i40e: Fix the inability to attach XDP program on downed interface

Whenever trying to load XDP prog on downed interface, function i40e_xdp
was passing vsi->rx_buf_len field to i40e_xdp_setup() which was equal 0.
i40e_open() calls i40e_vsi_configure_rx() which configures that field,
but that only happens when interface is up. When it is down, i40e_open()
is not being called, thus vsi->rx_buf_len is not set.

Solution for this is calculate buffer length in newly created
function - i40e_calculate_vsi_rx_buf_len() that return actual buffer
length. Buffer length is being calculated based on the same rules
applied previously in i40e_vsi_configure_rx() function.

Fixes: 613142b ("i40e: Log error for oversized MTU on device")
Fixes: 0c8493d ("i40e: add XDP support for pass and drop actions")
Signed-off-by: Bartosz Staszewski <bartoszx.staszewski@intel.com>
Signed-off-by: Mateusz Palczewski <mateusz.palczewski@intel.com>
Tested-by: Shwetha Nagaraju <Shwetha.nagaraju@intel.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Reviewed-by: Saeed Mahameed <saeed@kernel.com>
Link: https://lore.kernel.org/r/20221209185411.2519898-1-anthony.l.nguyen@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -3693,6 +3693,24 @@ static int i40e_vsi_configure_tx(struct i40e_vsi *vsi)
 	return err;
 }
 
+/**
+ * i40e_calculate_vsi_rx_buf_len - Calculates buffer length
+ *
+ * @vsi: VSI to calculate rx_buf_len from
+ */
+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;
+
+#if (PAGE_SIZE < 8192)
+	if (!I40E_2K_TOO_SMALL_WITH_PADDING && vsi->netdev->mtu <= ETH_DATA_LEN)
+		return I40E_RXBUFFER_1536 - NET_IP_ALIGN;
+#endif
+
+	return PAGE_SIZE < 8192 ? I40E_RXBUFFER_3072 : I40E_RXBUFFER_2048;
+}
+
 /**
  * i40e_vsi_configure_rx - Configure the VSI for Rx
  * @vsi: the VSI being configured
@@ -3704,20 +3722,14 @@ static int i40e_vsi_configure_rx(struct i40e_vsi *vsi)
 	int err = 0;
 	u16 i;
 
-	if (!vsi->netdev || (vsi->back->flags & I40E_FLAG_LEGACY_RX)) {
-		vsi->max_frame = I40E_MAX_RXBUFFER;
-		vsi->rx_buf_len = I40E_RXBUFFER_2048;
+	vsi->max_frame = I40E_MAX_RXBUFFER;
+	vsi->rx_buf_len = i40e_calculate_vsi_rx_buf_len(vsi);
+
 #if (PAGE_SIZE < 8192)
-	} else if (!I40E_2K_TOO_SMALL_WITH_PADDING &&
-		   (vsi->netdev->mtu <= ETH_DATA_LEN)) {
+	if (vsi->netdev && !I40E_2K_TOO_SMALL_WITH_PADDING &&
+	    vsi->netdev->mtu <= ETH_DATA_LEN)
 		vsi->max_frame = I40E_RXBUFFER_1536 - NET_IP_ALIGN;
-		vsi->rx_buf_len = I40E_RXBUFFER_1536 - NET_IP_ALIGN;
 #endif
-	} else {
-		vsi->max_frame = I40E_MAX_RXBUFFER;
-		vsi->rx_buf_len = (PAGE_SIZE < 8192) ? I40E_RXBUFFER_3072 :
-						       I40E_RXBUFFER_2048;
-	}
 
 	/* set up individual rings */
 	for (i = 0; i < vsi->num_queue_pairs && !err; i++)
@@ -13282,7 +13294,7 @@ static int i40e_xdp_setup(struct i40e_vsi *vsi, struct bpf_prog *prog,
 	int i;
 
 	/* Don't allow frames that span over multiple buffers */
-	if (frame_size > vsi->rx_buf_len) {
+	if (frame_size > i40e_calculate_vsi_rx_buf_len(vsi)) {
 		NL_SET_ERR_MSG_MOD(extack, "MTU too large to enable XDP");
 		return -EINVAL;
 	}