<p><b>@bergzand</b> commented on this pull request.</p>

<hr>

<p>In <a href="https://github.com/RIOT-OS/RIOT/pull/10916#discussion_r269294232">sys/include/usb/usbus.h</a>:</p>
<pre style='color:#555'>> +     * @param usbus     USBUS context
+     * @param handler   handler context
+     */
+    void (*init)(usbus_t *usbus, struct usbus_handler *handler);
+
+    /**
+     * @brief event handler function
+     *
+     * This function is passed USBUS events
+     *
+     * @param usbus     USBUS context
+     * @param handler   handler context
+     * @param event     @ref USBUS_MSG_TYPES "event" to handle
+     * @param arg       Additional argument
+     */
+    int (*event_handler)(usbus_t *usbus, struct usbus_handler *handler, uint16_t event, void *arg);
</pre>
<p>Splitted these, documentation can be improved and a few defines should be changed, but so far I'm happy with it and it reduces code size a bit</p>

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