Add jumbo frame support & fix Stream2UDPTX freeze-up

[meriac]

Why jumbo frames matter

Standard Ethernet frames are limited to a 1500-byte payload (1530 bytes including headers). For high-throughput streaming applications — such as SDR IQ sample transport — this means the CPU must process one interrupt and one DMA transaction per 1500 bytes of data. At 30.72 MSPS × 4 bytes/sample × 2 channels = 245 MB/s, that's over 160,000 packets per second, creating significant CPU overhead from interrupt handling, context switches, and per-packet protocol processing.

Jumbo frames (up to ~9000-byte payload) reduce this by ~6×: fewer packets, fewer interrupts, fewer DMA setup cycles, and better cache utilization. This directly translates to lower CPU usage and more headroom for real-time signal processing.

What changed

Previously, eth_mtu was a hardcoded global constant (1530) in liteeth/common.py, imported via from liteeth.common import * by all MAC/IP/UDP modules. There was no way to override it without patching the global at runtime — a fragile approach dependent on Python import ordering.

This commit makes eth_mtu a proper constructor parameter threaded through the entire class hierarchy, with the original value as the default for full backward compatibility.

Constants (liteeth/common.py)

• Renamed eth_mtu → eth_mtu_default (1530) to clarify it's a default, not a fixed value - and prevent accidental usage instead of the new parameter-passing approach
• Added eth_mtu_jumboframe (9022) as a standard jumbo frame constant
• Removed unused buffer_depth derived constant (would be dangerous to keep - must be calculated from the actual MTU size instead)

Parameter threading (6 source files)

Added eth_mtu=eth_mtu_default parameter to the following classes, each passing it down to its children:

• LiteEthUDPIPCore / LiteEthIPCore (core/__init__.py) → passes to LiteEthMAC
• LiteEthMAC (mac/__init__.py) → passes to LiteEthMACCore and LiteEthMACWishboneInterface
• LiteEthMACCore (mac/core.py) → passes to LiteEthMACPaddingChecker
• LiteEthMACPaddingChecker (mac/padding.py) → uses eth_mtu for length counter signal width
• LiteEthMACWishboneInterface (mac/wishbone.py) → uses eth_mtu for SRAM depth calculation and Wishbone address decoding; passes to LiteEthMACSRAM
• LiteEthMACSRAM / LiteEthMACSRAMWriter (mac/sram.py) → uses eth_mtu for the packet truncation threshold (If(length >= self.eth_mtu, DISCARD-REMAINING))

Tests (6 test files)

All test files now run at both eth_mtu_default (1530) and eth_mtu_jumboframe (9022) using unittest.subTest:

• test_arp.py, test_icmp.py, test_ip.py, test_udp.py, test_etherbone.py — DUT accepts eth_mtu parameter, test loops over both MTU values
• test_mac_wishbone.py — additionally replaces hardcoded SRAM slot offsets (0x200) with dynamically computed values based on eth_mtu and dw, since the Wishbone address decoder requires power-of-2 aligned slot boundaries that scale with MTU

[enjoy-digital]

Thanks @meriac! This looks good.