[v1,7/9] hw/mos6522: Avoid using discrepant QEMU clock values

Commit Message

Finn Thain Sept. 23, 2021, 10:49 p.m. UTC

mos6522_read() and mos6522_write() may call various functions to determine
timer irq state, timer counter value and QEMUTimer deadline. All called
functions must use the same value for the present time.

Signed-off-by: Finn Thain <[e-mail]>
Reviewed-by: Philippe Mathieu-Daudé <[e-mail]>
---
Changed since RFC
 - Moved the changes to QEMUTimer callbacks to the next patch.
 - Fix whitespace.
---
 hw/misc/mos6522.c | 47 +++++++++++++++++++++++------------------------
 1 file changed, 23 insertions(+), 24 deletions(-)

Patch

diff --git a/hw/misc/mos6522.c b/hw/misc/mos6522.c
index bfe1719b18..385ea81b62 100644
--- a/hw/misc/mos6522.c
+++ b/hw/misc/mos6522.c
@@ -39,9 +39,9 @@ 
 /* XXX: implement all timer modes */
 
 static void mos6522_timer1_update(MOS6522State *s, MOS6522Timer *ti,
-                                  int64_t current_time);
+                                  int64_t now);
 static void mos6522_timer2_update(MOS6522State *s, MOS6522Timer *ti,
-                                  int64_t current_time);
+                                  int64_t now);
 
 static void mos6522_update_irq(MOS6522State *s)
 {
@@ -52,13 +52,12 @@  static void mos6522_update_irq(MOS6522State *s)
     }
 }
 
-static unsigned int get_counter(MOS6522State *s, MOS6522Timer *ti)
+static unsigned int get_counter(MOS6522State *s, MOS6522Timer *ti, int64_t now)
 {
     int64_t d;
     unsigned int counter;
 
-    d = muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) - ti->load_time,
-                 ti->frequency, NANOSECONDS_PER_SECOND);
+    d = muldiv64(now - ti->load_time, ti->frequency, NANOSECONDS_PER_SECOND);
 
     if (ti->index == 0) {
         /* the timer goes down from latch to -1 (period of latch + 2) */
@@ -88,7 +87,7 @@  static void set_counter(MOS6522State *s, MOS6522Timer *ti, unsigned int val)
 }
 
 static int64_t get_next_irq_time(MOS6522State *s, MOS6522Timer *ti,
-                                 int64_t current_time)
+                                 int64_t now)
 {
     int64_t d, next_time;
     unsigned int counter;
@@ -98,8 +97,7 @@  static int64_t get_next_irq_time(MOS6522State *s, MOS6522Timer *ti,
     }
 
     /* current counter value */
-    d = muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) - ti->load_time,
-                 ti->frequency, NANOSECONDS_PER_SECOND);
+    d = muldiv64(now - ti->load_time, ti->frequency, NANOSECONDS_PER_SECOND);
 
     /* the timer goes down from latch to -1 (period of latch + 2) */
     if (d <= (ti->counter_value + 1)) {
@@ -122,20 +120,19 @@  static int64_t get_next_irq_time(MOS6522State *s, MOS6522Timer *ti,
     trace_mos6522_get_next_irq_time(ti->latch, d, next_time - d);
     next_time = muldiv64(next_time, NANOSECONDS_PER_SECOND, ti->frequency) +
                          ti->load_time;
-
-    if (next_time <= current_time) {
-        next_time = current_time + 1;
-    }
     return next_time;
 }
 
 static void mos6522_timer1_update(MOS6522State *s, MOS6522Timer *ti,
-                                 int64_t current_time)
+                                  int64_t now)
 {
     if (!ti->timer) {
         return;
     }
-    ti->next_irq_time = get_next_irq_time(s, ti, current_time);
+    ti->next_irq_time = get_next_irq_time(s, ti, now);
+    if (ti->next_irq_time <= now) {
+        ti->next_irq_time = now + 1;
+    }
     if ((s->ier & T1_INT) == 0 || (s->acr & T1MODE) != T1MODE_CONT) {
         timer_del(ti->timer);
     } else {
@@ -144,12 +141,15 @@  static void mos6522_timer1_update(MOS6522State *s, MOS6522Timer *ti,
 }
 
 static void mos6522_timer2_update(MOS6522State *s, MOS6522Timer *ti,
-                                 int64_t current_time)
+                                  int64_t now)
 {
     if (!ti->timer) {
         return;
     }
-    ti->next_irq_time = get_next_irq_time(s, ti, current_time);
+    ti->next_irq_time = get_next_irq_time(s, ti, now);
+    if (ti->next_irq_time <= now) {
+        ti->next_irq_time = now + 1;
+    }
     if ((s->ier & T2_INT) == 0) {
         timer_del(ti->timer);
     } else {
@@ -227,12 +227,12 @@  uint64_t mos6522_read(void *opaque, hwaddr addr, unsigned size)
         val = s->dira;
         break;
     case VIA_REG_T1CL:
-        val = get_counter(s, &s->timers[0]) & 0xff;
+        val = get_counter(s, &s->timers[0], now) & 0xff;
         s->ifr &= ~T1_INT;
         mos6522_update_irq(s);
         break;
     case VIA_REG_T1CH:
-        val = get_counter(s, &s->timers[0]) >> 8;
+        val = get_counter(s, &s->timers[0], now) >> 8;
         break;
     case VIA_REG_T1LL:
         val = s->timers[0].latch & 0xff;
@@ -241,12 +241,12 @@  uint64_t mos6522_read(void *opaque, hwaddr addr, unsigned size)
         val = (s->timers[0].latch >> 8) & 0xff;
         break;
     case VIA_REG_T2CL:
-        val = get_counter(s, &s->timers[1]) & 0xff;
+        val = get_counter(s, &s->timers[1], now) & 0xff;
         s->ifr &= ~T2_INT;
         mos6522_update_irq(s);
         break;
     case VIA_REG_T2CH:
-        val = get_counter(s, &s->timers[1]) >> 8;
+        val = get_counter(s, &s->timers[1], now) >> 8;
         break;
     case VIA_REG_SR:
         val = s->sr;
@@ -356,10 +356,9 @@  void mos6522_write(void *opaque, hwaddr addr, uint64_t val, unsigned size)
         }
         mos6522_update_irq(s);
         /* if IER is modified starts needed timers */
-        mos6522_timer1_update(s, &s->timers[0],
-                              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL));
-        mos6522_timer2_update(s, &s->timers[1],
-                              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL));
+        now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
+        mos6522_timer1_update(s, &s->timers[0], now);
+        mos6522_timer2_update(s, &s->timers[1], now);
         break;
     default:
         g_assert_not_reached();