Increase PPP stack size from 512 bytes to 768 bytes.

PPP is running close to the edge of its default thread stack size of 512 bytes. When it experiences an FCS error on the incoming data (for example. caused by character loss when the incoming serial rate is too high for it to process in time) it performs some additional work which overruns the thread's stack, hitting the OS "stack underflow" check. Increasing the PPP stack size to 768 bytes resolves this problem.

Author: RobMeades

features/FEATURE_LWIP/lwip-interface/lwipopts.h

@@ -108,7 +108,7 @@
 
 // Thread stack size for private PPP thread
 #ifndef MBED_CONF_LWIP_PPP_THREAD_STACKSIZE
-#define MBED_CONF_LWIP_PPP_THREAD_STACKSIZE    512
+#define MBED_CONF_LWIP_PPP_THREAD_STACKSIZE    768
 #endif
 
 #if LWIP_DEBUG

features/FEATURE_LWIP/lwip-interface/mbed_lib.json

@@ -68,7 +68,7 @@
         },
         "ppp-thread-stacksize": {
             "help": "Thread stack size for PPP",
-            "value": 512
+            "value": 768
         }
     }
 }