[Qemu-devel] [PATCH] Fix lost serial TX interrupts. Report receive overruns.

["Justin T. Gibbs" <gibbs@FreeBSD.org>]

This patch compliments the patch submitted by Jergen Lock and further
improves the performance of QEMU's serial emulation with FreeBSD's
uart(9) driver.

  o Implement receive overrun status.  The FreeBSD uart driver
    relies on this status in it's probe routine to determine the size
    of the FIFO supported.
  o As per the 16550 spec, do not overwrite the RX FIFO on an RX overrun.
  o Do not allow TX or RX FIFO overruns to increment the data valid count
    beyond the size of the FIFO.
  o For reads of the IIR register, only clear the "TX holding register
    emtpy interrupt" if the read reports this interrupt.  This is required
    by the specification and avoids losing TX interrupts when other, higher
    priority interrupts (usually RX) are reported first.

Signed-off-by: Justin T. Gibbs <gibbs@FreeBSD.org>

--- serial.c.orig    Thu Jan 14 15:18:00 2010
+++ serial.c    Thu Jan 28 15:36:04 2010
@@ -169,11 +169,19 @@
  {
      SerialFIFO *f = (fifo) ? &s->recv_fifo : &s->xmit_fifo;

-    f->data[f->head++] = chr;
+    /* Receive overruns do not overwrite FIFO contents. */
+    if (fifo == XMIT_FIFO || f->count < UART_FIFO_LENGTH) {

-    if (f->head == UART_FIFO_LENGTH)
-        f->head = 0;
-    f->count++;
+        f->data[f->head++] = chr;
+
+        if (f->head == UART_FIFO_LENGTH)
+            f->head = 0;
+    }
+
+    if (f->count < UART_FIFO_LENGTH)
+        f->count++;
+    else if (fifo == RECV_FIFO)
+        s->lsr |= UART_LSR_OE;

      return 1;
  }
@@ -533,8 +541,10 @@
          break;
      case 2:
          ret = s->iir;
+        if (ret & UART_IIR_THRI) {
              s->thr_ipending = 0;
-        serial_update_irq(s);
+            serial_update_irq(s);
+        }
          break;
      case 3:
          ret = s->lcr;
@@ -544,9 +554,9 @@
          break;
      case 5:
          ret = s->lsr;
-        /* Clear break interrupt */
-        if (s->lsr & UART_LSR_BI) {
-            s->lsr &= ~UART_LSR_BI;
+        /* Clear break and overrun interrupts */
+        if (s->lsr & (UART_LSR_BI|UART_LSR_OE)) {
+            s->lsr &= ~(UART_LSR_BI|UART_LSR_OE);
              serial_update_irq(s);
          }
          break;
@@ -629,6 +639,8 @@
          /* call the timeout receive callback in 4 char transmit time */
          qemu_mod_timer(s->fifo_timeout_timer, qemu_get_clock 
(vm_clock) + s->char_transmit_time * 4);
      } else {
+        if (s->lsr & UART_LSR_DR)
+            s->lsr |= UART_LSR_OE;
          s->rbr = buf[0];
          s->lsr |= UART_LSR_DR;
      }