netdev - Re: [PATCH net-next RFC v4] net: hdlc_x25: Queue outgoing LAPB frames

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Date:   Mon, 22 Feb 2021 08:14:07 +0100
From:   Martin Schiller <ms@....tdt.de>
To:     Xie He <xie.he.0141@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Leon Romanovsky <leon@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Linux X25 <linux-x25@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Krzysztof Halasa <khc@...waw.pl>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org
Subject: Re: [PATCH net-next RFC v4] net: hdlc_x25: Queue outgoing LAPB frames

On 2021-02-19 21:28, Xie He wrote:
> On Fri, Feb 19, 2021 at 10:39 AM Jakub Kicinski <kuba@...nel.org> 
> wrote:
>> 
>> Not entirely sure what the argument is about but adding constants 
>> would
>> certainly help.
> 
> Leon wants me to replace this:
> 
> dev->needed_headroom = 3 - 1;
> 
> with this:
> 
> /* 2 is the result of 3 - 1 */
> dev->needed_headroom = 2;
> 
> But I don't feel his way is better than my way.
> 
>> More fundamentally IDK if we can make such a fundamental change here.
>> When users upgrade from older kernel are all their scripts going to
>> work the same? Won't they have to bring the new netdev up?
> 
> Yes, this patch will break backward compatibility. Users with old
> scripts will find them no longer working.
> 
> However, it's hard for me to find a better way to solve the problem
> described in the commit message.
> 
> So I sent this as an RFC to see what people think about this. (Martin
> Schiller seems to be OK with this.)

Well, I said I would like to test it.

I'm not really happy with this change because it breaks compatibility.
We then suddenly have 2 interfaces; the X.25 routings are to be set via
the "new" hdlc<x>_x25 interfaces instead of the hdlc<x> interfaces.

I currently just don't have a nicer solution to fix this queueing
problem either. On the other hand, since the many years we have been
using the current state, I have never noticed any problems with
discarded frames. So it might be more a theoretical problem than a
practical one.


> 
> I think users who don't use scripts can adapt quickly and users who
> use scripts can also trivally fix their scripts.
> 
> Actually many existing commits in the kernel also (more or less) cause
> some user-visible changes. But I admit this patch is a really big
> change.