net: ppp_l2: Make prio of PPP TX thread configurable

The PPP TX thread handles the transmission of packets at PPP layer.
Make it's priority configurable, so it's priority can be configured higher
then higher protocol layers.

Signed-off-by: Sjors Hettinga <[email protected]>

Author: ssharks

subsys/net/l2/ppp/Kconfig

@@ -92,4 +92,20 @@ config NET_L2_PPP_TX_STACK_SIZE
 	help
 	  Set the TX handler stack size.
 
+config NET_L2_PPP_THREAD_PRIO
+	int "Priority of the PPP TX thread"
+	default 1
+	help
+	  Set the priority of the the PPP TX thread, that handles all
+	  transmission of PPP packets.
+	  Value 0 = highest priortity.
+	  When CONFIG_NET_TC_THREAD_COOPERATIVE = y, lowest priority is
+	  CONFIG_NUM_COOP_PRIORITIES-1 else lowest priority is
+	  CONFIG_NUM_PREEMPT_PRIORITIES-1.
+	  When using PPP in combination with TCP, make sure the priority
+	  is higher (or equal) than the TCP work queue, otherwise the
+	  TCP stack will consume all net_bufs before transferring
+	  execution to the lower layer network stack, with a high risk of
+	  running out of net_bufs.
+
 endif # NET_L2_PPP

subsys/net/l2/ppp/ppp_l2.c

@@ -26,9 +26,9 @@ static K_FIFO_DEFINE(tx_queue);
 
 #if defined(CONFIG_NET_TC_THREAD_COOPERATIVE)
 /* Lowest priority cooperative thread */
-#define THREAD_PRIORITY K_PRIO_COOP(CONFIG_NUM_COOP_PRIORITIES - 1)
+#define THREAD_PRIORITY K_PRIO_COOP(CONFIG_NET_L2_PPP_THREAD_PRIO)
 #else
-#define THREAD_PRIORITY K_PRIO_PREEMPT(CONFIG_NUM_PREEMPT_PRIORITIES - 1)
+#define THREAD_PRIORITY K_PRIO_PREEMPT(CONFIG_NET_L2_PPP_THREAD_PRIO)
 #endif
 
 static void tx_handler(void);