mac_via: always clear ADB interrupt when switching to A/UX mode

When the NetBSD kernel initialises it can leave the ADB interrupt asserted
depending upon where in the ADB poll cycle the MacOS ADB interrupt handler
is when the NetBSD kernel disables interrupts.

The NetBSD ADB driver uses the ADB interrupt state to determine if the ADB
is busy and refuses to send ADB commands unless it is clear. To ensure that
this doesn't happen, always clear the ADB interrupt when switching to A/UX
mode to ensure that the bus enumeration always occurs.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-ID: <20231004083806.757242-18-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/misc/mac_via.c

@@ -875,6 +875,15 @@ static void via1_auxmode_update(MOS6522Q800VIA1State *v1s)
     if (irq != oldirq) {
         trace_via1_auxmode(irq);
         qemu_set_irq(v1s->auxmode_irq, irq);
+
+        /*
+         * Clear the ADB interrupt. MacOS can leave VIA1B_vADBInt asserted
+         * (low) if a poll sequence doesn't complete before NetBSD disables
+         * interrupts upon boot. Fortunately NetBSD switches to the so-called
+         * "A/UX" interrupt mode after it initialises, so we can use this as
+         * a convenient place to clear the ADB interrupt for now.
+         */
+        s->b |= VIA1B_vADBInt;
     }
 }