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

[Martin Schiller]

On 2021-03-03 00:30, Jakub Kicinski wrote:
> On Tue, 02 Mar 2021 08:04:20 +0100 Martin Schiller wrote:
> > On 2021-03-01 09:56, Xie He wrote:
> > > On Sun, Feb 28, 2021 at 10:56 PM Martin Schiller <ms@xxxxxxxxxx> wrote:
> > >> I mean the change from only one hdlc<x> interface to both hdlc<x> and
> > >> hdlc<x>_x25.
> > >>
> > >> I can't estimate how many users are out there and how their setup
> > >> looks
> > >> like.
> > >
> > > I'm also thinking about solving this issue by adding new APIs to the
> > > HDLC subsystem (hdlc_stop_queue / hdlc_wake_queue) for hardware
> > > drivers to call instead of netif_stop_queue / netif_wake_queue. This
> > > way we can preserve backward compatibility.
> > >
> > > However I'm reluctant to change the code of all the hardware drivers
> > > because I'm afraid of introducing bugs, etc. When I look at the code
> > > of "wan/lmc/lmc_main.c", I feel I'm not able to make sure there are no
> > > bugs (related to stop_queue / wake_queue) after my change (and even
> > > before my change, actually). There are even serious style problems:
> > > the majority of its lines are indented by spaces.
> > >
> > > So I don't want to mess with all the hardware drivers. Hardware driver
> > > developers (if they wish to properly support hdlc_x25) should do the
> > > change themselves. This is not a problem for me, because I use my own
> > > out-of-tree hardware driver. However if I add APIs with no user code
> > > in the kernel, other developers may think these APIs are not
> > > necessary.
> >
> > I don't think a change that affects the entire HDLC subsystem is
> > justified, since the actual problem only affects the hdlc_x25 area.
> >
> > The approach with the additional hdlc<x>_x25 is clean and purposeful 
> > and
> > I personally could live with it.
> >
> > I just don't see myself in the position to decide such a change at the
> > moment.
> >
> > @Jakub: What is your opinion on this.
>
> Hard question to answer, existing users seem happy and Xie's driver
> isn't upstream, so the justification for potentially breaking backward
> compatibility isn't exactly "strong".
>
> Can we cop out and add a knob somewhere to control spawning the extra
> netdev? Let people who just want a newer kernel carry on without
> distractions and those who want the extra layer can flip the switch?

Yes, that would be a good compromise.
I think a compile time selection option is enough here.
We could introduce a new config option CONFIG_HDLC_X25_LEGACY (or
something like that) and then implement the new or the old behavior in
the driver accordingly.

A switch that can be toggled at runtime (e.g. via sethdlc) would also be
conceivable, but I don't think this is necessary.