Super Mario Advance 4: SMB3 Link with e-Reader communication error

[DesertWolf29]

As of version 10.2, the link function between Super Mario Advance 4 and the e-Reader will fail. The last known working version is 10.0. 10.1 will have the e-Reader ROM speed up when in a multiplayer window, but I successfully scanned a card over to SMA4. With 10.2, and the latest development build, the link fails to establish between the two ROMS, resulting in a communication error. The same speed-up issue persists as well.

Edit: Also of note, when using the installed version of 10.0, the communication fails. When using the portable version, it works.

SMA4 Bug Report.zip

[Testsr]

Can reproduce. Communication after getting the save is unaffected.

[Testsr]

I have bisected the connection issue to commit 395fa2d on 0.10 or f046596 on master. The speed up issue is bisected to 8e58dca on 0.10.

[Testsr]

@endrift Can we add this to a milestone,so that it gets fixed.

[Testsr]

I have traced down the regression to lines 517 - 534 of lockstep.c

...
      if (node->id < 3 && attached > node->id + 1) {
	      value = GBASIONormalSetSi(value, GBASIONormalGetIdleSo(node->p->players[node->id + 1]->d.p->siocnt));
      } else {
	      value = GBASIONormalFillSi(value);
      }
      
      enum mLockstepPhase transferActive;
      ATOMIC_LOAD(transferActive, node->p->d.transferActive);
      if (node->id > 0 && transferActive == TRANSFER_IDLE) {
	      int try;
	      for (try = 0; try < 3; ++try) {
		      GBASIONormal parentSiocnt;
		      ATOMIC_LOAD(parentSiocnt, node->p->players[node->id - 1]->d.p->siocnt);
		      if (ATOMIC_CMPXCHG(node->p->players[node->id - 1]->d.p->siocnt, parentSiocnt, GBASIONormalSetSi(parentSiocnt, GBASIONormalGetIdleSo(value)))) {
			      break;
		      }
	      }
...

(removing the lines fixes the bug), and know the regression is caused by the GBAs having different transfer modes. I don't know what causes the transfer mode desync, however. My best guess is a race condition in setting the SIOCNT.