Kernel CAPI 2.0 ISDN core
The in-kernel implementation of the Common ISDN Application Programming Interface (CAPI 2.0), a dispatch layer that let userspace talk to ISDN controller cards from vendors like AVM and Eicon throughout the late 1990s and 2000s. With every in-tree ISDN hardware driver already gone, the only remaining consumer is Bluetooth CMTP, which tunnels CAPI messages over Bluetooth to mobile phones.
recommendation
Already on the path to removal: an April 2026 patch series on netdev proposes deleting the entire ISDN subsystem, including this CAPI core. ISDN itself is being switched off by carriers worldwide (BT, Deutsche Telekom and others), no in-tree hardware drivers still depend on it, and the sole remaining user is the Bluetooth CMTP profile for an obscure phone-tethering use case. Users who still need ISDN connectivity have moved to SIP and IP telephony rather than to another kernel driver.
repository signals
sources
- lore.kernel.org
An April 21, 2026 netdev deletion patch explicitly proposes removing the ISDN subsystem and touches drivers/isdn/capi/capi.c.
- kernelconfig.io
Kernel config metadata says ISDN_CAPI requires hardware-specific drivers and points to BT_CMTP as the last remaining regular in-kernel user of the CAPI subsystem.
- kernel.org
Kernel documentation describes Kernel CAPI as a dispatch layer for ISDN hardware drivers rather than a modern standalone stack.
- en.wikipedia.org
ISDN has largely been replaced by newer network technologies, indicating legacy rather than growth deployments.
- business.bt.com
A major carrier migration notice says ISDN services are being switched off and customers must move to IP-based services, reinforcing obsolescence of new ISDN deployments.
codex reasoning notes (technical)
Local shell inspection showed this directory is the kernel CAPI core, built only for CONFIG_BT_CMTP; Kconfig/Makefile say BT_CMTP is the last remaining regular in-kernel user, so it is no longer serving a live family of in-tree ISDN hardware drivers. lore_file_timeline on drivers/isdn/capi/capi.c returned the 2026-04-21 removal series URL above, which is strong upstream-removal evidence, so this escalates from deprecate to remove. The kernelconfig.io, kernel.org, Wikipedia, and BT Business URLs were obtained via web search; the lore URL came from lore_file_timeline. No natural replacement driver exists inside Linux for CAPI ISDN hardware; the practical replacement is migration away from ISDN to IP telephony rather than another kernel driver.