netdev - Re: [PATCH] net: usb: pegasus: Do not drop long Ethernet frames

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Date:   Mon, 27 Dec 2021 12:56:41 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...il.com>
To:     Matthias-Christian Ott <ott@...ix.org>,
        Petko Manolov <petkan@...leusys.com>
Cc:     linux-usb@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] net: usb: pegasus: Do not drop long Ethernet frames

Hello!

On 26.12.2021 16:29, Matthias-Christian Ott wrote:

> The D-Link DSB-650TX (2001:4002) is unable to receive Ethernet frames
> that are longer than 1518 octets, for example, Ethernet frames that
> contain 802.1Q VLAN tags.
> 
> The frames are sent to the pegasus driver via USB but the driver
> discards them because they have the Long_pkt field set to 1 in the
> received status report. The function read_bulk_callback of the pegasus
> driver treats such received "packets" (in the terminology of the
> hardware) as errors but the field simply does just indicate that the
> Ethernet frame (MAC destination to FCS) is longer than 1518 octets.
> 
> It seems that in the 1990s there was a distinction between
> "giant" (> 1518) and "runt" (< 64) frames and the hardware includes
> flags to indicate this distinction. It seems that the purpose of the
> distinction "giant" frames was to not allow infinitely long frames due
> to transmission errors and to allow hardware to have an upper limit of
> the frame size. However, the hardware already has such limit with its
> 2048 octet receive buffer and, therefore, Long_pkt is merely a
> convention and should not be treated as a receive error.
> 
> Actually, the hardware is even able to receive Ethernet frames with 2048
> octets which exceeds the claimed limit frame size limit of the driver of
                                    ^^^^^            ^^^^^
    Too many limits. :-)

> 1536 octets (PEGASUS_MTU).
> 
> Signed-off-by: Matthias-Christian Ott <ott@...ix.org>
[...]

MBR, Sergey