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

From: Martin Schiller <[email-masked]>
Date: Mon, 01 Feb 2021 10:18:23 +0100

On 2021-01-31 04:16, Xie He wrote:
> On Sat, Jan 30, 2021 at 11:16 AM Jakub Kicinski <[email-masked]> 
> wrote:
>> 
>> Sounds like too much afford for a sub-optimal workaround.
>> The qdisc semantics are borken in the proposed scheme (double
>> counting packets) - both in term of statistics and if user decides
>> to add a policer, filter etc.
> 
> Hmm...
> 
> Another solution might be creating another virtual device on top of
> the HDLC device (similar to what "hdlc_fr.c" does), so that we can
> first queue L3 packets in the virtual device's qdisc queue, and then
> queue the L2 frames in the actual HDLC device's qdisc queue. This way
> we can avoid the same outgoing data being queued to qdisc twice. But
> this would significantly change the way the user uses the hdlc_x25
> driver.
> 
>> Another worry is that something may just inject a packet with
>> skb->protocol == ETH_P_HDLC but unexpected structure (IDK if
>> that's a real concern).
> 
> This might not be a problem. Ethernet devices also allow the user to
> inject raw frames with user constructed headers. "hdlc_fr.c" also
> allows the user to bypass the virtual circuit interfaces and inject
> raw frames directly on the HDLC interface. I think the receiving side
> should be able to recognize and drop invalid frames.
> 
>> It may be better to teach LAPB to stop / start the internal queue.
>> The lower level drivers just needs to call LAPB instead of making
>> the start/wake calls directly to the stack, and LAPB can call the
>> stack. Would that not work?
> 
> I think this is a good solution. But this requires changing a lot of
> code. The HDLC subsystem needs to be changed to allow HDLC Hardware
> Drivers to ask HDLC Protocol Drivers (like hdlc_x25.c) to stop/wake
> the TX queue. The hdlc_x25.c driver can then ask the LAPB module to
> stop/wake the queue.
> 
> So this means new APIs need to be added to both the HDLC subsystem and
> the LAPB module, and a number of HDLC Hardware Drivers need to be
> changed to call the new API of the HDLC subsystem.
> 
> Martin, do you have any suggestions?

I have thought about this issue again.

I also have to say that I have never noticed any problems in this area
before.

So again for (my) understanding:
When a hardware driver calls netif_stop_queue, the frames sent from
layer 3 (X.25) with dev_queue_xmit are queued and not passed "directly"
to x25_xmit of the hdlc_x25 driver.

So nothing is added to the write_queue anymore (except possibly
un-acked-frames by lapb_requeue_frames).

Shouldn't it actually be sufficient to check for netif_queue_stopped in
lapb_kick and then do "nothing" if necessary?

As soon as the hardware driver calls netif_wake_queue, the whole thing
should just continue running.

Or am I missing something?