linux-kernel - [PATCH 2/3] tty: n_gsm: add RING/CD control support

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Message-Id: <20230201080151.2068-2-daniel.starke@siemens.com>
Date:   Wed,  1 Feb 2023 09:01:50 +0100
From:   "D. Starke" <daniel.starke@...mens.com>
To:     linux-serial@...r.kernel.org, gregkh@...uxfoundation.org,
        jirislaby@...nel.org, ilpo.jarvinen@...ux.intel.com
Cc:     linux-kernel@...r.kernel.org,
        Daniel Starke <daniel.starke@...mens.com>
Subject: [PATCH 2/3] tty: n_gsm: add RING/CD control support

From: Daniel Starke <daniel.starke@...mens.com>

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@...mens.com>
---
 drivers/tty/n_gsm.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 98577b54f1fd..118511c1fa37 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -546,6 +546,11 @@ 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;
 }
 
-- 
2.34.1