netdev - Re: [PATCH v2] drivers/net/wan/lapbether: Use needed_headroom instead of hard_header

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Message-ID: <CAJht_EO1srhh68DifK61+hpY+zBRU8oOAbJOSpjOqePithc7gw@mail.gmail.com>
Date:   Thu, 30 Jul 2020 01:02:10 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     Martin Schiller <ms@....tdt.de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux X25 <linux-x25@...r.kernel.org>
Subject: Re: [PATCH v2] drivers/net/wan/lapbether: Use needed_headroom instead
 of hard_header_len

Hi Martin,

I'm currently working on a plan to make all X.25 drivers (lapbether.c,
x25_asy.c, hdlc_x25.c) to set dev->hard_header_len /
dev->needed_headroom correctly. So that upper layers no longer need to
guess how much headroom a X.25 device needs with a constant value (as
they currently do).

After studying af_packet.c, I found that X.25 drivers needed to set
needed_headroom to reserve the headroom instead of using
hard_header_len. Because hard_header_len should be the length of the
header that would be created by dev_hard_header, and in this case it
should be 0, according to the logic of af_packet.c.

So my first step is to fix the settings in lapbether.c. Could you
review this patch and extend your support via a "Reviewed-by" tag? If
this can be fixed, I'll go on and fix other X.25 drivers. Thanks!

It's very hard to find reviewers for X.25 code because it is
relatively unmaintained by people. I hope I can do some of the
maintenance work. I greatly appreciate your support!