Improved PIT IRQ handling. Now Windows 98 boots.

Prior to this commit, Windows 98 would crash with a "Protection Error" during boot or triple fault. After sifting through the log files, I found out that it sending too many IRQ0 events, which blocked the OS from receiving IRQ14 from an IDENTIFY command. This commit fixes several bugs in PIT timing handling code, and the new code is based on the ACPI timing functions.
With this bugfix, Windows 98 receives IRQ14 from the IDE controller again, and booting commences as normal.

src/hardware/pit.c

@@ -43,6 +43,8 @@ struct pit_channel {
     itick_t last_load_time, last_irq_time;
     uint32_t period;
 
+    uint32_t pit_last_count;
+
     int timer_flipflop;
 
     int timer_running;
@@ -58,8 +60,7 @@ static struct pit pit;
 static void pit_state(void)
 {
     // <<< BEGIN AUTOGENERATE "state" >>>
-// Auto-generated on Wed Oct 09 2019 13:00:43 GMT-0700 (PDT)
-    struct bjson_object* obj = state_obj("pit", (16 + 2) * 3);
+    struct bjson_object* obj = state_obj("pit", (18 + 2) * 3);
     state_field(obj, 4, "pit.chan[0].count", &pit.chan[0].count);
     state_field(obj, 4, "pit.chan[1].count", &pit.chan[1].count);
     state_field(obj, 4, "pit.chan[2].count", &pit.chan[2].count);
@@ -105,6 +106,12 @@ static void pit_state(void)
     state_field(obj, 4, "pit.chan[0].period", &pit.chan[0].period);
     state_field(obj, 4, "pit.chan[1].period", &pit.chan[1].period);
     state_field(obj, 4, "pit.chan[2].period", &pit.chan[2].period);
+    state_field(obj, 4, "pit.chan[0].pit_last_count", &pit.chan[0].pit_last_count);
+    state_field(obj, 4, "pit.chan[1].pit_last_count", &pit.chan[1].pit_last_count);
+    state_field(obj, 4, "pit.chan[2].pit_last_count", &pit.chan[2].pit_last_count);
+    state_field(obj, 4, "pit.chan[0].timer_flipflop", &pit.chan[0].timer_flipflop);
+    state_field(obj, 4, "pit.chan[1].timer_flipflop", &pit.chan[1].timer_flipflop);
+    state_field(obj, 4, "pit.chan[2].timer_flipflop", &pit.chan[2].timer_flipflop);
     state_field(obj, 4, "pit.chan[0].timer_running", &pit.chan[0].timer_running);
     state_field(obj, 4, "pit.chan[1].timer_running", &pit.chan[1].timer_running);
     state_field(obj, 4, "pit.chan[2].timer_running", &pit.chan[2].timer_running);
@@ -180,11 +187,11 @@ static int pit_get_count(struct pit_channel* pit)
     itick_t elapsed = get_now() - pit->last_load_time;
     uint32_t diff_in_ticks = (uint32_t)((double)elapsed * (double)PIT_CLOCK_SPEED / (double)ticks_per_second);
     uint32_t current = pit->count - diff_in_ticks;
-    if (current & 0x80000000) {
-        if (pit->count == 0)
-            return 0; // Avoid divide by zero errors for uninitialized timers.
-        current = (current % pit->count); // + pit->count;
-    }
+    if (pit->count == 0)
+        return 0; // Avoid divide by zero errors for uninitialized timers.
+    //if (current & 0x80000000) {
+    current = (current % pit->count); // + pit->count;
+    //}
     return current;
 }
 
@@ -194,11 +201,13 @@ static void pit_set_count(struct pit_channel* this, int v)
     this->count = (!v) << 16 | v; // 0x10000 if v is 0
     this->period = pit_counter_to_itick(this->count);
     this->timer_running = 1;
+    this->pit_last_count = pit_get_count(this); // should this be 0?
 }
 static void pit_channel_latch_counter(struct pit_channel* this)
 {
     if (!(this->whats_latched & COUNTER_LATCHED)) {
         uint16_t ct = pit_get_count(this);
+        PIT_LOG("Latch count: %ld\n", get_now());
         int mode = this->rw_mode;
         this->whats_latched = (mode << 2) | COUNTER_LATCHED;
         switch (mode) {
@@ -364,38 +373,40 @@ static void timer_cb(void)
     }
 }
 
-int pit_timer(itick_t now)
-{
-    // Only channel 0 does anything useful, so check if it's time to activate it.
-    if (pit.chan[0].timer_running && (pit.chan[0].last_irq_time + pit.chan[0].period) <= now) {
-        pit.chan[0].last_irq_time = now;
-        //pit.chan[0].last_irq_time = pit.chan[0].last_irq_time + pit.chan[0].period; // Make it more accurate
-        timer_cb();
-        if (pit.chan[0].mode != 2 && pit.chan[0].mode != 3)
-            pit.chan[0].timer_running = 0;
-        return 1;
-    }
-    return 0;
-}
-
 // Get the number of ticks, in the future, that the PIT needs to wait.
 int pit_next(itick_t now)
 {
+    UNUSED(now);
+    uint32_t count = pit_get_count(&pit.chan[0]), raise_irq = 0;
+    if (count > pit.chan[0].pit_last_count) {
+        // Count has gone from 0 --> 0xFFFF
+        raise_irq = 1;
+    }
     if (pit.chan[0].timer_running) {
-        pit_timer(now);
-        itick_t should_run = pit.chan[0].last_irq_time + pit.chan[0].period;
-        return should_run - now;
+
+        int refill_count = pit.chan[0].count;
+        if (raise_irq) {
+            timer_cb();
+            if (pit.chan[0].mode != 2 && pit.chan[0].mode != 3) {
+                pit.chan[0].timer_running = 0;
+                return -1;
+            }
+        }
+        pit.chan[0].pit_last_count = count;
+        return pit_counter_to_itick(refill_count - count);
     }
     return -1;
 }
 
-static uint32_t pit_speaker_readb(uint32_t port){
+static uint32_t pit_speaker_readb(uint32_t port)
+{
     UNUSED(port);
     // XXX: Use channel 2 for timing, not channel 0
     pit.chan[2].timer_flipflop ^= 1;
     return pit.chan[2].timer_flipflop << 4 | (pit_get_out(&pit.chan[0]) << 5);
 }
-static void pit_speaker_writeb(uint32_t port, uint32_t data){
+static void pit_speaker_writeb(uint32_t port, uint32_t data)
+{
     UNUSED(port | data);
     PIT_LOG("%sabled the pc speaker\n", data & 1 ? "En" : "Dis");
 }