1 files changed, 21 insertions, 10 deletions
diff --git a/ports/risc-v64/gnu/example_build/qemu_virt/hwtimer.c b/ports/risc-v64/gnu/example_build/qemu_virt/hwtimer.c
index 122e71a1..5cd21f1b 100644
--- a/ports/risc-v64/gnu/example_build/qemu_virt/hwtimer.c
+++ b/ports/risc-v64/gnu/example_build/qemu_virt/hwtimer.c
@@ -9,7 +9,7 @@
 /* SPDX-License-Identifier: MIT                                            */
 /***************************************************************************/
 
-#include "tx_port.h"
+#include "tx_api.h"
 #include "csr.h"
 #include "hwtimer.h"
 
@@ -17,20 +17,31 @@
 #define CLINT_TIME             (CLINT+0xBFF8)
 #define CLINT_TIMECMP(hart_id) (CLINT+0x4000+8*(hart_id))
 
+/* RV64: naturally aligned 64-bit MMIO accesses are single loads/stores;
+   volatile keeps the compiler from caching or reordering them.  */
+#define MTIME               (*(volatile uint64_t *)CLINT_TIME)
+#define MTIMECMP(hart)      (*(volatile uint64_t *)CLINT_TIMECMP(hart))
 
 int hwtimer_init(void)
 {
-	int hart = riscv_get_core();
-	uint64_t time = *((uint64_t*)CLINT_TIME);
-    	*((uint64_t*)CLINT_TIMECMP(hart)) = time + TICKNUM_PER_TIMER;
-	return 0;
+    int hart = riscv_get_core();
+
+    MTIMECMP(hart) = MTIME + TICKNUM_PER_TIMER;
+    return 0;
 }
 
 int hwtimer_handler(void)
 {
-	int hart = riscv_get_core();
-        uint64_t time = *((uint64_t*)CLINT_TIME);
-        *((uint64_t*)CLINT_TIMECMP(hart)) = time + TICKNUM_PER_TIMER;
-	return 0;
-}
+    int hart = riscv_get_core();
+
+    /* Advance from the previous compare value, so trap latency 
+    does not accumulate as tick drift.  */
+    uint64_t next = MTIMECMP(hart) + TICKNUM_PER_TIMER;
+    uint64_t now = MTIME;
 
+    if (next <= now)
+        next = now + TICKNUM_PER_TIMER;
+
+    MTIMECMP(hart) = next;
+    return 0;
+}