_data_source {
 	// 
     btstack_linked_item_t item;
     // file descriptor to watch for run loops that support file descriptors
     int  fd;
     // callback to call for enabled callback types
-    int  (*process)(struct btstack_data_source *ds);
+    void  (*process)(struct btstack_data_source *ds, btstack_data_source_callback_type_t callback_type);
     // flags storing enabled callback types
     uint16_t flags;
 } btstack_data_source_t;
@@ -82,7 +92,6 @@ typedef struct btstack_timer_source {
     void * context;
 } btstack_timer_source_t;
 
-// 
 typedef struct btstack_run_loop {
 	void (*init)(void);
 	void (*add_data_source)(btstack_data_source_t *dataSource);
@@ -99,15 +108,6 @@ void btstack_run_loop_timer_dump(void);
 
 /* API_START */
 
-/**
- * possible callback types for run loop data sources
- */
-typedef enum {
-	DATA_SOURCE_CALLBACK_POLL  = 1 << 0,
-	DATA_SOURCE_CALLBACK_READ  = 1 << 1,
-	DATA_SOURCE_CALLBACK_WRITE = 1 << 2,
-} btstack_data_source_callback_type_t;
-
 /**
  * @brief Init main run loop. Must be called before any other run loop call.
  *  
@@ -154,7 +154,7 @@ uint32_t btstack_run_loop_get_time_ms(void);
 /**
  * @brief Set data source callback.
  */
-void btstack_run_loop_set_data_source_handler(btstack_data_source_t *ds, int (*process)(btstack_data_source_t *_ds));
+void btstack_run_loop_set_data_source_handler(btstack_data_source_t *ds, void (*process)(btstack_data_source_t *_ds, btstack_data_source_callback_type_t callback_type));
 
 /**
  * @brief Set data source file descriptor.