netdev - Re: [PATCH net-next] net: wangxun: Implement the ndo change mtu interface

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Date:   Fri, 24 Feb 2023 16:57:45 +0100
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     "mengyuanlou@...-swift.com" <mengyuanlou@...-swift.com>
CC:     <netdev@...r.kernel.org>, Jiawen Wu <jiawenwu@...stnetic.com>,
        Andrew Lunn <andrew@...n.ch>
Subject: Re: [PATCH net-next] net: wangxun: Implement the ndo change mtu
 interface

From: Mengyuan Lou <mengyuanlou@...-swift.com>
Date: Mon, 20 Feb 2023 15:49:35 +0800

> 
> 
>> 2023年2月17日 01:38，Alexander Lobakin <aleksander.lobakin@...el.com> 写道：
>>
>> From: Mengyuan Lou <mengyuanlou@...-swift.com>
>> Date: Thu, 16 Feb 2023 16:44:13 +0800

[...]

>>> +static int ngbe_change_mtu(struct net_device *netdev, int new_mtu)
>>> +{
>>> + int max_frame = new_mtu + ETH_HLEN + ETH_FCS_LEN;
>>
>> You must also account `2 * VLAN_HLEN`. The difference between MTU and
>> frame size is `ETH_HLEN + 2 * VLAN_HLEN + ETH_FCS_LEN`, i.e. 26 bytes.
>> ...except for if your device doesn't handle VLANs, but I doubt so.
> 
> The code to support vlan has not been added, so VLAN_HLEN is not considered for now.

What do you mean by "code to support vlan"? You want to say that without
some code, your NIC/driver doesn't receive VLAN-tagged packets at all or
what? VLANs can be handled purely by software if you don't configure
anything on a device. Unless you have some strange HW.

>>
>>> + struct wx *wx = netdev_priv(netdev);
>>> +
>>> + if (max_frame > WX_MAX_JUMBO_FRAME_SIZE)
>>> + return -EINVAL;
>>
>> (Andrew already said that...)
>>
>>> +
>>> + netdev_info(netdev, "Changing MTU from %d to %d.\n",
>>> +     netdev->mtu, new_mtu);
[...]

Thanks,
Olek