mac_via: workaround NetBSD ADB bus enumeration issue

NetBSD assumes it can send its first ADB command after sending the ADB_BUSRESET
command in ADB_STATE_NEW without changing the state back to ADB_STATE_IDLE
first as detailed in the ADB protocol.

Add a workaround to detect this condition at the start of ADB enumeration
and send the next command written to SR after a ADB_BUSRESET onto the bus
regardless, even if we don't detect a state transition to ADB_STATE_NEW.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-ID: <20231004083806.757242-16-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

1 files changed, 34 insertions, 0 deletions

diff --git a/hw/misc/mac_via.c b/hw/misc/mac_via.c
index f2ccb12d64..9f9c2815d0 100644
--- a/hw/misc/mac_via.c
+++ b/hw/misc/mac_via.c
@@ -1001,6 +1001,8 @@ static void mos6522_q800_via1_write(void *opaque, hwaddr addr, uint64_t val,
 {
     MOS6522Q800VIA1State *v1s = MOS6522_Q800_VIA1(opaque);
     MOS6522State *ms = MOS6522(v1s);
+    int oldstate, state;
+    int oldsr = ms->sr;
 
     addr = (addr >> 9) & 0xf;
 
@@ -1016,6 +1018,38 @@ static void mos6522_q800_via1_write(void *opaque, hwaddr addr, uint64_t val,
 
         v1s->last_b = ms->b;
         break;
+
+    case VIA_REG_SR:
+        {
+            /*
+             * NetBSD assumes it can send its first ADB command after sending
+             * the ADB_BUSRESET command in ADB_STATE_NEW without changing the
+             * state back to ADB_STATE_IDLE first as detailed in the ADB
+             * protocol.
+             *
+             * Add a workaround to detect this condition at the start of ADB
+             * enumeration and send the next command written to SR after a
+             * ADB_BUSRESET onto the bus regardless, even if we don't detect a
+             * state transition to ADB_STATE_NEW.
+             *
+             * Note that in my tests the NetBSD state machine takes one ADB
+             * operation to recover which means the probe for an ADB device at
+             * address 1 always fails. However since the first device is at
+             * address 2 then this will work fine, without having to come up
+             * with a more complicated and invasive solution.
+             */
+            oldstate = (v1s->last_b & VIA1B_vADB_StateMask) >>
+                       VIA1B_vADB_StateShift;
+            state = (ms->b & VIA1B_vADB_StateMask) >> VIA1B_vADB_StateShift;
+
+            if (oldstate == ADB_STATE_NEW && state == ADB_STATE_NEW &&
+                    (ms->acr & VIA1ACR_vShiftOut) &&
+                    oldsr == 0 /* ADB_BUSRESET */) {
+                trace_via1_adb_netbsd_enum_hack();
+                adb_via_send(v1s, state, ms->sr);
+            }
+        }
+        break;
     }
 }