[stm32-lvgl] Fix an interrupt safety bug that caused occasional crashes

stm32-lvgl/Sources/Application/Interrupts.swift

@@ -69,13 +69,23 @@ func SystickTimerISR() {
   uptimeInMs += 1
 }
 
+// The following interrupt set up is trickier that it looks on the surface. The
+// ISR Swift code must be "trivial" directly and transitively, and namely it
+// must avoid destroying any heap objects (because we could be inside malloc
+// when the interrupt hits). For that, the expectation is that
+// lcdInterruptVerticalSyncHandler is only ever set once, and it not changing
+// after boot. The code inside the lcdInterruptVerticalSyncHandler closure is
+// expected to only perform trivial operations. lcdInterruptVerticalSyncEnabled
+// is allowed to change.
+
 var lcdInterruptVerticalSyncHandler: (() -> Void)? = nil
+var lcdInterruptVerticalSyncEnabled: Bool = false
 
 @_cdecl("LtdcIntHandlerISR")
 func LtdcIntHandlerISR() {
   let sr = ltdc.isr.read()
   ltdc.icr.write { $0.storage = sr.storage }
   if sr.raw.rrif != 0 {
-    lcdInterruptVerticalSyncHandler?()
+    if lcdInterruptVerticalSyncEnabled { lcdInterruptVerticalSyncHandler?() }
   }
 }

stm32-lvgl/Sources/Application/Main.swift

@@ -120,14 +120,16 @@ struct Main {
         // interrupt handler.   Once we get our VBI, we can tell
         // LVGL that we're good to go to switch again.
         Lcd.setFrameBuffer(bufferToShow!)
-        lcdInterruptVerticalSyncHandler = {
-          lv_display_flush_ready(disp)
-          lcdInterruptVerticalSyncHandler = nil
-        }
+        lcdInterruptVerticalSyncEnabled = true
         Lcd.reloadConfiguration()
         // the lv_display_flush_ready() will happen in the LCD frame interrupt.
       })
 
+    lcdInterruptVerticalSyncHandler = {
+      lv_display_flush_ready(disp)
+      lcdInterruptVerticalSyncEnabled = false
+    }
+
     let touch = lv_indev_create()
     lv_indev_set_type(touch, LV_INDEV_TYPE_POINTER)
     lv_indev_set_read_cb(
@@ -152,7 +154,7 @@ struct Main {
 
     while true {
       // If we're pending a render, wait.
-      while lcdInterruptVerticalSyncHandler != nil { /* busy wait */ nop() }
+      while lcdInterruptVerticalSyncEnabled { /* busy wait */ nop() }
 
       lv_timer_handler()
 

stm32-lvgl/toolset.json

@@ -38,7 +38,8 @@
       "-llvgl", "-llvgl_demos",
       "-static",
       "-e", "_start_elf",
-      "--orphan-handling=error"
+      "--orphan-handling=error",
+      "-Map", ".build/armv7em-none-none-eabi/release/Application.map"
     ]
   }
 }