liteeth/mac/core: Allow PHY to enforce with_preamble_crc/with_padding parameters.

Avoid exposing these parameters up to add_ethernet since appropriate behaviour is generally
directly related to the type of PHY (ex LiteEthPHYModel or custom/specialized PHY).

liteeth/mac/core.py

@@ -10,7 +10,6 @@
 
 from liteeth.common import *
 from liteeth.mac import gap, preamble, crc, padding, last_be
-from liteeth.phy.model import LiteEthPHYModel
 
 from migen.genlib.cdc import PulseSynchronizer
 
@@ -46,8 +45,12 @@ class LiteEthMACCore(Module, AutoCSR):
             cd_tx       = "eth_tx"
             cd_rx       = "eth_rx"
             datapath_dw = phy_dw
-        if isinstance(phy, LiteEthPHYModel):
-            with_preamble_crc = False # Disable Preamble/CRC with PHY Model for direct connection to the Ethernet tap.
+
+        # If the PHY specifies preamble, CRC, or padding behavior, use it.
+        if hasattr(phy, "with_preamble_crc"):
+            with_preamble_crc = phy.with_preamble_crc
+        if hasattr(phy, "with_padding"):
+            with_padding = phy.with_padding
 
         # CSRs.
         if with_preamble_crc:

liteeth/phy/model.py

@@ -35,6 +35,7 @@ class LiteEthPHYModelCRG(LiteXModule):
 
 class LiteEthPHYModel(LiteXModule):
     dw                = 8
+    with_preamble_crc = False # Disable Preamble/CRC with for direct connection to the Ethernet tap.
     def __init__(self, pads):
         self.crg    = LiteEthPHYModelCRG()
         self.sink   = sink   = stream.Endpoint(eth_phy_description(8))