linux-kernel - Re: [PATCH net] net: hdlc_x25: Use qdisc to queue outgoing LAPB frames

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Date:   Thu, 28 Jan 2021 14:06:58 -0800
From:   Xie He <xie.he.0141@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     "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>,
        Martin Schiller <ms@....tdt.de>,
        Krzysztof Halasa <khc@...waw.pl>
Subject: Re: [PATCH net] net: hdlc_x25: Use qdisc to queue outgoing LAPB frames

On Thu, Jan 28, 2021 at 11:47 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> Noob question - could you point at or provide a quick guide to layering
> here? I take there is only one netdev, and something maintains an
> internal queue which is not stopped when HW driver stops the qdisc?

Yes, there is only one netdev. The LAPB module (net/lapb/) (which is
used as a library by the netdev driver - hdlc_x25.c) is maintaining an
internal queue which is not stopped when the HW driver stops the
qdisc.

The queue is "write_queue" in "struct lapb_cb" in
"include/net/lapb.h". The code that takes skbs out of the queue and
feeds them to lower layers for transmission is at the "lapb_kick"
function in "net/lapb/lapb_out.c".

The layering is like this:

Upper layer (Layer 3) (net/x25/ or net/packet/)

^
| L3 packets (with control info)
v

The netdev driver (hdlc_x25.c)

^
| L3 packets
v

The LAPB Module (net/lapb/)

^
| LAPB (L2) frames
v

The netdev driver (hdlc_x25.c)

^
| LAPB (L2) frames
| (also called HDLC frames in the context of the HDLC subsystem)
v

HDLC core (hdlc.c)

^
| HDLC frames
v

HDLC Hardware Driver

> Sounds like we're optimizing to prevent drops, and this was not
> reported from production, rather thru code inspection. Ergo I think
> net-next will be more appropriate here, unless Martin disagrees.

Yes, I have no problem in targeting net-next instead. Thanks!