From 92d86af9552b48d4ce05b13823807131b99b1cce Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Mon, 30 Jan 2023 20:48:39 -0800
Subject: [PATCH] GBA SIO: Fix SIOCNT SI pin value after attaching player 2
 (fixes #2805)

---
 CHANGES                |  1 +
 src/gba/sio/lockstep.c | 12 ++++++++++++
 2 files changed, 13 insertions(+)

diff --git a/CHANGES b/CHANGES
index 36e2cefa3..7045e46b6 100644
--- a/CHANGES
+++ b/CHANGES
@@ -6,6 +6,7 @@ Features:
 Emulation fixes:
  - GBA Audio: Fix improperly deserializing GB audio registers (fixes mgba.io/i/2793)
  - GBA Memory: Make VRAM access stalls only apply to BG RAM
+ - GBA SIO: Fix SIOCNT SI pin value after attaching player 2 (fixes mgba.io/i/2805)
  - GBA Video: Disable BG target 1 blending when OBJ blending (fixes mgba.io/i/2722)
 Other fixes:
  - Core: Allow sending thread requests to a crashed core (fixes mgba.io/i/2784)
diff --git a/src/gba/sio/lockstep.c b/src/gba/sio/lockstep.c
index bc5a5fd33..a03e88e30 100644
--- a/src/gba/sio/lockstep.c
+++ b/src/gba/sio/lockstep.c
@@ -111,6 +111,18 @@ bool GBASIOLockstepNodeLoad(struct GBASIODriver* driver) {
 		if (node->id) {
 			node->d.p->rcnt |= 4;
 			node->d.p->siocnt = GBASIOMultiplayerFillSlave(node->d.p->siocnt);
+
+			int try;
+			for (try = 0; try < 3; ++try) {
+				uint16_t masterSiocnt;
+				ATOMIC_LOAD(masterSiocnt, node->p->players[0]->d.p->siocnt);
+				if (ATOMIC_CMPXCHG(node->p->players[0]->d.p->siocnt, masterSiocnt, GBASIOMultiplayerClearSlave(masterSiocnt))) {
+					break;
+				}
+			}
+		} else {
+			node->d.p->rcnt &= ~4;
+			node->d.p->siocnt = GBASIOMultiplayerClearSlave(node->d.p->siocnt);
 		}
 		break;
 	case SIO_NORMAL_8: