rebbarb/gc_bba_test

re-bba-rb — on-console test suite (GameCube)

A .dol you run from Swiss that drives the adapter in Serial Port 1 and reports pass/fail per test on the TV.

Build

Needs devkitPro's gamecube-dev group (devkitPPC + libogc). The devcontainer installs it — if DEVKITPPC is unset, rebuild the container, since the toolchain is added in .devcontainer/Dockerfile.

make            # -> gc_bba_test.dol

Copy the .dol to your SD card (SD Gecko / SD2SP2) and launch it from Swiss.

Controls

Button
D-pad up/down	move
A	open a suite / rerun the selected test
X	run the selected suite
START	run everything
Y	clear all results
B	back

What it tests

Suite	Proves
Identity	device enumerates: 04 02 02 00, HIPR, NWAYS
Registers	PAR0–5, IMR writable and read back; IR is write-1-clear
Reset/control	NCRA RESET self-clears, SR holds
EXI integrity	200 reads with no bit errors; which EXI clock indices work
Ethernet	MAC readback, 60-byte ARP transmit (TI), frame receive (RWP/RI)
SPRAM/DMA	ring descriptor is well-formed; reads ≥ 0x100 stream until CS

Things that will bite you

The device ID is a ONE-SHOT. Address 0 returns 04 02 02 00 only until the first NCRA reset clears id_phase in the gateware; after that it reads back NCRA. That is correct — it matches the GC's own init order — but it means the Identity test can only pass once per console power cycle. Run it before the Reset suite. On a re-run the app reports SKIP with an explanation rather than a misleading failure.

TI is not proof of transmission. It only means TXFrameDrain handed the frame to the W5100. A frame can set TI and never reach the wire. The TX test says so; to confirm the wire, capture on a PC or watch the RX test pick up a reply.

The RX test needs traffic. It waits ~2 s for any frame. On an isolated link with nothing talking it will fail legitimately — give the PC an address on the same subnet (the ARP the TX test sends is aimed at 10.0.0.50, so sudo ip addr add 10.0.0.50/24 dev <nic> makes the PC answer it and both tests pass together).

Never over-clock a register read. Asking for N bytes and clocking more leaves stale prefetched bytes queued and desynchronises the next transaction — a deterministic every-Nth-read corruption that looks like a signal-integrity fault. bba_read() clocks exactly len; don't "round up" for convenience.

TXDATA takes one byte per transaction. The register file's effective address auto-increments per data byte, so a 4-byte write to 0x48 would spill into 0x49–0x4B. bba_tx_bytes() does it correctly.

Build status

Builds clean (no warnings) with devkitPPC 16.1.0 / GCC 16 in the devcontainer: gc_bba_test.dol, ~172 KB, entry 0x80003100.

.gitignore excludes the build products — run make to produce the .dol.

Self-update over the BBA (Z button)

Press Z, then on the host:

sudo ./scripts/send-dol.py <iface> gc_bba_test/gc_bba_test.dol

The flow is deliberately receive → temp file → verify → rotate → boot Swiss:

1. The new build arrives over the adapter into a staging buffer.
2. It is written to <name>.tmp and read back and compared.
3. Only then is it rotated in: the existing build becomes <name>.prev.dol and the temp file becomes <name>.dol (default /gc_bba_test.dol, override with --dest).
4. The app then boots /boot.dol — your known-good Swiss.

Ordering is the point. If the transfer, the write or the verify fails, the build you are currently running is still on the card untouched, so you can just try again. And once an update succeeds, the previous build is kept as <name>.prev.dol — so if the new one turns out to be broken, boot the previous one from Swiss and transfer again from there. One generation back is enough to never get stranded needing an SD card reader.

The temp file deliberately has no .dol extension: Swiss lists .dol files, and a half-written image Swiss offers to boot is exactly what this avoids. The backup keeps .dol precisely because booting it is the point.

The received image is never launched. The only thing this ever jumps into is Swiss, which was already known good. So a broken test build costs nothing: Swiss comes up, and you load whatever you like from there, including the previous build. That is what makes the update recoverable without a power cycle or digging the SD card out.

If the SD write or the verify fails, nothing is launched at all — you stay in the menu with a working app.

sudo is needed for CAP_NET_RAW. The GC's MAC is discovered from its first ACK, so neither side needs an address configured.

Why it is not IP

Frames carry a private ethertype (0x88B5, IEEE 802 local experimental 1) and a 12-byte header: no ARP, no IP stack, no checksums, and no libogc network driver competing for the EXI channel. It reuses exactly the two paths the test suite already proves on hardware — bba_tx_bytes() for TX and the SPRAM ring at RRP for RX — so each update doubles as a few hundred round trips of soak testing through that path.

