From 9ccdd25e9f2937f43447e771a68492305c2bcbb6 Mon Sep 17 00:00:00 2001
From: Damien George <damien.p.george@gmail.com>
Date: Sat, 19 Sep 2015 11:45:45 +0100
Subject: [PATCH] microbit: Allow to run the idle task within the current
 fiber's stack.

If there is only 1 main fiber (plus the idle fiber) then enabling this
new feature for the main fiber (using fiber_allow_run_idle_within())
will mean that no heap is needed to swap the stack out.
---
 inc/MicroBitFiber.h      |  7 +++++++
 source/MicroBitFiber.cpp | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/inc/MicroBitFiber.h b/inc/MicroBitFiber.h
index 8e31f99..b560663 100644
--- a/inc/MicroBitFiber.h
+++ b/inc/MicroBitFiber.h
@@ -24,6 +24,7 @@
 #define MICROBIT_FIBER_FLAG_FOB             0x01 
 #define MICROBIT_FIBER_FLAG_PARENT          0x02 
 #define MICROBIT_FIBER_FLAG_CHILD           0x04 
+#define MICROBIT_FIBER_FLAG_RUN_IDLE_WITHIN 0x08
 
 /**
   *  Thread Context for an ARM Cortex M0 core.
@@ -118,6 +119,12 @@ Fiber *create_fiber(void (*entry_fn)(void), void (*completion_fn)(void) = releas
   */
 Fiber *create_fiber(void (*entry_fn)(void *), void *param, void (*completion_fn)(void *) = release_fiber);
 
+/**
+  * Allow the idle thread to run within the current thread's stack frame.
+  * This is useful to prevent paging of the thread's stack to the heap.
+  */
+void fiber_allow_run_idle_within();
+
 
 
 /**
diff --git a/source/MicroBitFiber.cpp b/source/MicroBitFiber.cpp
index 636f485..080206c 100644
--- a/source/MicroBitFiber.cpp
+++ b/source/MicroBitFiber.cpp
@@ -226,6 +226,15 @@ void scheduler_event(MicroBitEvent evt)
     }
 }
 
+/**
+  * Allow the idle thread to run within the current thread's stack frame.
+  * This is useful to prevent paging of the thread's stack to the heap.
+  */
+void fiber_allow_run_idle_within()
+{
+    currentFiber->flags |= MICROBIT_FIBER_FLAG_RUN_IDLE_WITHIN;
+}
+
 
 /**
   * Blocks the calling thread for the given period of time.
@@ -622,6 +631,29 @@ void schedule()
         // Otherwise, just pick the head of the run queue.
         currentFiber = runQueue;
         
+    if (currentFiber == idle && oldFiber->flags & MICROBIT_FIBER_FLAG_RUN_IDLE_WITHIN)
+    {
+        // Run the idle task right here using the old fiber's stack.
+        // Keep idling while the runqueue is empty, or there is data to process.
+        do
+        {
+            uBit.systemTasks();
+
+            if(scheduler_runqueue_empty())
+            {
+                if (uBit.ble)
+                    uBit.ble->waitForEvent();
+                else
+                    __WFI();
+            }
+        }
+        while (runQueue == NULL || fiber_flags & MICROBIT_FLAG_DATA_READY);
+
+        // Switch to a non-idle fiber.
+        // If this fiber is the same as the old one then there'll be no switching at all.
+        currentFiber = runQueue;
+    }
+
     // Swap to the context of the chosen fiber, and we're done.
     // Don't bother with the overhead of switching if there's only one fiber on the runqueue!
     if (currentFiber != oldFiber)