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From:   Xie He <xie.he.0141@gmail.com>
Date:   Mon, 15 Feb 2021 11:08:02 -0800
Message-ID: <CAJht_EPy1Us72YGMune2G3s1TLB4TOCBFJpZt+KbVUV8uoFbfA@mail.gmail.com>
Subject: Re: [PATCH net-next RFC v3] net: hdlc_x25: Queue outgoing LAPB frames
To:     Leon Romanovsky <leon@kernel.org>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        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>
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On Mon, Feb 15, 2021 at 10:54 AM Leon Romanovsky <leon@kernel.org> wrote:
>
> On Mon, Feb 15, 2021 at 09:23:32AM -0800, Xie He wrote:
> > On Mon, Feb 15, 2021 at 1:25 AM Leon Romanovsky <leon@kernel.org> wrote:
> > >
> > > > +     /* When transmitting data:
> > > > +      * first we'll remove a pseudo header of 1 byte,
> > > > +      * then the LAPB module will prepend an LAPB header of at most 3 bytes.
> > > > +      */
> > > > +     dev->needed_headroom = 3 - 1;
> > >
> > > 3 - 1 = 2
> > >
> > > Thanks
> >
> > Actually this is intentional. It makes the numbers more meaningful.
> >
> > The compiler should automatically generate the "2" so there would be
> > no runtime penalty.
>
> If you want it intentional, write it in the comment.
>
> /* When transmitting data, we will need extra 2 bytes headroom,
>  * which are 3 bytes of LAPB header minus one byte of pseudo header.
>  */
>  dev->needed_headroom = 2;

I think this is unnecessary. The current comment already explains the
meaning of the "1" and the "3". There's no need for a reader of this
code to understand what a "2" is. That is the job of the compiler, not
the human reader.