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

[Thangaraj.S]

> -----Original Message-----
> From: David Thompson <davthompson@xxxxxxxxxx>
> Sent: Saturday, May 30, 2026 2:33 AM
> To: Bryan Whitehead - C21958 <Bryan.Whitehead@xxxxxxxxxxxxx>;
> UNGLinuxDriver <UNGLinuxDriver@xxxxxxxxxxxxx>;
> andrew+netdev@xxxxxxx; davem@xxxxxxxxxxxxx; edumazet@xxxxxxxxxx;
> kuba@xxxxxxxxxx; pabeni@xxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; David Thompson
> <davthompson@xxxxxxxxxx>
> Subject: [PATCH net v2] net: lan743x: permit VLAN-tagged packets up to
> configured MTU
> 
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the
> content is safe
> 
> 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@xxxxxxxxxx>

Reviewed-by: Thangaraj Samynathan <Thangaraj.s@xxxxxxxxxxxxx>
> ---
> 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)
> 
> --
> 2.43.0