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Date:   Wed, 24 Apr 2019 14:29:04 +0200
From:   Arnd Bergmann <>
To:     Thomas Jarosch <>
Cc:     Karsten Keil <>,
        Networking <>,
        Tilman Schmidt <>,
        Paul Bolle <>,,,
        Al Viro <>,
        Holger Schurig <>
Subject: Re: [PATCH 5/5] isdn: move capi drivers to staging

On Wed, Apr 24, 2019 at 11:24 AM Thomas Jarosch
<> wrote:
> You wrote on Tue, Apr 23, 2019 at 05:11:43PM +0200:
> > I tried to find any indication of whether the capi drivers are
> > still in use, and have not found anything  from a long time ago.
> >
> > With public ISDN networks almost completely shut down over the past 12
> > months, there is very little you can actually do with this hardware. The
> > main remaining use case would be to connect ISDN voice phones to an
> > in-house installation with Asterisk or LCR, but anyone trying this in
> > turn seems to be using either the mISDN driver stack, or out-of-tree
> > drivers from the hardware vendors.
> >
> > I may of course have missed something, so I would suggest moving
> > these into drivers/staging/ just in case someone still uses one
> > of the three remaining in-kernel drivers (avm, hysdn, gigaset).
> >
> > If nobody complains, we can remove them entirely in six months,
> > or otherwise move the core code and any drivers that are still
> > needed back into drivers/isdn.
> thanks for your isdn4linux cleanup work!
> AFAIK the base capi drivers are still needed for mISDN, too.
> I quickly checked a machine that uses mISDNcapid for receiving fax
> and the modules "capi" and "kernelcapi" are loaded.

Ok, interesting. My understanding was that mISDN CAPI support
was done purely in user space, on top of the mISDN interface.

I don't see any interfaction between the two in the kernel code,
but if the capi module is required for mISDN, we clearly have to
keep it. We could still move the avm/gigaset/hysdn/cmtp drivers
to staging though, if there are no users for those.

> # lsmod |grep capi
> capi                   10751  2
> kernelcapi             33675  1 capi
> # capiinfo
> Number of Controllers : 1
> Controller 1:
> Manufacturer: mISDN
> CAPI Version: 2.0
> Manufacturer Version: 0.1
> Serial Number: 0000001
> BChannels: 2
> Global Options: 0x00000001
>    internal controller supported
> B1 protocols support: 0x00000013
>    64 kbit/s with HDLC framing
>    64 kbit/s bit-transparent operation
>    T.30 modem for fax group 3
> B2 protocols support: 0x00000013
>    ISO 7776 (X.75 SLP)
>    Transparent
>    T.30 for fax group 3
> B3 protocols support: 0x00000031
>    Transparent
>    T.30 for fax group 3
>    T.30 for fax group 3 with extensions
>   0100
>   0200
>   01000000
>   13000000
>   13000000
>   31000000
>   00000000 00000000 00000000 00000000 00000000 00000000
>   00000000 00000000 00000000 00000000 00000000
> Supplementary services support: 0x00000000
> -> I think the CAPI layer is still needed, but Karsten knows best.
> The ISDN network of Deutsche Telekom in Germany is still
> *in the process* of being shut down, business customers that use
> "direct inward dialing" (DID / "Anlagenanschlu├č") often still use ISDN.
> For example we use it here at the office, but preparations for
> a SIP migration are being done.

Right, this is what I'm trying to find out here. I realize that there
are (very few) remaining users of ISDN voice services, but this only
matters if someone uses them

1. with a modern Linux kernel, and planning to upgrade beyond linux-5.3
2. with a device driver that ships with the kernel
3. using the CAPI subsystem

I suspect that all three of the above are true in isolation, but onless
at least one person needs all three combined, that doesn't stop us
from staging them out.

> Vodafone Germany guarantees business customers to support ISDN until 2022.
> Quite a few people use a SIP -> ISDN converter like the Fritz!Box or LANCOM
> routers to keep their fax machines running. It's a bit crazy, but people seem
> to love it. We deal with this on a weekly basis to support
> our customers during the migration period.

This is certainly fine as well, as long as neither the router nor the
fax machine depend on the CAPI drivers remaining in future Linux
kernels. My understanding is that Fritz!Box routers are usually
on ancient kernels and do not use the mainline driver stack for ISDN,
so that's fine. I don't know about LANCOM, but looking at their
firmware images indicates that they are using NetBSD and are not
affected by Linux kernel changes either.

That leaves the question of whether there is anyone rolling their
own routers and/or fax machines based on future kernels with the
avm/gigaset/hysdn/cmtp CAPI drivers, and which drivers in particular
that would be.

> Intra2net is currently in the process of upgrading to kernel 4.19.x
> and Karsten recently did all the necessary adaptions for mISDN.
> So the ISDN code is used with modern (LTS) kernels ^^

Ok. If you are on 4.19 (or moving to that now), I would assume that
you probably upgrade to a future kernel later. Even if you don't
plan to, it's the safe bet.


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