Update Risc-V port to use environment call in place of software interrupt - still very much a work in progress.

FreeRTOS/Source/portable/GCC/RISC-V-RV32/port.c

@@ -94,6 +94,12 @@ StackType_t *pxPortInitialiseStack( StackType_t *pxTopOfStack, TaskFunction_t px
 		x28-31 		t3-6 			Temporaries 					Caller
 	*/
 
+	/* To ensure alignment. */
+	//_RB_	pxTopOfStack--;
+	//_RB_	pxTopOfStack--;
+	//_RB_pxTopOfStack--;
+
+	/* Numbers correspond to the x register number. */
 	pxTopOfStack--;
 	*pxTopOfStack = ( StackType_t ) 31;
 	pxTopOfStack--;
@@ -185,7 +191,7 @@ volatile uint32_t * const pulTimeLow = ( volatile uint32_t * const ) ( configCTR
 	ullNextTime += ullTimerIncrementsForOneTick;
 
 	/* Enable timer interrupt */
-	__asm volatile( "csrs mie, %0" :: "r"(0x80) );
+	__asm volatile( "csrs mie, %0" :: "r"(0x80) ); /* 1<<7 for timer interrupt. */
 }
 /*-----------------------------------------------------------*/
 
@@ -196,19 +202,21 @@ volatile uint32_t * const ulSoftInterrupt = ( uint32_t * ) configCTRL_BASE;
 	vTaskSwitchContext();
 
 	/* Clear software interrupt. */
-	*( ( uint32_t * ) configCTRL_BASE ) = 0UL;
+	*( ( uint32_t * ) configCTRL_BASE ) &= 0x08UL;
 }
 /*-----------------------------------------------------------*/
 
 void Timer_IRQHandler( void )
 {
+extern void vTaskSwitchContext( void );
+
 	/* Reload for the next timer interrupt. */
 	*pullMachineTimerCompareRegister = ullNextTime;
 	ullNextTime += ullTimerIncrementsForOneTick;
 
 	if( xTaskIncrementTick() != pdFALSE )
 	{
-		portYIELD();
+		vTaskSwitchContext();
 	}
 }
 /*-----------------------------------------------------------*/
@@ -217,6 +225,17 @@ BaseType_t xPortStartScheduler( void )
 {
 extern void xPortStartFirstTask( void );
 
+	#if( configASSERT_DEFINED == 1 )
+	{
+		volatile uint32_t mtvec = 0;
+
+		/* Check the least significant two bits of mtvec are 00 - indicating single
+		vector mode. */
+		__asm volatile( "csrr %0, mtvec" : "=r"( mtvec ) );
+		configASSERT( ( mtvec & 0x03UL ) == 0 );
+	}
+	#endif
+
 	vPortSetupTimerInterrupt();
 	xPortStartFirstTask();
 

FreeRTOS/Source/portable/GCC/RISC-V-RV32/portASM.S

@@ -82,14 +82,14 @@ xPortStartFirstTask:
     addi    sp, sp, CONTEXT_SIZE
     csrs    mstatus, 8              /* Enable machine interrupts. */
     csrs    mie, 8                  /* Enable soft interrupt. */
-     ret
+    ret
 
 /*-----------------------------------------------------------*/
 
 .align 8
 vPortTrapHandler:
 	addi sp, sp, -CONTEXT_SIZE
-	sw x1, 1( sp )
+	sw x1, 1 * WORD_SIZE( sp )
 	sw x5, 2 * WORD_SIZE( sp )
 	sw x6, 3 * WORD_SIZE( sp )
 	sw x7, 4 * WORD_SIZE( sp )
@@ -118,10 +118,6 @@ vPortTrapHandler:
 	sw x30, 27 * WORD_SIZE( sp )
 	sw x31, 28 * WORD_SIZE( sp )
 
-    /* Save exception return address. */
-    csrr t0, mepc
-    sw t0, 0( sp )
-
     lw  t0, pxCurrentTCB            /* Load pxCurrentTCB. */
     sw  sp, 0( t0 )                 /* Write sp from first TCB member. */
 
@@ -130,6 +126,9 @@ vPortTrapHandler:
 	mv a2, sp
 	jal handle_trap
 	csrw mepc, a0
+	/* Save exception return address. */
+	sw a0, 0( sp )
+
 
 	# Remain in M-mode after mret
 	li t0, 0x00001800 /* MSTATUS MPP */
@@ -138,11 +137,11 @@ vPortTrapHandler:
     lw  sp, pxCurrentTCB            /* Load pxCurrentTCB. */
     lw  sp, 0( sp )                 /* Read sp from first TCB member. */
 
-    /* Load mret with the address of the first task. */
+    /* Load mret with the address of the next task. */
     lw t0, 0( sp )
     csrw mepc, t0
 
-    lw  x1, 1( sp )
+    lw  x1, 1 * WORD_SIZE( sp )
     lw  x5, 2 * WORD_SIZE( sp )     /* t0 */
     lw  x6, 3 * WORD_SIZE( sp )     /* t1 */
     lw  x7, 4 * WORD_SIZE( sp )     /* t2 */

FreeRTOS/Source/portable/GCC/RISC-V-RV32/portmacro.h

@@ -70,7 +70,7 @@ not need to be guarded with a critical section. */
 
 
 /* Scheduler utilities. */
-#define portYIELD() *( ( uint32_t * ) configCTRL_BASE ) = 1UL
+#define portYIELD() __asm volatile( "ecall" ); // software interrupt alternative *( ( uint32_t * ) configCTRL_BASE ) |= 0x08UL
 #define portEND_SWITCHING_ISR( xSwitchRequired ) if( xSwitchRequired ) vPortYield()
 #define portYIELD_FROM_ISR( x ) portEND_SWITCHING_ISR( x )
 /*-----------------------------------------------------------*/