Re: [PATCH net-next RFC v4] net: hdlc_x25: Queue outgoing LAPB frames

[Martin Schiller]

On 2021-02-22 09:56, Xie He wrote:
> On Sun, Feb 21, 2021 at 11:14 PM Martin Schiller <ms@xxxxxxxxxx> wrote:
> > I'm not really happy with this change because it breaks compatibility.
> > We then suddenly have 2 interfaces; the X.25 routings are to be set 
> > via
> > the "new" hdlc<x>_x25 interfaces instead of the hdlc<x> interfaces.
> >
> > I currently just don't have a nicer solution to fix this queueing
> > problem either. On the other hand, since the many years we have been
> > using the current state, I have never noticed any problems with
> > discarded frames. So it might be more a theoretical problem than a
> > practical one.
>
> This problem becomes very serious when we use AF_PACKET sockets,
> because the majority of frames would be dropped by the hardware
> driver, which significantly impacts transmission speed. What I am
> really doing is to enable adequate support for AF_PACKET sockets,
> allowing users to use the bare (raw) LAPB protocol. If we take this
> into consideration, this problem is no longer just a theoretical
> problem, but a real practical issue.

I have now had a look at it. It works as expected.
I just wonder if it would not be more appropriate to call
the lapb_register() already in x25_hdlc_open(), so that the layer2
(lapb) can already "work" before the hdlc<x>_x25 interface is up.


Also, I have a hard time assessing if such a wrap is really enforceable.
Unfortunately I have no idea how many users there actually are.


> If we don't want to break backward compatibility, there is another 
> option:
> We can create a new API for the HDLC subsystem for stopping/restarting
> the TX queue, and replace all HDLC hardware drivers' netif_stop_queue
> and netif_wake_queue calls with calls to this new API. This new API
> would then call hdlc_x25 to stop/restart its internal queue.
>
> But this option would require modifying all HDLC hardware drivers'
> code, and frankly, not all HDLC hardware drivers' developers care
> about running X.25 protocols on their hardware. So this would cause
> both hardware driver instabilities and confusion for hardware driver
> developers.