Re: [PATCH net v2] net: lan743x: permit VLAN-tagged packets up to configured MTU

[Nicolai Buchwitz <nb@tipi-net.de>]

Hi David

On 29.5.2026 23:03, David Thompson wrote:
> VLAN-tagged interfaces on lan743x devices were previously unreachable 
> via
> SSH and failed to respond to large ping packets (e.g. "ping -s 1469" 
> given
> MTU=1500). In these scenarios, "ethtool -S" reports non-zero "RX 
> Oversize
> Frame Errors". According to Microchip AN2948, the MAC_RX FSE (VLAN 
> field
> size enforcement) bit determines whether frames with VLAN tags 
> exceeding
> the base MTU plus tag length are discarded.
> 
> The driver must set the MAC_RX.FSE bit before setting MAC_RX.RXEN to 
> allow
> VLAN-tagged frames up to the interface MTU, preventing them from being
> treated as oversized. As a result, both the base and VLAN-tagged 
> interfaces
> can use the same MTU without receive errors.
> 
> Fixes: 23f0703c125b ("lan743x: Add main source files for new lan743x 
> driver")
> Signed-off-by: David Thompson <davthompson@nvidia.com>
> ---
> v2:
> - Added helper function, lan743x_mac_rx_enable_fse(), to set FSE bit 
> properly
> - Added explicit comment about use of distinct write cycle to set FSE 
> bit
> - Moved setting of FSE from lan743x_mac_open() to lan743x_mac_init(), 
> allowing FSE
>   bit to be set upon probe() path or suspend/resume path. This change 
> fixes an
>   issue raised by Sashiko
> ---
>  drivers/net/ethernet/microchip/lan743x_main.c | 32 +++++++++++++++++++
>  drivers/net/ethernet/microchip/lan743x_main.h |  1 +
>  2 files changed, 33 insertions(+)
> 
> diff --git a/drivers/net/ethernet/microchip/lan743x_main.c 
> b/drivers/net/ethernet/microchip/lan743x_main.c
> index f3332417162e..ffac22883e49 100644
> --- a/drivers/net/ethernet/microchip/lan743x_main.c
> +++ b/drivers/net/ethernet/microchip/lan743x_main.c
> @@ -1219,6 +1219,36 @@ static void lan743x_mac_set_address(struct 
> lan743x_adapter *adapter,
>  		   "MAC address set to %pM\n", addr);
>  }
> 
> +static void lan743x_mac_rx_enable_fse(struct lan743x_adapter *adapter)
> +{
> +	u32 mac_rx;
> +	bool rxen;
> +
> +	mac_rx = lan743x_csr_read(adapter, MAC_RX);
> +	if (mac_rx & MAC_RX_FSE_)
> +		return;
> +
> +	rxen = mac_rx & MAC_RX_RXEN_;
> +	if (rxen) {
> +		mac_rx &= ~MAC_RX_RXEN_;
> +		lan743x_csr_write(adapter, MAC_RX, mac_rx);
> +		lan743x_csr_wait_for_bit(adapter, MAC_RX, MAC_RX_RXD_,
> +					 1, 1000, 20000, 100);
> +	}
> +
> +	/* Per AN2948, hardware prevents modification of the FSE bit while 
> the
> +	 * MAC receiver is enabled (RXEN bit set). Use separate register 
> write
> +	 * to assert the FSE bit before enabling the RXEN bit in MAC_RX
> +	 */
> +	mac_rx |= MAC_RX_FSE_;
> +	lan743x_csr_write(adapter, MAC_RX, mac_rx);
> +
> +	if (rxen) {
> +		mac_rx |= MAC_RX_RXEN_;
> +		lan743x_csr_write(adapter, MAC_RX, mac_rx);
> +	}
> +}
> +
>  static int lan743x_mac_init(struct lan743x_adapter *adapter)
>  {
>  	bool mac_address_valid = true;
> @@ -1258,6 +1288,8 @@ static int lan743x_mac_init(struct 
> lan743x_adapter *adapter)
>  	lan743x_mac_set_address(adapter, adapter->mac_address);
>  	eth_hw_addr_set(netdev, adapter->mac_address);
> 
> +	lan743x_mac_rx_enable_fse(adapter);
> +
>  	return 0;
>  }
> 
> diff --git a/drivers/net/ethernet/microchip/lan743x_main.h 
> b/drivers/net/ethernet/microchip/lan743x_main.h
> index 160d94a7cee6..1573c8f9c993 100644
> --- a/drivers/net/ethernet/microchip/lan743x_main.h
> +++ b/drivers/net/ethernet/microchip/lan743x_main.h
> @@ -182,6 +182,7 @@
>  #define MAC_RX				(0x104)
>  #define MAC_RX_MAX_SIZE_SHIFT_		(16)
>  #define MAC_RX_MAX_SIZE_MASK_		(0x3FFF0000)
> +#define MAC_RX_FSE_			BIT(2)
>  #define MAC_RX_RXD_			BIT(1)
>  #define MAC_RX_RXEN_			BIT(0)

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Tested-by: Nicolai Buchwitz <nb@tipi-net.de> # lan7430 on arm64 (RevPi 
Connect 5)

Thanks,
Nicolai