diff --git a/dev/include/dev/usb.h b/dev/include/dev/usb.h
index 17fd4fcf..b78b11a7 100644
--- a/dev/include/dev/usb.h
+++ b/dev/include/dev/usb.h
@@ -64,18 +64,17 @@ status_t usb_setup(usb_config *config);
 uint8_t usb_get_current_iface_num_highspeed(void);
 uint8_t usb_get_current_iface_num_lowspeed(void);
 
-/* apped new interface descriptors to the existing config if desired */
-status_t usb_append_interface_highspeed(const uint8_t *int_descr, size_t len);
-status_t usb_append_interface_lowspeed(const uint8_t *int_descr, size_t len);
+/* Append new interface descriptors to the existing config.*/
+/* Returns interface number selected or error. */
+int usb_append_interface_highspeed(const uint8_t *int_descr, size_t len);
+int usb_append_interface_lowspeed(const uint8_t *int_descr, size_t len);
 
+/* add a new string to the existing config */
 status_t usb_add_string(const char *string, uint8_t id);
 
 status_t usb_start(void);
 status_t usb_stop(void);
 
-/* callback api the usbc driver uses */
-status_t usbc_callback(usb_callback_op_t op, const union usb_callback_args *args);
-
 /* callback api that anyone can register for */
 status_t usb_register_callback(usb_callback_t, void *cookie);
 
diff --git a/dev/include/dev/usbc.h b/dev/include/dev/usbc.h
index 479226d1..365ae2ba 100644
--- a/dev/include/dev/usbc.h
+++ b/dev/include/dev/usbc.h
@@ -12,6 +12,7 @@
 #include <stdio.h>
 #include <sys/types.h>
 #include <hw/usb.h>
+#include <dev/usb.h>
 
 __BEGIN_CDECLS
 
@@ -58,6 +59,9 @@ status_t usbc_flush_ep(ep_t ep);
 status_t usbc_set_active(bool active);
 void usbc_set_address(uint8_t address);
 
+/* callback api the usbc driver uses */
+status_t usbc_callback(usb_callback_op_t op, const union usb_callback_args *args);
+
 /* called back from within a callback to handle setup responses */
 void usbc_ep0_ack(void);
 void usbc_ep0_stall(void);
diff --git a/dev/usb/usb.c b/dev/usb/usb.c
index e77fc122..0bb8ba4f 100644
--- a/dev/usb/usb.c
+++ b/dev/usb/usb.c
@@ -5,6 +5,8 @@
  * license that can be found in the LICENSE file or at
  * https://opensource.org/licenses/MIT
  */
+#include <dev/usb.h>
+
 #include <lk/debug.h>
 #include <lk/trace.h>
 #include <lk/err.h>
@@ -13,9 +15,16 @@
 #include <assert.h>
 #include <lk/list.h>
 #include <dev/usbc.h>
-#include <dev/usb.h>
 #include <lk/init.h>
 
+/*
+ * USB device side support. Handles configuration and main high level host requests.
+ * USB device controller must pump the stack via usbc_callback() and implement the
+ * usbc function interface.
+ */
+
+/* TODO: make big endian safe */
+
 #define LOCAL_TRACE 0
 
 #define MAX_STRINGS 8
@@ -38,8 +47,11 @@ typedef struct {
 
 static void usb_do_callbacks(usb_callback_op_t op, const union usb_callback_args *args);
 
-static void append_desc_data(usb_descriptor *desc, const void *dat, size_t len) {
+static status_t append_desc_data(usb_descriptor *desc, const void *dat, size_t len) {
     uint8_t *ptr = malloc(desc->len + len);
+    if (!ptr) {
+        return ERR_NO_MEMORY;
+    }
 
     memcpy(ptr, desc->desc, desc->len);
     memcpy(ptr + desc->len, dat, len);
@@ -51,6 +63,8 @@ static void append_desc_data(usb_descriptor *desc, const void *dat, size_t len)
 
     desc->desc = ptr;
     desc->len += len;
+
+    return NO_ERROR;
 }
 
 static uint8_t usb_get_current_iface_num(const usb_descriptor *desc) {
@@ -67,16 +81,18 @@ uint8_t usb_get_current_iface_num_lowspeed(void) {
     return usb_get_current_iface_num(&usb.config->lowspeed.config);
 }
 
-/* returns the interface number assigned */
+/* returns the interface number assigned or error */
 static int usb_append_interface(usb_descriptor *desc, const uint8_t *int_descr, size_t len) {
     uint8_t *ptr = malloc(len);
-    int interface_num;
+    if (!ptr) {
+        return ERR_NO_MEMORY;
+    }
 
     // create a temporary copy of the interface
     memcpy(ptr, int_descr, len);
 
     // find the last interface used
-    interface_num = usb_get_current_iface_num(desc);  // current interface
+    int interface_num = usb_get_current_iface_num(desc);  // current interface
 
     // patch our interface descriptor with the new id
     ptr[2] = interface_num;
@@ -90,6 +106,7 @@ static int usb_append_interface(usb_descriptor *desc, const uint8_t *int_descr,
     interface_num++;
     ((uint8_t *)desc->desc)[4] = interface_num;
 
+    DEBUG_ASSERT(interface_num > 0);
     return interface_num - 1;
 }
 
@@ -101,26 +118,6 @@ int usb_append_interface_lowspeed(const uint8_t *int_descr, size_t len) {
     return usb_append_interface(&usb.config->lowspeed.config, int_descr, len);
 }
 
-void usb_set_string_descriptor(usb_descriptor *desc, const char *string) {
-    int len = strlen(string);
-    ushort *data;
-    int datalen = len * 2 + 2;
-
-    data = malloc(datalen);
-
-    /* write length field */
-    data[0] = 0x0300 + datalen;
-
-    /* copy the string into the uint16_t based usb string */
-    int i;
-    for (i = 0; i < len; i++) {
-        data[i + 1] = string[i];
-    }
-
-    desc->desc = (void *)data;
-    desc->len = datalen;
-}
-
 static void set_usb_id(uint16_t vendor, uint16_t product) {
     // patch the current configuration to with the vendor/product id
     ((uint16_t *)usb.config->lowspeed.device.desc)[4] = vendor;
@@ -135,8 +132,9 @@ status_t usb_add_string(const char *string, uint8_t id) {
     size_t len = strlen(string);
 
     uint16_t *strbuf = malloc(len * 2 + 2);
-    if (!strbuf)
+    if (!strbuf) {
         return ERR_NO_MEMORY;
+    }
 
     /* build the usb string descriptor */
     strbuf[0] = 0x300 | (len * 2 + 2);
@@ -176,8 +174,9 @@ status_t usb_register_callback(usb_callback_t cb, void *cookie) {
     DEBUG_ASSERT(cb);
 
     usb_callback_container_t *c = malloc(sizeof(usb_callback_container_t));
-    if (!c)
+    if (!c) {
         return ERR_NO_MEMORY;
+    }
 
     c->cb = cb;
     c->cookie = cookie;