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

[Aurelien Jarno]

On Sat, Sep 12, 2009 at 06:52:22PM +0200, Juergen Lock wrote:
> 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:

Applied. In the future, could you please make sure to send patches with
a correct unified headers?

> 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 <address@hidden>
> 
> 
> 

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net