Make LWIP task priority configurable, document (#3100)

It is a good idea to leave the highes priority task slots for user
tasks with real-time needs, so move LWIP to MAX-2 and add a
configuration define to change it if needed.

Update docs with info on setting.

Author: earlephilhower

cores/rp2040/freertos/freertos-lwip.cpp

@@ -35,6 +35,10 @@ typedef struct {
 
 #define LWIP_WORK_ENTRIES 16
 
+#ifndef LWIP_TASK_PRIORITY
+#define LWIP_TASK_PRIORITY (configMAX_PRIORITIES - 2)
+#endif
+
 // The notify item we'll use to wake the calling process back up
 #define TASK_NOTIFY_LWIP_WAKEUP (configTASK_NOTIFICATION_ARRAY_ENTRIES - 1)
 
@@ -47,7 +51,7 @@ void __startLWIPThread() {
     if (!__lwipQueue) {
         panic("Unable to allocate LWIP work queue");
     }
-    if (pdPASS != xTaskCreate(lwipThread, "LWIP", 1024, 0, configMAX_PRIORITIES - 1, &__lwipTask)) {
+    if (pdPASS != xTaskCreate(lwipThread, "LWIP", 1024, 0, LWIP_TASK_PRIORITY, &__lwipTask)) {
         panic("Unable to create LWIP task");
     }
     vTaskCoreAffinitySet(__lwipTask, 1 << 0);

docs/freertos.rst

@@ -19,9 +19,11 @@ to your sketch and select ``Tools->Operating System->FreeRTOS SMP`` to enable it
 
 When using Platform.IO you need to add the following define to your .ini file:
 
-.. code::
+.. code:: ini
+
+    ; Enable FreeRTOS Support
+    build_flags = -DPIO_FRAMEWORK_ARDUINO_ENABLE_FREERTOS
 
-    -D__FREERTOS=1
 
 Configuration and Predefined Tasks
 ----------------------------------
@@ -33,6 +35,12 @@ quantum is 1 millisecond (i.e. 1,000 switches per second).
 ``setup()`` and ``loop()`` are assigned to only run on core 0, while ``setup1()`` and ``loop1()``
 only run in core 1 in this mode, the same as the default multithreading mode.
 
+There is an LWIP worker thread running on core 0 at priority ``(configMAX_PRIORITIES - 2)``
+This should allow for LWIP to always make progress even with applications that don't
+``yield`` or ``delay``, while leaving the highest priority available for hard
+real time processes.  This setting can be changed by defining ``LWIP_TASK_PRIORITY``
+in your build process.
+
 You can launch and manage additional processes using the standard FreeRTOS routines.
 
 ``delay()`` and ``yield()`` free the CPU for other tasks, while ``delayMicroseconds()`` does not.