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

[David Thompson <davthompson@nvidia.com>]

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)
 
-- 
2.43.0