CDTV with netplay desyncs on every boot regardless of CD presence

[ztronzo]

CDTV with netplay desyncs on every boot regardless of CD presence, before any game loads.
Root cause is the CDTV CD controller thread writing to emulated hardware registers asynchronously during initialization.
These register values are read by the Amiga ROM during boot and written to chip RAM at different emulation frame counts on each client.
Fix requires deferring CDTV controller register writes to frame boundaries in deterministic mode, similar to the existing hard drive desync fix.

Looking at the log, the exact sequence is:

uae_start_tread "cdtv" function at 0000000000fa5830 arg 0000000000000000
CDTV: CD thread started
CDTV: reset

↓ thread starts and runs asynchronously

Card 1: Z2 0x00e90000   64K IO  CDTV DMAC/CD Controller

↓ Amiga ROM autoconfig probes the CDTV controller

CDTV: command register read while empty
CDTV: command register read while empty
WARNING: ERROR 8 from netplay server

[ztronzo]

Additional finding: CDTV netplay desync — confirmed fix via binary patching

CDTV games desync on every boot regardless of CD presence. After extensive binary patching and testing, the root cause has been identified and confirmed fixed on boot. (prior reading CD-ROM)

Root cause

In cdtv_wget() in cdtv.cpp, a global state variable is read to determine the dispatch path for register reads. This variable is written by the CDTV CD thread during initialization. Because the thread runs asynchronously, a race condition exists during boot:

• The Amiga ROM probes CDTV DMAC registers during autoconfig
• On one client the thread has already written the variable (non-zero → different dispatch path)
• On the other client it hasn't yet (zero → safe path)
• Different register values are returned → different chip RAM writes → checksum mismatch → ERROR 8

Log signature:

uae_start_tread "cdtv" function at ...
CDTV: CD thread started
CDTV: reset
Card 1: Z2 0x00e90000  CDTV DMAC/CD Controller
CDTV: command register read while empty
WARNING: ERROR 8 from netplay server

Binary confirmation

Patching cdtv_bget and cdtv_wget to return 0 eliminated the desync (blank screen, both clients in sync). Patching just the conditional branch in cdtv_wget that checks the asynchronously-written global — changing JLE to JMP at file offset 0x236771 in fs-uae.exe 3.1.66 — fixes the desync while keeping the CDTV fully functional.

Suggested source fix in cdtv.cpp

In cdtv_wget(), find the state variable check before the async dispatch and guard it with a deterministic mode check:

if (!currprefs.deterministic && state_variable > 0) {
    // async dispatch path (existing behavior)
} else {
    // deterministic/safe register read path
}

Tested: CDTV previously desynced on every boot, now boots and runs in netplay correctly.
But the fix needs to be more targeted — only bypass the async state check during the race window (early boot), not permanently. Currently doesnt load any CD-ROM images

In src/cdtv.cpp, in cdtv_wget(), find the counter check and add:
// Before the existing counter check:

if (addr < 0x48) {
    // Autoconfig range - always return 0 in deterministic mode
    // to avoid race condition with CDTV thread initialization
    if (currprefs.deterministic)
        return 0;
}

// Existing counter check follows...

if (cdrom_command_cnt <= 0)
    return 0;

// ... dispatch to handler