move rv32i port code under gcc

arch/risc-v/rv32i/gcc/port_c.c

@@ -0,0 +1,49 @@
+/*
+ * port_c.c
+ *
+ *  Created on: Sep 9, 2019
+ *      Author: ace
+ */
+#include "riscv_encoding.h"
+#include <tos.h>
+
+#define CLINT_CTRL_ADDR 0x2000000
+#define CLINT_MSIP      0x0000
+#define CLINT_MTIMECMP  0x4000
+#define CLINT_MTIME     0xBFF8
+
+__PORT__ void port_systick_config(uint32_t cycle_per_tick)
+{
+
+    // it cost cpu read two times, first mtime_lo and then mtime_hi
+    // if mtime_lo == 0xFFFFFFFF and mtime_hi = 0 at first read
+    // then mtime_lo == 0 and mtime_hi = 1 at next read
+    // the result will be  0x1FFFFFFFF, not 0x100000000
+    uint64_t mtime = 0;
+    while(1) {
+        uint32_t mtime_hi = *(volatile uint32_t *)(CLINT_CTRL_ADDR + CLINT_MTIME + 4);
+        uint32_t mtime_lo = *(volatile uint32_t *)(CLINT_CTRL_ADDR + CLINT_MTIME + 0);
+        uint32_t mtime_hn = *(volatile uint32_t *)(CLINT_CTRL_ADDR + CLINT_MTIME + 4);
+        if(mtime_hi == mtime_hn) {
+            mtime += ((uint64_t)mtime_hi << 32) | mtime_lo;
+            break;
+        }
+    }
+
+    // just set mtime to mtimecmp does not accurately reflect the passage of time
+    // cause some time cost on the path to deal with the interrupt
+    // so, we need to to fix the value with a multiple of cycle_per_tick
+    uint64_t tick = mtime / cycle_per_tick;
+    uint64_t mtimecmp = (tick + 1) * cycle_per_tick;
+
+
+    // write to mtimecmp register
+    *(volatile uint32_t *)(CLINT_CTRL_ADDR + CLINT_MTIMECMP + 0) = 0xFFFFFFFF;
+    *(volatile uint32_t *)(CLINT_CTRL_ADDR + CLINT_MTIMECMP + 4) = 0xFFFFFFFF & (mtimecmp >> 32);
+    *(volatile uint32_t *)(CLINT_CTRL_ADDR + CLINT_MTIMECMP + 0) = 0xFFFFFFFF & (mtimecmp >>  0);
+}
+
+__PORT__ void port_systick_priority_set(uint32_t prio)
+{
+    //NVIC_SetPriority(SysTick_IRQn, prio);
+}