x86/hpet: simplify hpet_get_channel()

There's no need for a lock here, elimination of which makes the
function a leaf one, thus allowing for better (and smaller) code.

Further, use the variable next_channel according to its name - so far
it represented the most recently used channel rather than the next one
to use.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hpet.c
+++ b/xen/arch/x86/hpet.c
@@ -402,30 +402,35 @@ static void __init hpet_fsb_cap_lookup(v
 static struct hpet_event_channel *hpet_get_channel(unsigned int cpu)
 {
     static unsigned int next_channel;
-    static spinlock_t next_lock = SPIN_LOCK_UNLOCKED;
     unsigned int i, next;
     struct hpet_event_channel *ch;
 
     if ( num_hpets_used == 0 )
         return hpet_events;
 
-    spin_lock(&next_lock);
-    next = next_channel = (next_channel + 1) % num_hpets_used;
-    spin_unlock(&next_lock);
+    do {
+        next = next_channel;
+        if ( (i = next + 1) == num_hpets_used )
+            i = 0;
+    } while ( cmpxchg(&next_channel, next, i) != next );
 
     /* try unused channel first */
-    for ( i = next; i < next + num_hpets_used; i++ )
-    {
-        ch = &hpet_events[i % num_hpets_used];
+    ch = &hpet_events[i = next];
+    do {
         if ( !test_and_set_bit(HPET_EVT_USED_BIT, &ch->flags) )
         {
             ch->cpu = cpu;
             return ch;
         }
-    }
+        ++ch;
+        if ( ++i == num_hpets_used )
+        {
+            i = 0;
+            ch = hpet_events;
+        }
+    } while ( i != next );
 
-    /* share a in-use channel */
-    ch = &hpet_events[next];
+    /* Share an in-use channel. */
     if ( !test_and_set_bit(HPET_EVT_USED_BIT, &ch->flags) )
         ch->cpu = cpu;