netboot: make the transfer round-trip efficient

Transfer now works on hardware -- the block counter climbs -- but slowly, with
some ACKs missing.  It is ROUND-TRIP bound, not bandwidth bound: stop-and-wait
means one block per round trip, so three things dominated and all three were
conservative bring-up choices rather than considered ones.

  - Block size 256 -> 1024.  A 298 KB image is 1166 blocks at 256 but only 292
    at 1024.  256 was chosen to stay near the only multi-page frame ever proven
    on hardware (a 342-byte DHCP broadcast); large frames are now safe because
    RBFI stops a full ring being overwritten and stop-and-wait keeps exactly
    one frame in flight, so five pages of a fourteen-page budget is never
    contended.  --block 256 remains as a fallback.

  - Host ACK timeout 0.5 s -> 0.1 s.  The round trip is a few milliseconds on
    a LAN, so half a second meant every lost ACK cost half a second.

  - The GC's TI poll slept 500 us up to 20 times, costing up to 10 ms per ACK
    even when the transmit succeeded immediately -- on every block.  It now
    polls without sleeping; a bba_r8 is itself tens of microseconds, so a bare
    loop samples often enough and returns as soon as TI sets.

Absence of TI is explicitly NOT fatal.  TI only means TXFrameDrain handed the
frame to the W5100, so the frame may well have gone out; and if it did not,
the host's retransmit covers it.  Blocking longer would slow the common case
to protect against the rare one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

gc_bba_test/README.md

@@ -149,6 +149,22 @@ 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. `txto=N` in the status line counts ACKs where TI never set.
+
 ### Stop-and-wait is load-bearing
 
 The gateware does not consume RRP, so `RXFrameAssembler` can overwrite ring

gc_bba_test/source/netboot.c

@@ -172,14 +172,20 @@ static int send_ack(u16 seq)
 	if (bba_tx_bytes(f, sizeof(f)) < 0) return -1;
 	bba_w8(REG_NCRA, NCRA_SR | NCRA_ST0);
 
-	/* Wait briefly for TI.  Not proof it reached the wire, but its absence
+	/* Poll for TI WITHOUT sleeping.  usleep(500) x 20 meant every ACK cost
-	 * is a clear failure, and blocking here is fine: stop-and-wait means the
+	 * up to 10 ms even when the transmit succeeded immediately, and the
-	 * host is idle until this ACK lands anyway. */
+	 * transfer is round-trip bound -- that is 10 ms on every one of ~292
-	for (i = 0; i < 20; i++) {
+	 * blocks.  A bba_r8 is itself tens of microseconds, so a bare loop polls
+	 * often enough and finishes as soon as TI sets.
+	 *
+	 * Absence of TI is NOT treated as fatal: the frame may still have gone
+	 * out (TI only means TXFrameDrain handed it to the W5100), and if it did
+	 * not, the host's retransmit covers it.  Blocking longer here would only
+	 * slow the common case to protect against the rare one. */
+	for (i = 0; i < 200; i++) {
 		ir = bba_r8(REG_IR);
 		if (ir >= 0 && (ir & IR_TI))
 			return 0;
-		usleep(500);
 	}
 	s_tx_timeouts++;
 	return -1;

scripts/send-dol.py

@@ -25,13 +25,18 @@ import time
 ETHERTYPE = 0x88B5
 MAGIC = b"GCUP"
 OP_DATA, OP_ACK, OP_DONE = 1, 2, 3
-# Default payload 256 bytes -> a ~296-byte frame, which with the 4-byte
+# 1024-byte payloads -> ~1050-byte frames, five of the SPRAM ring's 15 pages.
-# descriptor occupies TWO of the SPRAM ring's 15 pages.  That is deliberately
+#
-# close to the only multi-page frame ever proven on this hardware (a 342-byte
+# 256 was the conservative bring-up value, chosen to stay near the only
-# DHCP broadcast).  1024-byte payloads make ~1050-byte frames spanning FIVE
+# multi-page frame ever proven on hardware (a 342-byte DHCP broadcast).  Small
-# pages, which has never been exercised -- raise --block once small blocks are
+# blocks are now known to work, and two things make the large ones safe:
-# known to work, not before.
+# RBFI stops a full ring being overwritten, and stop-and-wait keeps exactly one
-BLOCK_DEFAULT = 256
+# frame in flight, so five pages of a fourteen-page budget is never contended.
+#
+# This matters for speed: a 298 KB image is 1166 blocks at 256 bytes but only
+# 292 at 1024, and the transfer is round-trip bound, not bandwidth bound.  Drop
+# back with --block 256 if large frames misbehave.
+BLOCK_DEFAULT = 1024
 HDR = 26
 # The GC's MAC comes from PAR0-5, which the gateware pre-populates with a
 # Nintendo OUI.  Discovered from the first ACK, so this is only the initial
@@ -53,8 +58,10 @@ def main():
     ap = argparse.ArgumentParser()
     ap.add_argument("iface", help="network interface facing the GameCube")
     ap.add_argument("dol", help=".dol to send")
-    ap.add_argument("--timeout", type=float, default=0.5,
+    ap.add_argument("--timeout", type=float, default=0.1,
-                    help="per-block ACK timeout in seconds (default 0.5)")
+                    help="per-block ACK timeout in seconds (default 0.1). "
+                         "The round trip is a few ms on a LAN, so 0.5 made "
+                         "every lost ACK cost half a second.")
     ap.add_argument("--retries", type=int, default=20,
                     help="per-block retransmits before giving up (default 20)")
     ap.add_argument("--dest", default="/gc_bba_test.dol",