TXFrameDrain sat in IDLE until it had a length, and the length only reached
tx_ctrl when the NCRA ST bits were written -- after the GC had streamed the
whole frame. So the entire frame had to fit in the 16-deep tx_bytes FIFO, and
BBARegisterFile silently drops bytes when w_rdy is low (SPI cannot
back-pressure). On hardware this truncated frames at exactly 16 bytes: a
captured ARP had 16 correct bytes followed by the frame restarting over itself.
The length is now handed to the drain one cycle after the TWD_HI (0x35) write,
which the GC issues BEFORE streaming data. The drain starts early and blocks
on tx_bytes_r_rdy, consuming bytes as they arrive, so tx_bytes becomes a
rate-matching buffer instead of whole-frame storage. The ST write remains a
fallback trigger for software that skips TWD, guarded by tx_queued so one frame
is never queued twice.
Adds BBATop T8: streams a 34-byte frame (> the 16-deep FIFO) over EXI and
checks the W5100 model received it byte-for-byte. Verified to FAIL without the
fix with exactly the hardware symptom -- 16/34 bytes, first bad byte at index
16. The gap that let this ship: TXFrameDrain's own tests drive the FIFO
signals directly, and the BBATop test only covered the RX loop.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Roflin
Dennis Brentjes