Outputs an error message within send_recv, giving details of the fail…

…ure.

src/usb.c

@@ -33,32 +33,47 @@ ssize_t send_recv(struct stlink_libusb* handle, int terminate,
         unsigned char* rxbuf, size_t rxsize) {
     /* note: txbuf and rxbuf can point to the same area */
     int res = 0;
+    int t;
 
-    if (libusb_bulk_transfer(handle->usb_handle, handle->ep_req,
+    t = libusb_bulk_transfer(handle->usb_handle, handle->ep_req,
             txbuf,
             txsize,
             &res,
-            3000))
+            3000);
+    if (t) {
+        printf("[!] send_recv send request failed: %s\n", libusb_error_name(t));
         return -1;
+    } else if ((size_t)res != txsize) {
+        printf("[!] send_recv send request wrote %d bytes (instead of %d).\n",
+                res, txsize);
+    }
 
     if (rxsize != 0) {
-        if (libusb_bulk_transfer(handle->usb_handle, handle->ep_rep,
+        t = libusb_bulk_transfer(handle->usb_handle, handle->ep_rep,
                 rxbuf,
                 rxsize,
                 &res,
-                3000))
-            return -1;
+                3000);
+        if (t) {
+                printf("[!] send_recv read reply failed: %s\n",
+                        libusb_error_name(t));
+                return -1;
+        }
     }
 
     if ((handle->protocoll == 1) && terminate) {
         /* Read the SG reply */
         unsigned char sg_buf[13];
-        if (libusb_bulk_transfer(handle->usb_handle, handle->ep_rep,
+        t = libusb_bulk_transfer(handle->usb_handle, handle->ep_rep,
                 sg_buf,
                 13,
                 &res,
-                3000))
+                3000);
+        if (t) {
+            printf("[!] send_recv read storage failed: %s\n",
+                    libusb_error_name(t));
             return -1;
+        }
         /* The STLink doesn't seem to evaluate the sequence number */
         handle->sg_transfer_idx++;
     }