qemu serial: lost tx irqs (affectig FreeBSD's new uart(4) driver)

[Juergen Lock]

On Sat, Sep 12, 2009 at 02:26:51PM +0200, Jan Kiszka wrote:
> Juergen Lock wrote:
> > 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? :)
> 
> The reordering violates the 16550A spec in that RX event overrules TX in
> the IRQ status register. Maybe something else is wrong but it's not the
> ordering in serial_update_irq.

Well one problem seems to be the rx condition,
 	... if ((s->ier & UART_IER_RDI) && (s->lsr & UART_LSR_DR))
is not enough to trigger an irq, yet still causes the following
conditions not to be checked anymore at all.  And ideed, fixing that
seems to get my FreeBSD 8 guest back to working order as well:

Index: qemu/hw/serial.c
@@ -196,12 +195,10 @@ static void serial_update_irq(SerialStat
          * this is not in the specification but is observed on existing
          * hardware.  */
         tmp_iir = UART_IIR_CTI;
-    } else if ((s->ier & UART_IER_RDI) && (s->lsr & UART_LSR_DR)) {
-        if (!(s->fcr & UART_FCR_FE)) {
-           tmp_iir = UART_IIR_RDI;
-        } else if (s->recv_fifo.count >= s->recv_fifo.itl) {
-           tmp_iir = UART_IIR_RDI;
-        }
+    } else if ((s->ier & UART_IER_RDI) && (s->lsr & UART_LSR_DR) &&
+               (!(s->fcr & UART_FCR_FE) ||
+                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)) {

 Signed-off-by: Juergen Lock <nox at jelal.kn-bremen.de>