Move USBDevice processing out of user callbacks

Perform processing triggered by user callbacks when USB is being
unlocked rather than synchronously. This prevents recursive callbacks
which reduces stack usage. This also prevents state change inside
the user callback which makes the code easier to reason about.

usb/device/USBDevice/USBDevice.cpp

@@ -299,6 +299,18 @@ void USBDevice::complete_request_xfer_done(bool success)
     lock();
 
     MBED_ASSERT(_transfer.user_callback == RequestXferDone);
+    _transfer.args.status = success;
+    _run_later(&USBDevice::_complete_request_xfer_done);
+
+    unlock();
+}
+
+void USBDevice::_complete_request_xfer_done()
+{
+    assert_locked();
+
+    bool success = _transfer.args.status;
+
     _transfer.user_callback = None;
     if (_abort_control) {
         _control_abort();
@@ -320,8 +332,6 @@ void USBDevice::complete_request_xfer_done(bool success)
         _transfer.stage = Status;
         _phy->ep0_read();
     }
-
-    unlock();
 }
 
 bool USBDevice::_request_set_address()
@@ -363,6 +373,18 @@ void USBDevice::complete_set_configuration(bool success)
     lock();
 
     MBED_ASSERT(_transfer.user_callback == SetConfiguration);
+    _transfer.args.status = success;
+    _run_later(&USBDevice::_complete_set_configuration);
+
+    unlock();
+}
+
+void USBDevice::_complete_set_configuration()
+{
+    assert_locked();
+
+    bool success = _transfer.args.status;
+
     _transfer.user_callback = None;
     if (_abort_control) {
         _control_abort();
@@ -379,7 +401,6 @@ void USBDevice::complete_set_configuration(bool success)
         _phy->ep0_stall();
     }
 
-    unlock();
 }
 
 bool USBDevice::_request_get_configuration()
@@ -425,6 +446,18 @@ void USBDevice::complete_set_interface(bool success)
     lock();
 
     MBED_ASSERT(_transfer.user_callback == SetInterface);
+    _transfer.args.status = success;
+    _run_later(&USBDevice::_complete_set_interface);
+
+    unlock();
+}
+
+void USBDevice::_complete_set_interface()
+{
+    assert_locked();
+
+    bool success = _transfer.args.status;
+
     _transfer.user_callback = None;
     if (_abort_control) {
         _control_abort();
@@ -439,8 +472,6 @@ void USBDevice::complete_set_interface(bool success)
     } else {
         _phy->ep0_stall();
     }
-
-    unlock();
 }
 
 bool USBDevice::_request_set_feature()
@@ -643,6 +674,22 @@ void USBDevice::complete_request(RequestResult direction, uint8_t *data, uint32_
     lock();
 
     MBED_ASSERT(_transfer.user_callback == Request);
+    _transfer.args.request.result = direction;
+    _transfer.args.request.data = data;
+    _transfer.args.request.size = size;
+    _run_later(&USBDevice::_complete_request);
+
+    unlock();
+}
+
+void USBDevice::_complete_request()
+{
+    assert_locked();
+
+    RequestResult direction = _transfer.args.request.result;
+    uint8_t *data = _transfer.args.request.data;
+    uint32_t size = _transfer.args.request.size;
+
     _transfer.user_callback = None;
     if (_abort_control) {
         if ((direction == Receive) || (direction == Send)) {
@@ -674,8 +721,6 @@ void USBDevice::complete_request(RequestResult direction, uint8_t *data, uint32_
         _transfer.direction = direction;
         _control_setup_continue();
     }
-
-    unlock();
 }
 
 void USBDevice::_control_abort_start()
@@ -1151,6 +1196,7 @@ USBDevice::USBDevice(USBPhy *phy, uint16_t vendor_id, uint16_t product_id, uint1
     _current_interface = 0;
     _current_alternate = 0;
     _locked = 0;
+    _post_process = NULL;
 
     /* Set initial device state */
     _device.state = Powered;
@@ -1176,6 +1222,7 @@ USBDevice::USBDevice(uint16_t vendor_id, uint16_t product_id, uint16_t product_r
     _current_interface = 0;
     _current_alternate = 0;
     _locked = 0;
+    _post_process = NULL;
 
     /* Set initial device state */
     _device.state = Powered;
@@ -1390,6 +1437,16 @@ void USBDevice::lock()
 void USBDevice::unlock()
 {
     MBED_ASSERT(_locked > 0);
+
+    if (_locked == 1) {
+        // Perform post processing before fully unlocking
+        while (_post_process != NULL) {
+            void (USBDevice::*call)() = _post_process;
+            _post_process = NULL;
+            (this->*call)();
+        }
+    }
+
     _locked--;
     core_util_critical_section_exit();
 }
@@ -1424,3 +1481,8 @@ void USBDevice::_change_state(DeviceState new_state) {
 
     callback_state_change(new_state);
 }
+
+void USBDevice::_run_later(void (USBDevice::*function)())
+{
+    _post_process = function;
+}

usb/device/USBDevice/USBDevice.h

@@ -509,6 +509,12 @@ private:
     bool _request_get_interface();
     bool _request_set_interface();
     void _change_state(DeviceState state);
+    void _run_later(void (USBDevice::*function)());
+
+    void _complete_request();
+    void _complete_request_xfer_done();
+    void _complete_set_configuration();
+    void _complete_set_interface();
 
     struct endpoint_info_t {
         void (USBDevice::*callback)(usb_ep_t endpoint);
@@ -538,6 +544,17 @@ private:
         SetInterface
     };
 
+    struct complete_request_t {
+        RequestResult result;
+        uint8_t *data;
+        uint32_t size;
+    };
+
+    union complete_args_t {
+        complete_request_t request;
+        bool status;
+    };
+
     struct control_transfer_t {
         setup_packet_t setup;
         uint8_t *ptr;
@@ -547,6 +564,7 @@ private:
         bool notify;
         ControlState stage;
         UserCallback user_callback;
+        complete_args_t args;
     };
 
     endpoint_info_t _endpoint_info[32 - 2];
@@ -556,6 +574,7 @@ private:
     control_transfer_t _transfer;
     usb_device_t _device;
     uint32_t _max_packet_size_ep0;
+    void (USBDevice::*_post_process)();
 
     bool _setup_ready;
     bool _abort_control;