Stop random crashes while writing to flash (#730)

FreeRTOS SMP was updated to:
a) Move ths SYSTICK handler, which cannot be disabled and can fire
   even with IRQs disabled, to RAM
b) Add a flag from the core to the SYSTICK handler to hold off on
   any PendSV (task switch) calls while we are doing the idleOtherCore.

The core now sets this flag, _holdPendSV, and adds add'l FreeRTOS SMP
calls to really, really tell the OS we can't, don't, and better not
be swapped out while writing to flash.

Fixes #719

cores/rp2040/RP2040Support.h

@@ -32,6 +32,18 @@
 
 extern "C" volatile bool __otherCoreIdled;
 
+// Halt the FreeRTOS PendSV task switching magic
+extern "C" int __holdUpPendSV;
+
+// FreeRTOS weak functions, to be overridden when we really are running FreeRTOS
+extern "C" {
+    extern void vTaskSuspendAll() __attribute__((weak));
+    extern int32_t xTaskResumeAll() __attribute__((weak));
+    typedef struct tskTaskControlBlock * TaskHandle_t;
+    extern void vTaskPreemptionDisable(TaskHandle_t p) __attribute__((weak));
+    extern void vTaskPreemptionEnable(TaskHandle_t p) __attribute__((weak));
+}
+
 class _MFIFO {
 public:
     _MFIFO() { /* noop */ };
@@ -86,6 +98,11 @@ public:
         if (!_multicore) {
             return;
         }
+        __holdUpPendSV = 1;
+        if (__isFreeRTOS) {
+            vTaskPreemptionDisable(nullptr);
+            vTaskSuspendAll();
+        }
         mutex_enter_blocking(&_idleMutex);
         __otherCoreIdled = false;
         multicore_fifo_push_blocking(_GOTOSLEEP);
@@ -98,6 +115,12 @@ public:
         }
         mutex_exit(&_idleMutex);
         __otherCoreIdled = false;
+        if (__isFreeRTOS) {
+            xTaskResumeAll();
+            vTaskPreemptionEnable(nullptr);
+        }
+        __holdUpPendSV = 0;
+
         // Other core will exit busy-loop and return to operation
         // once __otherCoreIdled == false.
     }

cores/rp2040/main.cpp

@@ -28,6 +28,7 @@
 RP2040 rp2040;
 extern "C" {
     volatile bool __otherCoreIdled = false;
+    int __holdUpPendSV = 0;
 };
 
 mutex_t _pioMutex;

FreeRTOS-Kernel @ 0b55ee70

-Subproject commit f5fe79dacd723b490ae9afe1b4f5ae45aae620c6
+Subproject commit 0b55ee70ad08ab10af77f49e48a7381ac81a6827

libraries/FreeRTOS/src/FreeRTOSConfig.h

@@ -26,6 +26,7 @@
 #define configSUPPORT_DYNAMIC_ALLOCATION 1
 #define configSUPPORT_STATIC_ALLOCATION 0
 #define configSTACK_DEPTH_TYPE			uint32_t
+#define configUSE_TASK_PREEMPTION_DISABLE 1
 
 #define configUSE_NEWLIB_REENTRANT 1
 #define configNEWLIB_REENTRANT_IS_DYNAMIC 0 /* Note that we have a different config option, portSET_IMPURE_PTR */

libraries/FreeRTOS/src/variantHooks.cpp

@@ -29,7 +29,6 @@
 #include <Arduino.h>
 #include <RP2040USB.h>
 #include "tusb.h"
-#define USB_TASK_IRQ 31
 
 
 /* Raspberry PI Pico includes */
@@ -382,19 +381,11 @@ void vApplicationAssertHook() {
 #endif
 
 
-static void __usb_irq() {
-    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
-    vTaskNotifyGiveFromISR( __usbTask, &xHigherPriorityTaskWoken );
-    portYIELD_FROM_ISR( xHigherPriorityTaskWoken );
-}
-
 static void __usb(void *param)
 {
     (void) param;
 
     tusb_init();
-    irq_set_exclusive_handler(USB_TASK_IRQ, __usb_irq);
-    irq_set_enabled(USB_TASK_IRQ, true);
 
     Serial.begin(115200);