netdev - Re: [PATCH net-next RFC v3] net: hdlc_x25: Queue outgoing LAPB frames

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Message-ID: <YCrDcMYgSgdKp4eX@unreal>
Date:   Mon, 15 Feb 2021 20:54:40 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Xie He <xie.he.0141@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux X25 <linux-x25@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Martin Schiller <ms@....tdt.de>,
        Krzysztof Halasa <khc@...waw.pl>
Subject: Re: [PATCH net-next RFC v3] net: hdlc_x25: Queue outgoing LAPB frames

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@...nel.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;