isdn: deprecate non-mISDN drivers

When isdn4linux came up in the context of another patch series, I
remembered that we had discussed removing it a while ago.

It turns out that the suggestion from Karsten Keil wa to remove I4L
in 2018 after the last public ISDN networks are shut down. This has
happened now (with a very small number of exceptions), so I guess it's
time to try again.

We currently have three ISDN stacks in the kernel: the original
isdn4linux (with the hisax driver), the newer CAPI (with four drivers),
and finally the mISDN stack (supporting roughly the same hardware as
hisax).

As far as I can tell, anyone using ISDN with mainline kernel drivers in
the past few years uses mISDN, and this is typically used for voice-only
PBX installations that don't require a public network.

The older stacks support additional features for data networks, but those
typically make no sense any more if there is no network to connect to.

My proposal for this time is to kill off isdn4linux entirely, as it seems
to have been unusable for quite a while. This code has been abandoned
for many years and it does cause problems for treewide maintenance as
it tends to do everything that we try to stop doing.
Birger Harzenetter mentioned that is is still using i4l in order to
make use of the 'divert' feature that is not part of mISDN, but has
otherwise moved on to mISDN for normal operation, like apparently
everyone else.

CAPI in turn is not quite as obsolete, but two of the drivers (avm
and hysdn) don't seem to be used at all, while another one (gigaset)
will stop being maintained as Paul Bolle is no longer able to
test it after the network gets shut down in September.
All three are now moved into drivers/staging to let others speak
up in case there are remaining users.
This leaves Bluetooth CMTP as the only remaining user of CAPI, but
Marcel Holtmann wishes to keep maintaining it.

For the discussion on version 1, see [2]
Unfortunately, Karsten Keil as the maintainer has not participated in
the discussion.

      Arnd

[1] https://patchwork.kernel.org/patch/8484861/#17900371
[2] https://listserv.isdn4linux.de/pipermail/isdn4linux/2019-April/thread.html
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isdn: move capi drivers to staging

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
three drivers (avm, hysdn, gigaset) into drivers/staging/ just in case
someone still uses them.

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.

As Paul Bolle notes, he is still testing the gigaset driver as long as
he can, but the Dutch ISDN network will be shut down in September 2019,
which puts an end to that.

Marcel Holtmann still maintains the Bluetooth CMTP profile and wants to
keep that alive, so the actual CAPI subsystem code remains in place for
now, after all other drivers are gone, CMTP and CAPI can be merged into
a single driver directory.

Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
52 files changed