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

[Jan Kiszka]

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.

Jan

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