tty: n_gsm: add RING/CD control support

The status lines ring and carrier detect are used by the modem to signal
incoming calls (RING) or an established connection (CD). This is
implemented as physical lines on a standard RS232 connection. However,
the muxer protocol encodes these status lines as modem bits IC and DV.
These incoming lines are masked by tty driver (see tty_io.c) and cannot be
set by a user application.

Allow setting RING via TIOCM_OUT1 and CD via TIOCM_OUT2 to allow
implementation of a modem or modem emulator.

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230206114606.2133-3-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 5783801d652475e832a4b95a3bb46bf6c8ea1971..c6c8ee9cbdc3131f39a4a5b8474c09e8f11990b3 100644 (file)
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -540,6 +540,11 @@ static u8 gsm_encode_modem(const struct gsm_dlci *dlci)
                modembits |= MDM_IC;
        if (dlci->modem_tx & TIOCM_CD || dlci->gsm->initiator)
                modembits |= MDM_DV;
+       /* special mappings for passive side to operate as UE */
+       if (dlci->modem_tx & TIOCM_OUT1)
+               modembits |= MDM_IC;
+       if (dlci->modem_tx & TIOCM_OUT2)
+               modembits |= MDM_DV;
        return modembits;
 }