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CXS Configuration, Control Word and Packing

CXSConfig

A frozen dataclass, one per interface; with_(**changes) returns a modified copy.

Field	Spec property	Default
issue	(issue; CXSIssue, int 1-4 or 'A'-'D')	D
data_width	CXSDATAFLITWIDTH	256
max_pkt_per_flit	CXSMAXPKTPERFLIT (1 = CXS-Lite)	2
start_align	CXS_START_ALIGNMENT	16
has_last / has_prcltype	CXS_LAST / CXS_PROTOCOL_TYPE	False
srcid_width / tgtid_width / user_width	CXS_SRCID_WIDTH / CXS_TGTID_WIDTH / CXS_USER_WIDTH	0
max_credit	CXS_MAX_CREDIT	15
link_control	CXSLINKCONTROL = Explicit_Credit_Return	True
check	CXSCHECKTYPE = Odd_Byte_Parity	False
continuous	CXSCONTINUOUSDATA	False

Derived: cntl_width, cntl_fields() (the rows of IHI 0079D Table 4-5 or 4-7), check_widths() (Table 3-2), is_compatible_rx(rx) (Table 2-3), and rtl_parameters() (the cxs_master/cxs_slave parameter dict). tests/unit/test_cxs_config.py checks cntl_width and the END field position against every row of both tables.

CXSCntl

c = CXSCntl(starts=(0, 1), ends=(2, 7))       # pointers in spec units
word = c.encode(cfg)
CXSCntl.decode(cfg, word).violations(cfg, open_before=False)  # [] when legal

Start pointers are in alignment units (16 or 4 bytes). End pointers are in 4-byte units and point at the last aligned 4 bytes of the packet. violations() reports non-thermometer START/END, ENDERROR without END, non-increasing pointers, a packet ending that never started, too many packets in a flit, overlap, and (with 16-byte alignment) a start that is not the first free boundary.

Packing

CXSPacketizer(cfg).add_all(packets).flush() returns flits. It follows 4.1:

• A packet starts at the first free alignment boundary and fills every following byte until it ends or the flit ends.
• A packet that runs past the flit continues at byte 0 of the next flit.
• At most CXSMAXPKTPERFLIT packets have bytes in one flit, and all of them share one stream (protocol type, source ID, target ID).
• CXSLAST is low while a packet is open, and after a packet with keep_with_next.
• With 4-byte alignment, an optional gap_hook inserts the legal 4-byte gaps.

A stream field the interface has no wire for (for example prcltype=1 with has_prcltype=False) raises, because it would be dropped silently. CXSDepacketizer reverses the packing and records every rule a received flit breaks. tests/unit/test_cxs_packet.py reproduces the IHI 0079B Table 4-3 example exactly and round-trips random traffic for every issue.