[Qemu-devel] qemu serial: lost tx irqs (affectig FreeBSD's new uart(4) driver)

[Juergen Lock]

Hi!

 I got a report of FreeBSD guest's new uart(4) driver misbehaving in
qemu again(?) (output stopping for no apparent reason), and now found
out the problem is tx irqs (UART_IIR_THRI) are getting lost because
serial_update_irq() checks for the rx condtion,
        ... if ((s->ier & UART_IER_RDI) && (s->lsr & UART_LSR_DR))
first before checking for the tx irq condition,
        ... if ((s->ier & UART_IER_THRI) && s->thr_ipending)
which at least in this case (FreeBSD 8 guest after doing
        set console="comconsole"
at the loader prompt or when simply echo'ing text to /dev/ttyu0
or typing to the serial port from cu(1) on a `regular' vga console)
causes the second condition (.. && s->thr_ipending) to be never
reached anymore, or only after a very long delay.  Moving that
condition up so it is checked first like this,

Index: qemu/hw/serial.c
@@ -189,7 +188,9 @@ static void serial_update_irq(SerialStat
 {
     uint8_t tmp_iir = UART_IIR_NO_INT;
 
-    if ((s->ier & UART_IER_RLSI) && (s->lsr & UART_LSR_INT_ANY)) {
+    if ((s->ier & UART_IER_THRI) && s->thr_ipending) {
+        tmp_iir = UART_IIR_THRI;
+    } else if ((s->ier & UART_IER_RLSI) && (s->lsr & UART_LSR_INT_ANY)) {
         tmp_iir = UART_IIR_RLSI;
     } else if ((s->ier & UART_IER_RDI) && s->timeout_ipending) {
         /* Note that(s->ier & UART_IER_RDI) can mask this interrupt,
@@ -202,8 +203,6 @@ static void serial_update_irq(SerialStat
         } else if (s->recv_fifo.count >= s->recv_fifo.itl) {
            tmp_iir = UART_IIR_RDI;
         }
-    } else if ((s->ier & UART_IER_THRI) && s->thr_ipending) {
-        tmp_iir = UART_IIR_THRI;
     } else if ((s->ier & UART_IER_MSI) && (s->msr & UART_MSR_ANY_DELTA)) {
         tmp_iir = UART_IIR_MSI;
     }

...fixes the issue for me, but I'm not 100% sure if this might cause
rx irqs to come (too?) late when a guest keeps sending while its
receiving at the same time.  Anyone care to comment? :)

 Thanx,
        Juergen