Detect USB requests and reset or enter error state

Per the DFU spec, a USB reset should cause the device to either reenumerate as the application or enter an error state if invalid.
Register a USB reset callback to detect and handle USB resets, taking care not to trigger the first time the bootloader is enumerated.
Run the manifest callback if present when manifesting via USB reset.

Author: devanlai

src/dfu.c

@@ -331,6 +331,10 @@ dfu_control_class_request(usbd_device *usbd_dev,
     return status;
 }
 
+/* Track dfu enumeration status to distinguish between the first USB
+   reset after bootup versus a subsequent USB reset and re-enumeration */
+static bool dfu_enumerated = false;
+
 static void dfu_set_config(usbd_device* usbd_dev, uint16_t wValue) {
     (void)wValue;
 
@@ -339,6 +343,35 @@ static void dfu_set_config(usbd_device* usbd_dev, uint16_t wValue) {
         USB_REQ_TYPE_CLASS | USB_REQ_TYPE_INTERFACE,
         USB_REQ_TYPE_TYPE | USB_REQ_TYPE_RECIPIENT,
         dfu_control_class_request);
+
+    dfu_enumerated = true;
+}
+
+static void dfu_on_usb_reset(void) {
+    /* Ignore the first USB reset after boot, before anyone has had
+       a chance to do anything to the device; otherwise we'd never
+       enter DFU mode when the application is valid */
+    if (!dfu_enumerated) {
+        return;
+    }
+
+    /* On subsequent USB reset requests, either manifest by resetting
+       or enter the error state if firmware is invalid, per the spec */
+    if (validate_application()) {
+        /* Manifest by resetting after responding */
+        dfu_set_state(STATE_DFU_MANIFEST);
+
+        if (dfu_manifest_request_callback) {
+            dfu_manifest_request_callback();
+        }
+
+        /* Reset and launch the application if the manifest callback
+           didn't already do it */
+        scb_reset_system();
+    } else {
+        /* Enter the error state and await further commands */
+        dfu_set_status(DFU_STATUS_ERR_FIRMWARE);
+    }
 }
 
 void dfu_setup(usbd_device* usbd_dev,
@@ -349,6 +382,7 @@ void dfu_setup(usbd_device* usbd_dev,
     dfu_state_change_callback = on_state_change;
     dfu_status_change_callback = on_status_change;
 
+    usbd_register_reset_callback(usbd_dev, dfu_on_usb_reset);
     usbd_register_set_config_callback(usbd_dev, dfu_set_config);
     current_dfu_state = STATE_DFU_IDLE;
     current_dfu_status = DFU_STATUS_OK;