diff --git a/src/usbd_stm32f103_devfs.c b/src/usbd_stm32f103_devfs.c
index 575d6b0..bd41ad5 100644
--- a/src/usbd_stm32f103_devfs.c
+++ b/src/usbd_stm32f103_devfs.c
@@ -345,7 +345,7 @@ static void ep_deconfig(uint8_t ep) {
 }
 
 static uint16_t pma_read (uint8_t *buf, uint16_t blen, pma_rec *rx) {
-    uint16_t tmp;
+    uint16_t tmp = 0;
     uint16_t *pma = PMA(rx->addr);
     uint16_t rxcnt = rx->cnt & 0x03FF;
     rx->cnt &= ~0x3FF;
diff --git a/src/usbd_stm32f105_otgfs.c b/src/usbd_stm32f105_otgfs.c
index e3c0f4b..455baf3 100644
--- a/src/usbd_stm32f105_otgfs.c
+++ b/src/usbd_stm32f105_otgfs.c
@@ -321,7 +321,7 @@ static void ep_deconfig(uint8_t ep) {
 }
 
 static int32_t ep_read(uint8_t ep, void* buf, uint16_t blen) {
-    uint32_t len, tmp;
+    uint32_t len, tmp = 0;
     volatile uint32_t *fifo = EPFIFO(0);
     /* no data in RX FIFO */
     if (!(OTG->GINTSTS & USB_OTG_GINTSTS_RXFLVL)) return -1;
@@ -416,6 +416,7 @@ static void evt_poll(usbd_device *dev, usbd_evt_callback callback) {
             case 0x03:  /* OUT completed */
             case 0x04:  /* SETUP completed */
                 _BST(EPOUT(ep)->DOEPCTL, USB_OTG_DOEPCTL_CNAK | USB_OTG_DOEPCTL_EPENA);
+                // fall through
             default:
                 /* pop GRXSTSP */
                 OTG->GRXSTSP;
@@ -436,7 +437,7 @@ static void evt_poll(usbd_device *dev, usbd_evt_callback callback) {
             /* no more supported events */
             return;
         }
-        return callback(dev, evt, ep);
+        callback(dev, evt, ep);
     }
 }
 
diff --git a/src/usbd_stm32f429_otgfs.c b/src/usbd_stm32f429_otgfs.c
index 059a541..5e8d237 100644
--- a/src/usbd_stm32f429_otgfs.c
+++ b/src/usbd_stm32f429_otgfs.c
@@ -318,7 +318,7 @@ static void ep_deconfig(uint8_t ep) {
 }
 
 static int32_t ep_read(uint8_t ep, void* buf, uint16_t blen) {
-    uint32_t len, tmp;
+    uint32_t len, tmp = 0;
     volatile uint32_t *fifo = EPFIFO(0);
     /* no data in RX FIFO */
     if (!(OTG->GINTSTS & USB_OTG_GINTSTS_RXFLVL)) return -1;
@@ -413,6 +413,7 @@ static void evt_poll(usbd_device *dev, usbd_evt_callback callback) {
             case 0x03:  /* OUT completed */
             case 0x04:  /* SETUP completed */
                 _BST(EPOUT(ep)->DOEPCTL, USB_OTG_DOEPCTL_CNAK | USB_OTG_DOEPCTL_EPENA);
+                // fall through
             default:
                 /* pop GRXSTSP */
                 OTG->GRXSTSP;
@@ -433,7 +434,7 @@ static void evt_poll(usbd_device *dev, usbd_evt_callback callback) {
             /* no more supported events */
             return;
         }
-        return callback(dev, evt, ep);
+        callback(dev, evt, ep);
     }
 }
 
diff --git a/src/usbd_stm32f429_otghs.c b/src/usbd_stm32f429_otghs.c
index 996c701..10a48dc 100644
--- a/src/usbd_stm32f429_otghs.c
+++ b/src/usbd_stm32f429_otghs.c
@@ -320,7 +320,7 @@ static void ep_deconfig(uint8_t ep) {
 }
 
 static int32_t ep_read(uint8_t ep, void* buf, uint16_t blen) {
-    uint32_t len, tmp;
+    uint32_t len, tmp = 0;
     volatile uint32_t *fifo = EPFIFO(0);
     /* no data in RX FIFO */
     if (!(OTG->GINTSTS & USB_OTG_GINTSTS_RXFLVL)) return -1;
@@ -418,6 +418,7 @@ static void evt_poll(usbd_device *dev, usbd_evt_callback callback) {
             case 0x03:  /* OUT completed */
             case 0x04:  /* SETUP completed */
                 _BST(EPOUT(ep)->DOEPCTL, USB_OTG_DOEPCTL_CNAK | USB_OTG_DOEPCTL_EPENA);
+                // fall through
             default:
                 /* pop GRXSTSP */
                 OTG->GRXSTSP;
@@ -438,7 +439,7 @@ static void evt_poll(usbd_device *dev, usbd_evt_callback callback) {
             /* no more supported events */
             return;
         }
-        return callback(dev, evt, ep);
+        callback(dev, evt, ep);
     }
 }
 
