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From:   Xie He <xie.he.0141@gmail.com>
Date:   Fri, 19 Feb 2021 12:28:12 -0800
Message-ID: <CAJht_EOru3pW6AHN4QVjiaERpLSfg-0G0ZEaqU_hkhX1acv0HQ@mail.gmail.com>
Subject: Re: [PATCH net-next RFC v4] net: hdlc_x25: Queue outgoing LAPB frames
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     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>,
        Martin Schiller <ms@dev.tdt.de>,
        Krzysztof Halasa <khc@pm.waw.pl>,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org
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On Fri, Feb 19, 2021 at 10:39 AM Jakub Kicinski <kuba@kernel.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.)

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.