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Bound the TXDATA chunk to half the tx_bytes depth

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a2e8e5c batched TXDATA writes into 64-byte chunks and broke ACKs outright --
"no ACK after 20 tries".  My regression, and CLAUDE.md warns about exactly
this: tx_bytes is 16 deep and BBARegisterFile silently drops bytes when
w_rdy is low, because SPI cannot back-pressure.

Within a single transaction the bytes arrive back-to-back at EXI speed, far
faster than TXFrameDrain starts consuming, so a 64-byte chunk overran the FIFO,
the frame went out truncated, and no valid ACK ever reached the host.
Byte-by-byte worked only because the per-byte EXI overhead happened to pace
the drain -- which is also why it was slow.

Chunk is now 8, half the FIFO depth: it cannot overflow even if the drain
consumes nothing for the whole transaction, while still cutting a 60-byte ACK
from ~60 transactions to 8.  Noted in both the source and CLAUDE.md not to
raise it without deepening tx_bytes or giving the TX path real back-pressure.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Dennis Brentjes
2026-10-09 19:16:28 +02:00
parent a2e8e5ceaf
commit 1e370deb49
2 changed files with 34 additions and 14 deletions
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gc_bba_test/source/bba.c
+20 -10
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@@ -125,21 +125,31 @@ int bba_tx_bytes(const u8 *buf, u32 len)
{
u32 off = 0;
/* Stream in 64-byte chunks, ONE EXI transaction each.
/* Stream in 8-byte chunks, ONE EXI transaction each.
*
* This used to write byte by byte with bba_w8(), which means a full
* EXI_Lock / EXI_Select / header / EXI_Deselect / EXI_Unlock per BYTE --
* about sixty transactions to send one 60-byte ACK. That latency is why
* roughly a third of netboot blocks were retried: the ACK was always
* transmitted (txto=0 on hardware, so TI always set) but did not get back
* to the host inside its timeout. The data phase of a register-space
* write runs until CS, so there is no reason to pay the per-byte cost.
* Writing byte by byte with bba_w8() costs a full EXI_Lock / EXI_Select /
* header / EXI_Deselect / EXI_Unlock PER BYTE -- about sixty transactions
* for one 60-byte ACK. That latency is why ~1/3 of netboot blocks were
* retried: the ACK always reached the W5100 (txto=0, TI always set) but
* missed the host's timeout.
*
* 64 is bba_write()'s internal buffer limit, not a protocol limit. */
* ⚠️ BUT THE CHUNK SIZE IS BOUNDED BY tx_bytes, WHICH IS 16 DEEP.
* BBARegisterFile silently drops bytes when tx_bytes.w_rdy is low,
* because SPI cannot back-pressure. Within one transaction the bytes
* arrive back-to-back at EXI speed -- far faster than TXFrameDrain can
* start consuming -- so a 64-byte chunk overran the FIFO, the frame went
* out truncated, and the host saw no valid ACK at all ("no ACK after 20
* tries"). Byte-by-byte worked only because the per-byte EXI overhead
* happened to give the drain time.
*
* 8 is half the FIFO depth: it cannot overflow even if the drain consumes
* nothing for the whole transaction, while still cutting a 60-byte ACK
* from ~60 transactions to 8. Do not raise it without deepening
* tx_bytes or giving the TX path real back-pressure. */
while (off < len) {
u32 n = len - off;
int rc;
if (n > 64) n = 64;
if (n > 8) n = 8;
rc = bba_write(REG_TXDATA, buf + off, n);
if (rc < 0) return rc;
off += n;