Re: [PATCH net] net: airoha: Fix max TX packet length configuration

[Simon Horman <horms@kernel.org>]

From: 'Simon Horman' <horms@kernel.org>

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---
net: airoha: Fix max TX packet length configuration

This commit fixes the MTU configuration for the Airoha driver by removing
ETH_FCS_LEN from the maximum TX packet length, matching the hardware
documentation which states the register does not include the FCS length.

> diff --git a/drivers/net/ethernet/airoha/airoha_eth.c b/drivers/net/ethernet/airoha/airoha_eth.c
> index c14cdce588a7c..a81ffda72b39f 100644
> --- a/drivers/net/ethernet/airoha/airoha_eth.c
> +++ b/drivers/net/ethernet/airoha/airoha_eth.c
> @@ -1629,9 +1629,9 @@ static void airoha_update_hw_stats(struct airoha_gdm_port *port)
>  
>  static int airoha_dev_open(struct net_device *dev)
>  {
> -	int err, len = ETH_HLEN + dev->mtu + ETH_FCS_LEN;
>  	struct airoha_gdm_port *port = netdev_priv(dev);
>  	struct airoha_qdma *qdma = port->qdma;
> +	int err, len = ETH_HLEN + dev->mtu;

Does this regression cause MTU-sized VLAN-tagged frames to be dropped by the
hardware?

By limiting the frame size to ETH_HLEN + dev->mtu, the maximum allowed length
becomes 1514 bytes for a standard 1500 MTU. An Ethernet frame containing a
standard 802.1Q VLAN tag and a maximum-sized 1500-byte IP payload requires an
18-byte L2 header, making the total frame size 1518 bytes (excluding FCS).

Previously, the inclusion of ETH_FCS_LEN inadvertently provided the 4-byte
headroom needed to accommodate a single VLAN tag. 

Could the configuration explicitly account for VLAN headers using VLAN_ETH_HLEN
instead?

[ ... ]