Fix bug in prussd.py that causes kernel oops while sending message th…

…rough rpmsg channel

A kernel oops (NULL pointer dereference) was thrown when the pruss_api driver hadn't already been probed because there was no check that the rpmsg device file in /dev was being created before sending messages to the PRU.
Sometimes the oops wasn't thrown but there was a loss of some initial data sent to PRU before the rpmsg device file gets initialized.
Now, the code waits before sending messages to PRU until the appropriate device file is created to ensure no loss of data. No kernel oops is thrown now.
The device file creation is timed out after 3 seconds otherwise the function might get stuck in an infinite loop if rpmsg isn't being used and function gets called.

prussd/prussd.py

@@ -20,6 +20,7 @@
 import threading
 import mmap     # For memory functions
 import struct   # For memory functions 
+import time
 
 PRU_ICSS     = 0x4a300000     # This is the address of the PRU Subsystem on the RAM
 PRU_ICSS_SIZE = 512*1024      # This is the length of the PRU subsystem ie it is of 512 K
@@ -168,8 +169,15 @@ def send_msg(cmd):
     rpmsg_dev = rpmsg_devnode(chan_name, chan_port)
     if chan_name not in paths.RPMSG_CHANNELS:
         return -errno.EPERM
-    if not os.path.exists(rpmsg_dev):
-        return -errno.ENODEV
+
+    time_to_wait = 3
+    time_counter = 0
+
+    while not os.path.exists(rpmsg_dev):
+        time.sleep(1)
+        time_counter += 1
+        if (time_counter >= time_to_wait):
+            return -errno.ENODEV
 
     if mode == 's':
         try: