From 398f6722fca0b3cdc96f50c806c9c4c1260c1c87 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Fin=20Maa=C3=9F?= <f.maass@vogl-electronic.com>
Date: Mon, 3 Feb 2025 18:49:37 +0100
Subject: [PATCH] mac: LiteEthMACPaddingChecker
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implement LiteEthMACPaddingChecker, to drop
packet when payload size < minimum ethernet payload size

Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>
---
 liteeth/mac/padding.py | 38 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 36 insertions(+), 2 deletions(-)

diff --git a/liteeth/mac/padding.py b/liteeth/mac/padding.py
index 9ea37a0c..44ceab46 100644
--- a/liteeth/mac/padding.py
+++ b/liteeth/mac/padding.py
@@ -5,6 +5,7 @@
 # Copyright (c) 2021 David Sawatzke <d-git@sawatzke.dev>
 # Copyright (c) 2015 Sebastien Bourdeauducq <sb@m-labs.hk>
 # Copyright (c) 2018 whitequark <whitequark@whitequark.org>
+# Copyright (c) 2025 Fin Maaß <f.maass@vogl-electronic.com>
 # SPDX-License-Identifier: BSD-2-Clause
 
 import math
@@ -74,7 +75,40 @@ def __init__(self, dw, packet_min_length):
 
         # # #
 
-        # TODO: see if we should drop the packet when
+        # drop the packet when
         # payload size < minimum ethernet payload size
-        self.comb += sink.connect(source)
 
+        length     = Signal(max=eth_mtu)
+        length_inc = Signal(4)
+
+        # Decode Length increment from from last_be.
+        self.comb += Case(sink.last_be, {
+            0b00000001 : length_inc.eq(1),
+            0b00000010 : length_inc.eq(2),
+            0b00000100 : length_inc.eq(3),
+            0b00001000 : length_inc.eq(4),
+            0b00010000 : length_inc.eq(5),
+            0b00100000 : length_inc.eq(6),
+            0b01000000 : length_inc.eq(7),
+            "default"  : length_inc.eq(dw//8)
+        })
+
+        self.sync += [
+            If(sink.valid & sink.ready,
+                If(sink.last,
+                    length.eq(0),
+                ).Else(
+                    length.eq(length + length_inc)
+                )
+            )
+        ]
+
+        self.comb += [
+            sink.connect(source, omit={"error"}),
+
+            If(sink.valid & sink.last & ((length + length_inc) < packet_min_length),
+                source.error.eq(Replicate(1, dw//8)),
+            ).Else(
+                source.error.eq(sink.error),
+            )
+        ]