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From: =?iso-8859-1?Q?Sanju=E1n_Garc=EDa=2C_Jorge?=
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To: "andrew@lunn.ch" <andrew@lunn.ch>, "davem@davemloft.net"
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CC: "grygorii.strashko@ti.com" <grygorii.strashko@ti.com>,
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Subject: [PATCH net v2 0/1] net: ethernet: ti: am65-cpsw: Allow for MTU values
Thread-Topic: [PATCH net v2 0/1] net: ethernet: ti: am65-cpsw: Allow for MTU
 values
Thread-Index: AQHaP7T0fi4egi9ldE6vGFwYnBU0hQ==
Date: Fri, 5 Jan 2024 08:55:39 +0000
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The am65-cpsw-nuss driver has a fixed definition for the maximum ethernet
frame length of 1522 bytes (AM65_CPSW_MAX_PACKET_SIZE). This limits the swi=
tch
ports to only operate at a maximum MTU of 1500 bytes. When combining this C=
PSW
switch with a DSA switch connected to one of its ports this limitation show=
s up.
The extra 8 bytes the DSA subsystem adds internally to the ethernet frame
create resulting frames bigger than 1522 bytes (1518 for non VLAN + 8 for D=
SA
stuff) so they get dropped by the switch.

One of the issues with the the am65-cpsw-nuss driver is that the network de=
vice
max_mtu was being set to the same fixed value defined for the max total fra=
me
length (1522 bytes). This makes the DSA subsystem believe that the MTU of t=
he
interface can be set to 1508 bytes to make room for the extra 8 bytes of th=
e DSA
headers. However, all packages created assuming the 1500 bytes payload get
dropped by the switch as oversized.

This series offers a solution to this problem. The max_mtu advertised on th=
e
network device and the actual max frame size configured on the switch regis=
ters
are made consistent by letting the extra room needed for the ethernet heade=
rs
and the frame checksum (22 bytes including VLAN).

Changes in v2:
 - Just set the max_mtu to a fixed value and calculate the overheads.
 - Define the max packet size as part of the only commit of this series.

Jorge Sanjuan Garcia (1):
  net: ethernet: ti: am65-cpsw: Fix max mtu to fit ethernet frames

 drivers/net/ethernet/ti/am65-cpsw-nuss.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--=20
2.34.1