netdev - RE: [PATCH net 1/2] net/mlx4_en: Change min MTU size to ETH_MIN

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Date:   Tue, 4 Dec 2018 16:59:17 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Tariq Toukan' <tariqt@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Eran Ben Elisha <eranbe@...lanox.com>
Subject: RE: [PATCH net 1/2] net/mlx4_en: Change min MTU size to ETH_MIN_MTU

From: Tariq Toukan
> Sent: 02 December 2018 12:35
> From: Eran Ben Elisha <eranbe@...lanox.com>
> 
> NIC driver minimal MTU size shall be set to ETH_MIN_MTU, as defined in
> the RFC791 and in the network stack. Remove old mlx4_en only define for
> it, which was set to wrong value.
...
> 
> -	/* MTU range: 46 - hw-specific max */
> -	dev->min_mtu = MLX4_EN_MIN_MTU;
> +	/* MTU range: 68 - hw-specific max */
> +	dev->min_mtu = ETH_MIN_MTU;
>  	dev->max_mtu = priv->max_mtu;

Where does 68 come from?
The minimum size of an ethernet packet including the mac addresses
and CRC is 64 bytes - but that would never be an 'mtu'.

Since 64 - 46 = 18, the 46 probably excludes both MAC addresses,
the ethertype/length and the CRC.
This is 'sort of' the minimum mtu for an ethernet frame.

I'm not sure which values are supposed to be in dev->min/max_mtu.

	David

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