On Wed, Mar 13, 2024 at 10:07:16AM +0100, Erwan Velu wrote:
> Commit 6871a7de705 ("[intelxl] Use admin queue to set port MAC address
> and maximum frame size") from iPXE project set the MFS to 0x600 = 1536.
> See https://github.com/ipxe/ipxe/commit/6871a7de705
>
> At boot time the i40e driver complains about it with
> the following message but continues.
>
> MFS for port 1 has been set below the default: 600
>
> If the MTU size is increased, the driver accepts it but large packets will
> not be processed by the firmware generating tx_errors. The issue is pretty
> silent for users. i.e doing TCP in such context will generates lots of
> retransmissions until the proper window size (below 1500) will be used.
>
> To fix this case, it would have been ideal to increase the MFS,
> via i40e_aqc_opc_set_mac_config, incoming patch will take care of it.
>
> At least, commit prevents setting up an MTU greater than the current MFS.
> It will avoid being in the position of having an MTU set to 9000 on the
> netdev with a firmware refusing packets larger than 1536.
>
> A typical trace looks like:
> [ 377.548696] i40e 0000:5d:00.0 eno5: Error changing mtu to 9000, Max is
> 1500. MFS is too small.
>
Hi Erwan, all,
As a fix, I think this patch warrants a fixes tag.
Perhaps this one is appropriate?
Fixes: 41c445ff0f48 ("i40e: main driver core")
> Signed-off-by: Erwan Velu <e.v...@criteo.com>
> ---
> drivers/net/ethernet/intel/i40e/i40e_main.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c
> b/drivers/net/ethernet/intel/i40e/i40e_main.c
> index f86578857e8a..85ecf2f3de18 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_main.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
> @@ -2946,7 +2946,7 @@ static int i40e_change_mtu(struct net_device *netdev,
> int new_mtu)
> struct i40e_netdev_priv *np = netdev_priv(netdev);
> struct i40e_vsi *vsi = np->vsi;
> struct i40e_pf *pf = vsi->back;
> - int frame_size;
> + int frame_size, mfs, max_mtu;
>
> frame_size = i40e_max_vsi_frame_size(vsi, vsi->xdp_prog);
> if (new_mtu > frame_size - I40E_PACKET_HDR_PAD) {
I am fine with this patch, so please take what follows as a suggestion
for improvement, possibly as a follow-up. Not as a hard requirement from
my side.
The part of this function between the two hunks of this patch is:
netdev_err(netdev, "Error changing mtu to %d, Max is %d\n",
new_mtu, frame_size - I40E_PACKET_HDR_PAD);
My reading is that with this patch two different limits are
checked wrt maximum MTU size:
1. A VSI level limit, which relates to RX buffer size
2. A PHY level limit that relates to the MFS
That seems fine to me. But the log message for 1 (above) does
not seem particularly informative wrt which limit has been exceeded.
> @@ -2955,6 +2955,14 @@ static int i40e_change_mtu(struct net_device *netdev,
> int new_mtu)
> return -EINVAL;
> }
>
> + mfs = pf->hw.phy.link_info.max_frame_size;
> + max_mtu = mfs - I40E_PACKET_HDR_PAD;
> + if (new_mtu > max_mtu) {
> + netdev_err(netdev, "Error changing mtu to %d, Max is %d. MFS is
> too small.\n",
> + new_mtu, max_mtu);
> + return -EINVAL;
> + }
> +
> netdev_dbg(netdev, "changing MTU from %d to %d\n",
> netdev->mtu, new_mtu);
> netdev->mtu = new_mtu;
> --
> 2.44.0
>
>
