[PATCH net-next v4 1/3] net: wwan: core: propagate modem control signals to port drivers

From: Peter Hunt <hidden>
Date: 2026-08-16 12:18:46
Also in: linux-arm-msm, lkml
Subsystem: networking drivers, the rest, wwan drivers · Maintainers: Andrew Lunn, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds, Loic Poulain, Sergey Ryazanov

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, gated on WWAN_PORT_AT to
match the open/close raise/drop behaviour below. The boolean passed
reflects DTR only, since DTR is the line that governs URC gating on
modems supporting this signalling; RTS is tracked in mdmbits but driven
together with DTR on open/close.

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.

at_data.mdmbits is protected by data_lock; the ->dtr_rts call is made
after releasing data_lock and re-acquiring ops_lock so that it is
serialised against port removal (which nulls port->ops under ops_lock).

Signed-off-by: Peter Hunt <redacted>
---
v4: Protect at_data.mdmbits in wwan_port_op_start/stop under data_lock;
    release data_lock and acquire ops_lock with a NULL check before calling
    ->dtr_rts from the ioctl path; gate ioctl ->dtr_rts on WWAN_PORT_AT to
    match open/close behaviour; reduce boolean to TIOCM_DTR only
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 | 31 ++++++++++++++++++++++++++++++-
 include/linux/wwan.h         |  3 +++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wwan/wwan_core.c b/drivers/net/wwan/wwan_core.c
index ffbcf11e4e68..90ca85a63b5a 100644
--- a/drivers/net/wwan/wwan_core.c
+++ b/drivers/net/wwan/wwan_core.c

@@ -759,8 +759,17 @@ 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) {
+			mutex_lock(&port->data_lock);
+			port->at_data.mdmbits |= TIOCM_DTR | TIOCM_RTS;
+			mutex_unlock(&port->data_lock);
+			port->ops->dtr_rts(port, true);
+		}
+	}
 
 out_unlock:
 	mutex_unlock(&port->ops_lock);

@@ -773,6 +782,13 @@ 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) {
+			mutex_lock(&port->data_lock);
+			port->at_data.mdmbits &= ~(TIOCM_DTR | TIOCM_RTS);
+			mutex_unlock(&port->data_lock);
+			port->ops->dtr_rts(port, false);
+		}
 		if (port->ops)
 			port->ops->stop(port);
 		skb_queue_purge(&port->rxq);

@@ -980,6 +996,8 @@ static long wwan_port_fops_at_ioctl(struct wwan_port *port, unsigned int cmd,
 				    unsigned long arg)
 {
 	int ret = 0;
+	bool call_dtr_rts = false;
+	bool dtr_on = false;
 
 	mutex_lock(&port->data_lock);

@@ -1036,6 +1054,10 @@ 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->type == WWAN_PORT_AT) {
+			dtr_on = !!(port->at_data.mdmbits & TIOCM_DTR);
+			call_dtr_rts = true;
+		}
 		break;
 	}

@@ -1061,6 +1083,13 @@ static long wwan_port_fops_at_ioctl(struct wwan_port *port, unsigned int cmd,
 
 	mutex_unlock(&port->data_lock);
 
+	if (call_dtr_rts) {
+		mutex_lock(&port->ops_lock);
+		if (port->ops && port->ops->dtr_rts)
+			port->ops->dtr_rts(port, dtr_on);
+		mutex_unlock(&port->ops_lock);
+	}
+
 	return ret;
 }

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

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