From mboxrd@z Thu Jan  1 00:00:00 1970
From: Leon Romanovsky <leon@kernel.org>
Subject: Re: [PATCH net-next RFC v3] net: hdlc_x25: Queue outgoing LAPB frames
Date: Mon, 15 Feb 2021 20:54:40 +0200
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To: Xie He <xie.he.0141@gmail.com>
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>

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;