Re: [Qemu-devel] [PATCH v2] hw/char/escc: Lower irq when transmit buffer is filled

[Philippe Mathieu-Daudé]

On 4/17/19 2:50 AM, Stephen Checkoway wrote:
> The SCC/ESCC will briefly stop asserting an interrupt when the
> transmit FIFO is filled.
> 
> This code doesn't model the transmit FIFO/shift register so the
> pending transmit interrupt is never deasserted which means that an
> edge-triggered interrupt controller will never see the low-to-high
> transition it needs to raise another interrupt. The practical
> consequence of this is that guest firmware with an interrupt service
> routine for the ESCC that does not send all of the data it has
> immediately will stop sending data if the following sequence of
> events occurs:
> 1. Disable processor interrupts
> 2. Write a character to the ESCC
> 3. Add additional characters to a buffer which is drained by the ISR
> 4. Enable processor interrupts
> 
> In this case, the first character will be sent, the interrupt will
> fire and the ISR will output the second character. Since the pending
> transmit interrupt remains asserted, no additional interrupts will
> ever fire.

You might want to add a line expliciting the chipset model which forced
you to do that patch (Z85C30).

> This fixes that situation by explicitly lowering the IRQ when a
> character is written to the buffer and no other interrupts are currently
> pending.

> 
> Signed-off-by: Stephen Checkoway <address@hidden>
> ---
>  hw/char/escc.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/hw/char/escc.c b/hw/char/escc.c
> index 628f5f81f7..c5b05a63f1 100644
> --- a/hw/char/escc.c
> +++ b/hw/char/escc.c
> @@ -509,6 +509,13 @@ static void escc_mem_write(void *opaque, hwaddr addr,
>          break;
>      case SERIAL_DATA:
>          trace_escc_mem_writeb_data(CHN_C(s), val);
> +        /*
> +         * Lower the irq when data is written to the Tx buffer and no other
> +         * interrupts are currently pending. The irq will be raised again 
> once
> +         * the Tx buffer becomes empty below.
> +         */
> +        s->txint = 0;
> +        escc_update_irq(s);

Way better, thanks :)

Reviewed-by: Philippe Mathieu-Daudé <address@hidden>

Note: the Z85C30 has a 1-byte transmit FIFO and 3-byte receive FIFO.
Our model default is the ESCC, and we use ESCC_SERIO_QUEUE_SIZE=256 byte
for rx FIFO... If needed we can add properties and use a white list of
chipset with their tx/rx FIFO size.

>          s->tx = val;
>          if (s->wregs[W_TXCTRL2] & TXCTRL2_TXEN) { // tx enabled
>              if (qemu_chr_fe_backend_connected(&s->chr)) {
>