os: use trap exception to make task context switch request in FreeRTOS port for SecureShield

TRAP exception is used by FreeRTOS kernel port, so applicaiton should not use trap exception.

Author: wayne ren

os/freertos/portable/Synopsys/ARC_EM_SECURESHIELD/arc_freertos_exceptions.c

@@ -79,14 +79,22 @@
 #ifdef __GNU__
 extern void gnu_printf_setup(void);
 #endif
+
+extern unsigned int context_switch_reqflg;
+
+static void trap_task_switch_handler(void)
+{
+    context_switch_reqflg = 1; // require a context switch
+}
+
 /**
  * \brief  freertos related cpu exception initialization, all the interrupts handled by freertos must be not
  * fast irqs. If fiq is needed, please install the default firq_exc_entry or your own fast irq entry into
  * the specific interrupt exception.
  */
 void freertos_exc_init(void)
 {
-
+    exc_handler_install(EXC_NO_TRAP, (EXC_HANDLER)trap_task_switch_handler);
 #ifdef __GNU__
 	gnu_printf_setup();
 #endif

os/freertos/portable/Synopsys/ARC_EM_SECURESHIELD/arc_support.s

@@ -82,27 +82,6 @@
 #include "arc.h"
 #include "arc_asm_common.h"
 
-/*
- *  task dispatcher
- *
- */
-	.text
-	.align 4
-	.global dispatch
-dispatch:
-/*
- *  the pre-conditions of this routine are task context, CPU is
- *  locked, dispatch is enabled.
- */	
-	sr 	blink, [AUX_ERRET]
-	lr 	blink, [AUX_STATUS32]
-	sr 	blink, [AUX_ERSTATUS]
-	EXCEPTION_PROLOGUE
-	SAVE_CALLEE_REGS
-	ld	r0, [pxCurrentTCB]
-	b	dispatcher
-
-
 /*
  *  start dispatch
  */

os/freertos/portable/Synopsys/ARC_EM_SECURESHIELD/port.c

@@ -286,11 +286,7 @@ void vPortYieldFromIsr(void)
 /* ----------------------------------------------------------------------------*/
 void vPortYield(void)
 {
-	unsigned int status32;
-
-	status32 = cpu_lock_save();
-	dispatch();
-	cpu_unlock_restore(status32);
+	Asm("trap_s 1");
 }
 
 /* --------------------------------------------------------------------------*/