i40e: change Rx buffer size for legacy-rx to support XDP multi-buffer

Adding support for XDP multi-buffer entails adding information of all
the fragments of the packet in the xdp_buff. This approach implies that
underlying buffer has to provide tailroom for skb_shared_info.

In the legacy-rx mode, driver can only configure up to 2k sized Rx buffers
and with the current configuration of 2k sized Rx buffers there is no way
to do tailroom reservation for skb_shared_info. Hence size of Rx buffers
is now lowered to 2048 - sizeof(skb_shared_info). Also, driver can only
chain up to 5 Rx buffers and this means max MTU supported for legacy-rx
is now 8614 (5 * rx_buffer_len  - ETH header with VLAN).

Signed-off-by: Tirthendu Sarkar <tirthendu.sarkar@intel.com>
Tested-by: Chandan Kumar Rout <chandanx.rout@intel.com> (A Contingent Worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/i40e/i40e_ethtool.c

@@ -5402,6 +5402,13 @@ static int i40e_set_priv_flags(struct net_device *dev, u32 flags)
 		return -EOPNOTSUPP;
 	}
 
+	if ((changed_flags & I40E_FLAG_LEGACY_RX) &&
+	    I40E_2K_TOO_SMALL_WITH_PADDING) {
+		dev_warn(&pf->pdev->dev,
+			 "2k Rx buffer is too small to fit standard MTU and skb_shared_info\n");
+		return -EOPNOTSUPP;
+	}
+
 	if ((changed_flags & new_flags &
 	     I40E_FLAG_LINK_DOWN_ON_CLOSE_ENABLED) &&
 	    (new_flags & I40E_FLAG_MFP_ENABLED))

drivers/net/ethernet/intel/i40e/i40e_main.c

@@ -2903,7 +2903,7 @@ static void i40e_sync_filters_subtask(struct i40e_pf *pf)
 static u16 i40e_calculate_vsi_rx_buf_len(struct i40e_vsi *vsi)
 {
 	if (!vsi->netdev || (vsi->back->flags & I40E_FLAG_LEGACY_RX))
-		return I40E_RXBUFFER_2048;
+		return SKB_WITH_OVERHEAD(I40E_RXBUFFER_2048);
 
 	return PAGE_SIZE < 8192 ? I40E_RXBUFFER_3072 : I40E_RXBUFFER_2048;
 }
@@ -3661,10 +3661,16 @@ static int i40e_configure_rx_ring(struct i40e_ring *ring)
 	}
 
 	/* configure Rx buffer alignment */
-	if (!vsi->netdev || (vsi->back->flags & I40E_FLAG_LEGACY_RX))
+	if (!vsi->netdev || (vsi->back->flags & I40E_FLAG_LEGACY_RX)) {
+		if (I40E_2K_TOO_SMALL_WITH_PADDING) {
+			dev_info(&vsi->back->pdev->dev,
+				 "2k Rx buffer is too small to fit standard MTU and skb_shared_info\n");
+			return -EOPNOTSUPP;
+		}
 		clear_ring_build_skb_enabled(ring);
-	else
+	} else {
 		set_ring_build_skb_enabled(ring);
+	}
 
 	ring->rx_offset = i40e_rx_offset(ring);