Date: Sat, 12 Sep 2009 18:52:22 +0200 From: Juergen Lock <nox@jelal.kn-bremen.de> To: Jan Kiszka <jan.kiszka@web.de> Cc: Olivier =?iso-8859-1?Q?Cochard-Labb=E9?= <olivier@cochard.me>, freebsd-current@FreeBSD.org, Juergen Lock <nox@jelal.kn-bremen.de>, qemu-devel@nongnu.org Subject: Re: qemu serial: lost tx irqs (affectig FreeBSD's new uart(4) driver) Message-ID: <20090912165222.GA38048@triton8.kn-bremen.de> In-Reply-To: <4AAB938B.9080004@web.de> References: <20090911213508.GA97446@triton8.kn-bremen.de> <4AAB938B.9080004@web.de>
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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@jelal.kn-bremen.de>
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