The WWAN character device emulates the TTY modem-control ioctls
(TIOCMGET/TIOCMSET/TIOCMBIC/TIOCMBIS) for AT and QCDM ports, but the
result is only stored in port->at_data.mdmbits and never reaches the port
driver. A driver therefore cannot act on the host raising or dropping
DTR/RTS, even though some modems depend on it (e.g. they withhold
unsolicited AT result codes until the host asserts DTR).

Add an optional ->dtr_rts(port, on) operation to struct wwan_port_ops,
mirroring tty_port_operations.dtr_rts. Drivers that implement it receive
a simple assert/de-assert signal while the TIOCM bitmask state is tracked
by the wwan core. TIOCMSET/TIOCMBIC/TIOCMBIS resolve the new bitmask and
call ->dtr_rts when the driver implements it.

Also raise DTR/RTS in wwan_port_op_start on first open of an AT port when
the driver implements ->dtr_rts, and drop them in wwan_port_op_stop on
last close. This mirrors TTY semantics (DTR is asserted on open) and means
individual drivers do not need to implement this themselves.

Signed-off-by: Peter Hunt <peter.hunt@opengear.com>
---
v3: Replace ->tiocmget/->tiocmset with ->dtr_rts(port, bool on) modelled
    on tty_port_operations.dtr_rts; raise/drop DTR/RTS in
    wwan_port_op_start/stop rather than in the driver (Loic Poulain)

 drivers/net/wwan/wwan_core.c | 18 +++++++++++++++++-
 include/linux/wwan.h         |  3 +++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wwan/wwan_core.c b/drivers/net/wwan/wwan_core.c
index ffbcf11e4e68..f8ca9cda3c9a 100644
--- a/drivers/net/wwan/wwan_core.c
+++ b/drivers/net/wwan/wwan_core.c
@@ -759,8 +759,15 @@ static int wwan_port_op_start(struct wwan_port *port)
 	if (!port->start_count)
 		ret = port->ops->start(port);
 
-	if (!ret)
+	if (!ret) {
 		port->start_count++;
+		/* Mirror TTY semantics: raise DTR/RTS on first open of an AT port */
+		if (port->start_count == 1 && port->type == WWAN_PORT_AT &&
+		    port->ops->dtr_rts) {
+			port->at_data.mdmbits |= TIOCM_DTR | TIOCM_RTS;
+			port->ops->dtr_rts(port, true);
+		}
+	}
 
 out_unlock:
 	mutex_unlock(&port->ops_lock);
@@ -773,6 +780,11 @@ static void wwan_port_op_stop(struct wwan_port *port)
 	mutex_lock(&port->ops_lock);
 	port->start_count--;
 	if (!port->start_count) {
+		/* Mirror TTY semantics: drop DTR/RTS on last close of an AT port */
+		if (port->ops && port->type == WWAN_PORT_AT && port->ops->dtr_rts) {
+			port->at_data.mdmbits &= ~(TIOCM_DTR | TIOCM_RTS);
+			port->ops->dtr_rts(port, false);
+		}
 		if (port->ops)
 			port->ops->stop(port);
 		skb_queue_purge(&port->rxq);
@@ -1036,6 +1048,10 @@ static long wwan_port_fops_at_ioctl(struct wwan_port *port, unsigned int cmd,
 			port->at_data.mdmbits |= mdmbits;
 		else
 			port->at_data.mdmbits = mdmbits;
+		if (port->ops->dtr_rts)
+			port->ops->dtr_rts(port,
+					   !!(port->at_data.mdmbits &
+					      (TIOCM_DTR | TIOCM_RTS)));
 		break;
 	}
 
diff --git a/include/linux/wwan.h b/include/linux/wwan.h
index 1e0e2cb53579..a7684950d73b 100644
--- a/include/linux/wwan.h
+++ b/include/linux/wwan.h
@@ -57,6 +57,8 @@ struct wwan_port;
  * @tx_blocking: Optional blocking routine that sends WWAN port protocol data
  *               to the device.
  * @tx_poll: Optional routine that sets additional TX poll flags.
+ * @dtr_rts: Optional routine that asserts (on=true) or de-asserts (on=false)
+ *           the DTR and RTS modem control lines.
  *
  * The wwan_port_ops structure contains a list of low-level operations
  * that control a WWAN port device. All functions are mandatory unless specified.
@@ -70,6 +72,7 @@ struct wwan_port_ops {
 	int (*tx_blocking)(struct wwan_port *port, struct sk_buff *skb);
 	__poll_t (*tx_poll)(struct wwan_port *port, struct file *filp,
 			    poll_table *wait);
+	void (*dtr_rts)(struct wwan_port *port, bool on);
 };
 
 /** struct wwan_port_caps - The WWAN port capbilities
-- 
2.43.0