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

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Date:   Mon, 1 Feb 2021 06:02:44 -0800
From:   Xie He <xie.he.0141@...il.com>
To:     Martin Schiller <ms@....tdt.de>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "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>,
        Krzysztof Halasa <khc@...waw.pl>
Subject: Re: [PATCH net] net: hdlc_x25: Use qdisc to queue outgoing LAPB frames

On Mon, Feb 1, 2021 at 5:14 AM Martin Schiller <ms@....tdt.de> wrote:
>
> But control frames are currently sent past the lapb write_queue.
> So another queue would have to be created.
>
> And wouldn't it be better to have it in the hdlc_x25 driver, leaving
> LAPB unaffected?

Hmm.. Indeed. I agree.

I also think the queue needs to be the qdisc queue, so that it'll be
able to respond immediately to hardware drivers' netif_wake_queue
call.

Initially I was considering using the qdisc of the HDLC device to
queue the outgoing L2 frames again (after their corresponding L3
packets having already gone through the queue). But Jakub didn't like
the idea of queuing the same data twice. I also found that if an L3
packet was sent through the qdisc without being queued, and LAPB
didn't queue it either, then the emitted L2 frame must be queued in
the qdisc. This is both not optimal and causing problems when using
the "noqueue" qdisc.

Maybe the only way is to create a virtual device on top of the HDLC
device, using the virtual device to queue L3 packets and using the
actual HDLC device to queue L2 frames.