risc-v/mpfs: usb: fix infinite loop issue

mpfs_write_tx_fifo() gets stuck in the following case:
  - CDCACM is used
  - ttyACM0 is opened and then closed from the remote end,
    such as Linux or Windows
  - data is written into ttyACM0 from NuttX
  - tx fifo will never get empty and the system is stuck

Fix this by issuing an error code if the transmit fifo doesn't
proceed as expected. The error code is then propagated into
higher level keeping the system functional.

Signed-off-by: Eero Nurkkala <eero.nurkkala@offcode.fi>

arch/risc-v/src/mpfs/mpfs_usb.c

@@ -81,6 +81,7 @@
 #define MPFS_TRACEERR_EP0SETUPOUTSIZE      0x0015
 #define MPFS_TRACEERR_EPOUTQEMPTY          0x0016
 #define MPFS_TRACEERR_EP0PREMATURETERM     0x0017
+#define MPFS_TRACEERR_TXHALT               0x0018
 
 /* USB trace interrupt codes */
 
@@ -570,12 +571,12 @@ static void mpfs_req_cancel(struct mpfs_ep_s *privep, int16_t result)
  *   epno       - Endpoint number
  *
  * Returned Value:
- *   None
+ *   OK, or error
  *
  ****************************************************************************/
 
-static void mpfs_write_tx_fifo(const void *in_data, uint32_t length,
-                               uint8_t epno)
+static int mpfs_write_tx_fifo(const void *in_data, uint32_t length,
+                              uint8_t epno)
 {
   uint32_t i;
   uint32_t *temp;
@@ -584,6 +585,7 @@ static void mpfs_write_tx_fifo(const void *in_data, uint32_t length,
   uint16_t words = length / 4;
   uint16_t bytes = length - words * 4;
   uint16_t offset;
+  uint16_t retries = 10000;
 
   temp      = (uint32_t *)in_data;
   temp_8bit = (uint8_t *)in_data;
@@ -597,7 +599,13 @@ static void mpfs_write_tx_fifo(const void *in_data, uint32_t length,
           tx_csr = getreg16(MPFS_USB_ENDPOINT(epno) +
                             MPFS_USB_ENDPOINT_TX_CSR_OFFSET);
         }
-      while (tx_csr & TXCSRL_REG_EPN_TX_FIFO_NE_MASK);
+      while ((tx_csr & TXCSRL_REG_EPN_TX_FIFO_NE_MASK) && --retries);
+    }
+
+  if (retries == 0)
+    {
+      usbtrace(TRACE_DEVERROR(MPFS_TRACEERR_TXHALT), epno);
+      return -EIO;
     }
 
   /* Send 32-bit words first */
@@ -615,6 +623,8 @@ static void mpfs_write_tx_fifo(const void *in_data, uint32_t length,
     {
       mpfs_putreg8((uint8_t)temp_8bit[i], MPFS_USB_FIFO(epno));
     }
+
+  return OK;
 }
 
 /****************************************************************************
@@ -629,18 +639,19 @@ static void mpfs_write_tx_fifo(const void *in_data, uint32_t length,
  *   privreq    - The actual write request
  *
  * Returned Value:
- *   None
+ *   OK if success, error otherwise
  *
  ****************************************************************************/
 
-static void mpfs_req_wrsetup(struct mpfs_usbdev_s *priv,
-                             struct mpfs_ep_s *privep,
-                             struct mpfs_req_s *privreq)
+static int mpfs_req_wrsetup(struct mpfs_usbdev_s *priv,
+                            struct mpfs_ep_s *privep,
+                            struct mpfs_req_s *privreq)
 {
   const uint8_t *buf;
   uint32_t packetsize;
   uint8_t epno;
   int nbytes;
+  int ret;
 
   epno = USB_EPNO(privep->ep.eplog);
 
@@ -693,7 +704,11 @@ static void mpfs_req_wrsetup(struct mpfs_usbdev_s *priv,
 
   if (nbytes > packetsize)
     {
-      mpfs_write_tx_fifo(buf, packetsize, epno);
+      ret = mpfs_write_tx_fifo(buf, packetsize, epno);
+      if (ret != OK)
+        {
+          return ret;
+        }
 
       if (epno == EP0)
         {
@@ -709,11 +724,15 @@ static void mpfs_req_wrsetup(struct mpfs_usbdev_s *priv,
         }
 
       privreq->inflight = packetsize;
-      return;
+      return OK;
     }
   else
     {
-      mpfs_write_tx_fifo(buf, nbytes, epno);
+      ret = mpfs_write_tx_fifo(buf, nbytes, epno);
+      if (ret != OK)
+        {
+          return ret;
+        }
     }
 
   privreq->req.xfrd += nbytes;
@@ -734,6 +753,8 @@ static void mpfs_req_wrsetup(struct mpfs_usbdev_s *priv,
                        TXCSRL_REG_EPN_UNDERRUN_MASK,
                        TXCSRL_REG_EPN_TX_PKT_RDY_MASK);
     }
+
+  return OK;
 }
 
 /****************************************************************************
@@ -835,6 +856,7 @@ static int mpfs_req_write(struct mpfs_usbdev_s *priv,
   struct mpfs_req_s *privreq;
   uint8_t epno;
   int bytesleft;
+  int ret;
 
   epno = USB_EPNO(privep->ep.eplog);
 
@@ -881,7 +903,11 @@ static int mpfs_req_write(struct mpfs_usbdev_s *priv,
         {
           /* Perform the write operation. */
 
-          mpfs_req_wrsetup(priv, privep, privreq);
+          ret = mpfs_req_wrsetup(priv, privep, privreq);
+          if (ret != OK)
+            {
+              return ret;
+            }
         }
       else if ((privreq->req.len == 0) && !privep->zlpsent)
         {