diff --git a/src/usbd_stm32f446_otgfs.c b/src/usbd_stm32f446_otgfs.c
index ea1d586..d933d58 100644
--- a/src/usbd_stm32f446_otgfs.c
+++ b/src/usbd_stm32f446_otgfs.c
@@ -309,7 +309,7 @@ static void ep_deconfig(uint8_t ep) {
 }
 
 static int32_t ep_read(uint8_t ep, void* buf, uint16_t blen) {
-    uint32_t len, tmp;
+    uint32_t len, tmp = 0;
     ep &= 0x7F;
     volatile uint32_t *fifo = EPFIFO(0);
     USB_OTG_OUTEndpointTypeDef* epo = EPOUT(ep);
@@ -418,7 +418,7 @@ static void evt_poll(usbd_device *dev, usbd_evt_callback callback) {
             /* no more supported events */
             return;
         }
-        return callback(dev, evt, ep);
+        callback(dev, evt, ep);
     }
 }
 
diff --git a/src/usbd_stm32f446_otghs.c b/src/usbd_stm32f446_otghs.c
index 4e4a8b3..102db9d 100644
--- a/src/usbd_stm32f446_otghs.c
+++ b/src/usbd_stm32f446_otghs.c
@@ -309,7 +309,7 @@ static void ep_deconfig(uint8_t ep) {
 }
 
 static int32_t ep_read(uint8_t ep, void* buf, uint16_t blen) {
-    uint32_t len, tmp;
+    uint32_t len, tmp = 0;
     ep &= 0x7F;
     volatile uint32_t *fifo = EPFIFO(0);
     USB_OTG_OUTEndpointTypeDef* epo = EPOUT(ep);
@@ -418,7 +418,7 @@ static void evt_poll(usbd_device *dev, usbd_evt_callback callback) {
             /* no more supported events */
             return;
         }
-        return callback(dev, evt, ep);
+        callback(dev, evt, ep);
     }
 }
 
diff --git a/src/usbd_stm32h743_otgfs.c b/src/usbd_stm32h743_otgfs.c
index 7ea6236..d3494d5 100644
--- a/src/usbd_stm32h743_otgfs.c
+++ b/src/usbd_stm32h743_otgfs.c
@@ -309,7 +309,7 @@ static void ep_deconfig(uint8_t ep) {
 }
 
 static int32_t ep_read(uint8_t ep, void* buf, uint16_t blen) {
-    uint32_t len, tmp;
+    uint32_t len, tmp = 0;
     ep &= 0x7F;
     volatile uint32_t *fifo = EPFIFO(0);
     USB_OTG_OUTEndpointTypeDef* epo = EPOUT(ep);
@@ -418,7 +418,7 @@ static void evt_poll(usbd_device *dev, usbd_evt_callback callback) {
             /* no more supported events */
             return;
         }
-        return callback(dev, evt, ep);
+        callback(dev, evt, ep);
     }
 }
 
diff --git a/src/usbd_stm32l052_devfs.c b/src/usbd_stm32l052_devfs.c
index 6bc65ad..d6004c3 100644
--- a/src/usbd_stm32l052_devfs.c
+++ b/src/usbd_stm32l052_devfs.c
@@ -283,7 +283,7 @@ static void ep_deconfig(uint8_t ep) {
 }
 
 static uint16_t pma_read (uint8_t *buf, uint16_t blen, pma_rec *rx) {
-    uint16_t tmp;
+    uint16_t tmp = 0;
     uint16_t *pma = (void*)(USB_PMAADDR + rx->addr);
     uint16_t rxcnt = rx->cnt & 0x03FF;
     rx->cnt &= ~0x3FF;
diff --git a/src/usbd_stm32l100_devfs.c b/src/usbd_stm32l100_devfs.c
index 9da29fe..f502476 100644
--- a/src/usbd_stm32l100_devfs.c
+++ b/src/usbd_stm32l100_devfs.c
@@ -255,7 +255,7 @@ static void ep_deconfig(uint8_t ep) {
 }
 
 static uint16_t pma_read (uint8_t *buf, uint16_t blen, pma_rec *rx) {
-    uint16_t tmp;
+    uint16_t tmp = 0;
     uint16_t *pma = (void*)(USB_PMAADDR + 2 * rx->addr);
     uint16_t rxcnt = rx->cnt & 0x03FF;
     rx->cnt &= ~0x3FF;
diff --git a/src/usbd_stm32l476_otgfs.c b/src/usbd_stm32l476_otgfs.c
index 66294bc..6b0eda7 100644
--- a/src/usbd_stm32l476_otgfs.c
+++ b/src/usbd_stm32l476_otgfs.c
@@ -342,7 +342,7 @@ static void ep_deconfig(uint8_t ep) {
 }
 
 static int32_t ep_read(uint8_t ep, void* buf, uint16_t blen) {
-    uint32_t len, tmp;
+    uint32_t len, tmp = 0;
     ep &= 0x7F;
     volatile uint32_t *fifo = EPFIFO(0);
     USB_OTG_OUTEndpointTypeDef* epo = EPOUT(ep);
@@ -451,7 +451,7 @@ static void evt_poll(usbd_device *dev, usbd_evt_callback callback) {
             /* no more supported events */
             return;
         }
-        return callback(dev, evt, ep);
+        callback(dev, evt, ep);
     }
 }