From mboxrd@z Thu Jan 1 00:00:00 1970 From: Martin Schiller Subject: Re: [PATCH net] net: hdlc_x25: Use qdisc to queue outgoing LAPB frames Date: Mon, 01 Feb 2021 10:18:23 +0100 Message-ID: <36a6c0769c57cd6835d32cc0fb95bca6@dev.tdt.de> References: <20210127090747.364951-1-xie.he.0141@gmail.com> <20210128114659.2d81a85f@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> <3f67b285671aaa4b7903733455a730e1@dev.tdt.de> <20210129173650.7c0b7cda@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> <20210130111618.335b6945@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Xie He Cc: Jakub Kicinski , "David S. Miller" , Linux X25 , Linux Kernel Network Developers , LKML , Krzysztof Halasa On 2021-01-31 04:16, Xie He wrote: > On Sat, Jan 30, 2021 at 11:16 AM Jakub Kicinski > 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?