aspeed/timer: Fix behaviour running Linux

The Linux kernel driver was updated in commit 4451d3f59f2a
("clocksource/drivers/fttmr010: Fix set_next_event handler) to fix an
issue observed on hardware:

 > RELOAD register is loaded into COUNT register when the aspeed timer
 > is enabled, which means the next event may be delayed because timer
 > interrupt won't be generated until <0xFFFFFFFF - current_count +
 > cycles>.

When running under Qemu, the system appeared "laggy". The guest is now
scheduling timer events too regularly, starving the host of CPU time.

This patch modifies the timer model to attempt to schedule the timer
expiry as the guest requests, but if we have missed the deadline we
re interrupt and try again, which allows the guest to catch up.

Provides expected behaviour with old and new guest code.

Fixes: c04bd47db6b9 ("hw/timer: Add ASPEED timer device model")
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20190618165311.27066-8-clg@kaod.org
[clg: - merged a fix from Andrew Jeffery <andrew@aj.id.au>
        "Fire interrupt on failure to meet deadline"
        https://lists.ozlabs.org/pipermail/openbmc/2019-January/014641.html
      - adapted commit log
      - checkpatch fixes ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/timer/aspeed_timer.c

@@ -109,37 +109,40 @@ static inline uint64_t calculate_time(struct AspeedTimer *t, uint32_t ticks)
 
 static uint64_t calculate_next(struct AspeedTimer *t)
 {
-    uint64_t next = 0;
-    uint32_t rate = calculate_rate(t);
+    uint64_t now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
+    uint64_t next;
 
-    while (!next) {
-        /* We don't know the relationship between the values in the match
-         * registers, so sort using MAX/MIN/zero. We sort in that order as the
-         * timer counts down to zero. */
-        uint64_t seq[] = {
-            calculate_time(t, MAX(t->match[0], t->match[1])),
-            calculate_time(t, MIN(t->match[0], t->match[1])),
-            calculate_time(t, 0),
-        };
-        uint64_t reload_ns;
-        uint64_t now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
-
-        if (now < seq[0]) {
-            next = seq[0];
-        } else if (now < seq[1]) {
-            next = seq[1];
-        } else if (now < seq[2]) {
-            next = seq[2];
-        } else if (t->reload) {
-            reload_ns = muldiv64(t->reload, NANOSECONDS_PER_SECOND, rate);
-            t->start = now - ((now - t->start) % reload_ns);
-        } else {
-            /* no reload value, return 0 */
-            break;
-        }
+    /*
+     * We don't know the relationship between the values in the match
+     * registers, so sort using MAX/MIN/zero. We sort in that order as
+     * the timer counts down to zero.
+     */
+
+    next = calculate_time(t, MAX(t->match[0], t->match[1]));
+    if (now < next) {
+        return next;
+    }
+
+    next = calculate_time(t, MIN(t->match[0], t->match[1]));
+    if (now < next) {
+        return next;
+    }
+
+    next = calculate_time(t, 0);
+    if (now < next) {
+        return next;
+    }
+
+    /* We've missed all deadlines, fire interrupt and try again */
+    timer_del(&t->timer);
+
+    if (timer_overflow_interrupt(t)) {
+        t->level = !t->level;
+        qemu_set_irq(t->irq, t->level);
     }
 
-    return next;
+    t->start = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
+    return calculate_time(t, MAX(MAX(t->match[0], t->match[1]), 0));
 }
 
 static void aspeed_timer_mod(AspeedTimer *t)