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Date:   Wed, 03 Mar 2021 14:26:35 +0100
From:   Martin Schiller <ms@dev.tdt.de>
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     Xie He <xie.he.0141@gmail.com>, Leon Romanovsky <leon@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Linux X25 <linux-x25@vger.kernel.org>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Krzysztof Halasa <khc@pm.waw.pl>,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org
Subject: Re: [PATCH net-next RFC v4] net: hdlc_x25: Queue outgoing LAPB frames
Organization: TDT AG
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On 2021-03-03 00:30, Jakub Kicinski wrote:
> On Tue, 02 Mar 2021 08:04:20 +0100 Martin Schiller wrote:
>> On 2021-03-01 09:56, Xie He wrote:
>> > On Sun, Feb 28, 2021 at 10:56 PM Martin Schiller <ms@dev.tdt.de> wrote:
>> >> I mean the change from only one hdlc<x> interface to both hdlc<x> and
>> >> hdlc<x>_x25.
>> >>
>> >> I can't estimate how many users are out there and how their setup
>> >> looks
>> >> like.
>> >
>> > I'm also thinking about solving this issue by adding new APIs to the
>> > HDLC subsystem (hdlc_stop_queue / hdlc_wake_queue) for hardware
>> > drivers to call instead of netif_stop_queue / netif_wake_queue. This
>> > way we can preserve backward compatibility.
>> >
>> > However I'm reluctant to change the code of all the hardware drivers
>> > because I'm afraid of introducing bugs, etc. When I look at the code
>> > of "wan/lmc/lmc_main.c", I feel I'm not able to make sure there are no
>> > bugs (related to stop_queue / wake_queue) after my change (and even
>> > before my change, actually). There are even serious style problems:
>> > the majority of its lines are indented by spaces.
>> >
>> > So I don't want to mess with all the hardware drivers. Hardware driver
>> > developers (if they wish to properly support hdlc_x25) should do the
>> > change themselves. This is not a problem for me, because I use my own
>> > out-of-tree hardware driver. However if I add APIs with no user code
>> > in the kernel, other developers may think these APIs are not
>> > necessary.
>> 
>> I don't think a change that affects the entire HDLC subsystem is
>> justified, since the actual problem only affects the hdlc_x25 area.
>> 
>> The approach with the additional hdlc<x>_x25 is clean and purposeful 
>> and
>> I personally could live with it.
>> 
>> I just don't see myself in the position to decide such a change at the
>> moment.
>> 
>> @Jakub: What is your opinion on this.
> 
> Hard question to answer, existing users seem happy and Xie's driver
> isn't upstream, so the justification for potentially breaking backward
> compatibility isn't exactly "strong".
> 
> Can we cop out and add a knob somewhere to control spawning the extra
> netdev? Let people who just want a newer kernel carry on without
> distractions and those who want the extra layer can flip the switch?

Yes, that would be a good compromise.
I think a compile time selection option is enough here.
We could introduce a new config option CONFIG_HDLC_X25_LEGACY (or
something like that) and then implement the new or the old behavior in
the driver accordingly.

A switch that can be toggled at runtime (e.g. via sethdlc) would also be
conceivable, but I don't think this is necessary.