⚠️ Quiet the link first — the ring has no overflow protection

sudo ./scripts/gc-testlink.sh up <iface>     # static IP, no DHCP, NM off

Nothing in the gateware consumes RRP, so RXFrameAssembler keeps writing and laps the reader — the real BBA has RBFI for this and this design does not. The poll loop costs ~5 EXI transactions per frame (~200 µs) while a 100 Mbit link delivers a 342-byte frame every ~30 µs, and the ring is only 15 pages (~3–11 frames). Under continuous traffic pages are overwritten before they are read, and the "descriptor" you then parse is really frame data from the frame that replaced it.

That is where a descriptor of 00 00 00 01 comes from. It is not a dropped write — it is someone else's bytes.

The biggest offender is usually the host's own DHCP client broadcasting on that interface (a 342-byte broadcast). gc-testlink.sh up sets a static address and takes the device out of NetworkManager, which stops it. Disabling IPv6 and mDNS on that interface helps further.

netboot_receive() detects being lapped (writer more than two thirds of the ring ahead), resyncs and continues rather than parsing untrustworthy bytes, and reports the count on timeout — but detection is not prevention. A busy link will still cost retransmissions.

Throughput

The transfer is round-trip bound, not bandwidth bound: stop-and-wait means one block per round trip, so block size sets the speed.

• --block defaults to 1024 (292 blocks for a ~298 KB image). 256 was the conservative bring-up value; drop back to it if large frames misbehave.
• --timeout defaults to 0.1 s. The round trip is a few ms on a LAN, so the original 0.5 s made every lost ACK cost half a second.
• The GC polls for TI without sleeping. A usleep(500) × 20 loop cost up to 10 ms per ACK even when the transmit succeeded immediately — on every block.

A missing ACK is not fatal: the host retransmits, and the GC re-ACKs a duplicate block rather than ignoring it, so a lost ACK recovers in one timeout.

The GC stops with its summary before booting Swiss — press a button to continue. Those counters are the only place the reason for a retry shows up:

desc@+0=N +1=M  bad=N  lapped=N  txto=N

• txto — ACKs where TI never set, i.e. the frame never reached the W5100. High txto with many host-side retries means the TX path is the problem.
• txto near zero with many retries means ACKs were sent but lost on the wire or at the host.
• desc@+1 — frames whose ring descriptor landed at page offset +1 instead of +0. Still unexplained; the reader handles both.

Measured 2026-10-09: 299908 bytes in 293 blocks took 15.7 s with roughly a third of blocks retried — the retries, not the data, dominate the time.

Stop-and-wait is load-bearing

The gateware does not consume RRP, so RXFrameAssembler can overwrite ring pages the GC has not read yet — there is no overflow protection. One block in flight means the ring never holds more than one unread frame. Do not turn this into a sliding window without adding RBFI-style protection first.

SD interfaces

netboot_sd_write() auto-detects, trying SD2SP2, then SD Gecko slot B, then slot A. Note the SD Gecko in slot A shares EXI channel 0 with the BBA (channel 0, device 2); libogc arbitrates, but SD2SP2 sits on its own channel and avoids the question.

Why the copier lives in assembly at the top of MEM1

A DOL's sections load from 0x80003100 upward — exactly where this program's own .text sits. A copier compiled as an ordinary C function would overwrite its own instructions partway through the copy and hang. So:

• netboot_reserve() carves the top of MEM1 and hands it to SYS_SetArena1Hi(), so neither libogc nor malloc will touch it.
• netboot_stub.S is copied into that region and executed there. It must stay strictly position independent: no globals, no literal pool, no TOC references, no calls — otherwise it reads from the address it was linked at instead of the one it is running at.
• netboot_launch() checks every section (and BSS) against the reserved region and refuses if any would land on it.

Failure behaviour

• 30 s with no frames, or 5 s mid-transfer, aborts back to the menu.
• A duplicate block is re-ACKed rather than ignored, so a lost ACK recovers.
• netboot_receive() re-asserts the NCRA SR bit first: an earlier test may have left receive disabled, and it would otherwise wait forever for frames the gateware is not storing.
• A failed SD write or verify launches nothing.
• The residual risk is confined to booting /boot.dol. If that is a healthy Swiss, this is the safe part; if it is missing or corrupt, you get a message and stay in the menu — the new build is already on the card either way.