[PATCH 2/2] hw/timer/xilinx_timer: Reload at the expiry

The callback of a timer with the auto reload and hold bit set restarted
the count.  It restarted it at the moment the callback ran, so every
period gained the latency of the callback and the period moved with the
load of the host.

Run the ptimer in the periodic mode, so that it reloads at the expiry.
A write of the load register while the count runs sets the limit for
the next reload, which keeps the load value live.

Signed-off-by: Sebastian Huber <[email protected]>
---
 hw/timer/xilinx_timer.c | 36 ++++++++++++++++++++++++++++--------
 1 file changed, 28 insertions(+), 8 deletions(-)

diff --git a/hw/timer/xilinx_timer.c b/hw/timer/xilinx_timer.c
index dff439c623..7120c339ec 100644
--- a/hw/timer/xilinx_timer.c
+++ b/hw/timer/xilinx_timer.c
@@ -139,6 +139,14 @@ timer_read(void *opaque, hwaddr addr, unsigned int size)
     return r;
 }
 
+static uint64_t timer_limit(struct xlx_timer *xt)
+{
+    if (xt->regs[R_TCSR] & TCSR_UDT)
+        return xt->regs[R_TLR];
+    else
+        return ~0 - xt->regs[R_TLR];
+}
+
 /* Must be called inside ptimer transaction block */
 static void timer_enable(struct xlx_timer *xt)
 {
@@ -149,12 +157,14 @@ static void timer_enable(struct xlx_timer *xt)
 
     ptimer_stop(xt->ptimer);
 
-    if (xt->regs[R_TCSR] & TCSR_UDT)
-        count = xt->regs[R_TLR];
-    else
-        count = ~0 - xt->regs[R_TLR];
+    count = timer_limit(xt);
     ptimer_set_limit(xt->ptimer, count, 1);
-    ptimer_run(xt->ptimer, 1);
+    /*
+     * The auto reload and hold bit makes the count periodic.  Let the ptimer
+     * carry the period, so that it reloads at the expiry rather than when the
+     * callback runs.
+     */
+    ptimer_run(xt->ptimer, !(xt->regs[R_TCSR] & TCSR_ARHT));
 }
 
 static void
@@ -195,6 +205,19 @@ timer_write(void *opaque, hwaddr addr,
             break;
         }
  
+        case R_TLR:
+            xt->regs[addr] = value;
+            /*
+             * A running periodic count takes the new load value at its next
+             * reload.
+             */
+            if (xt->regs[R_TCSR] & TCSR_ENT) {
+                ptimer_transaction_begin(xt->ptimer);
+                ptimer_set_limit(xt->ptimer, timer_limit(xt), 0);
+                ptimer_transaction_commit(xt->ptimer);
+            }
+            break;
+
         default:
             if (addr < ARRAY_SIZE(xt->regs))
                 xt->regs[addr] = value;
@@ -226,9 +249,6 @@ static void timer_hit(void *opaque)
     XpsTimerState *t = xt->parent;
     D(fprintf(stderr, "%s %d\n", __func__, xt->nr));
     xt->regs[R_TCSR] |= TCSR_TINT;
-
-    if (xt->regs[R_TCSR] & TCSR_ARHT)
-        timer_enable(xt);
     timer_update_irq(t);
 }
 
-- 
2.51.0