[PATCH net-next v5 1/3] net: wwan: core: propagate modem control signals to port drivers

[Peter Hunt <peter.hunt@opengear.com>]

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, mdmbits) operation to struct wwan_port_ops.
Drivers that implement it receive the full TIOCM bitmask so they can assert
or de-assert DTR and RTS independently. The wwan core tracks the full TIOCM
bitmask in port->at_data.mdmbits and calls ->dtr_rts when it changes, gated
on WWAN_PORT_AT to match the open/close raise/drop behaviour.

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. In the ioctl path the ->dtr_rts
call is deferred until ops_lock is held, where mdmbits is re-read under
data_lock, so the value passed to the driver always reflects the committed
bitmask under ops_lock and is serialised against concurrent ioctls and
against port removal (which nulls port->ops under ops_lock).

wwan_remove_port() is also updated to call ->dtr_rts(port, 0) before
->stop() when a port is removed while still open, mirroring the last-close
de-assert path.

Signed-off-by: Peter Hunt <peter.hunt@opengear.com>
---
v5: Change ->dtr_rts signature from bool to unsigned int mdmbits so DTR
    and RTS can be driven independently; re-read mdmbits inside ops_lock
    in the ioctl path to close a concurrent-ioctl ordering race; add
    de-assert call to wwan_remove_port() for the hot-unplug case; update
    kernel-doc to note the op is AT-only and describe the mdmbits argument
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 | 40 +++++++++++++++++++++++++++++++++++-
 include/linux/wwan.h         |  3 +++
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wwan/wwan_core.c b/drivers/net/wwan/wwan_core.c
index ffbcf11e4e68..3a6a0c5a3acf 100644
--- a/drivers/net/wwan/wwan_core.c
+++ b/drivers/net/wwan/wwan_core.c
@@ -685,6 +685,12 @@ void wwan_remove_port(struct wwan_port *port)
 
 	mutex_lock(&port->ops_lock);
 	if (port->start_count) {
+		if (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, 0);
+		}
 		port->ops->stop(port);
 		port->start_count = 0;
 	}
@@ -759,8 +765,17 @@ 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, port->at_data.mdmbits);
+		}
+	}
 
 out_unlock:
 	mutex_unlock(&port->ops_lock);
@@ -773,6 +788,13 @@ 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, port->at_data.mdmbits);
+		}
 		if (port->ops)
 			port->ops->stop(port);
 		skb_queue_purge(&port->rxq);
@@ -980,6 +1002,7 @@ 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;
 
 	mutex_lock(&port->data_lock);
 
@@ -1036,6 +1059,8 @@ 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)
+			call_dtr_rts = true;
 		break;
 	}
 
@@ -1061,6 +1086,19 @@ static long wwan_port_fops_at_ioctl(struct wwan_port *port, unsigned int cmd,
 
 	mutex_unlock(&port->data_lock);
 
+	if (call_dtr_rts) {
+		unsigned int bits;
+
+		mutex_lock(&port->ops_lock);
+		if (port->ops && port->ops->dtr_rts) {
+			mutex_lock(&port->data_lock);
+			bits = port->at_data.mdmbits;
+			mutex_unlock(&port->data_lock);
+			port->ops->dtr_rts(port, bits);
+		}
+		mutex_unlock(&port->ops_lock);
+	}
+
 	return ret;
 }
 
diff --git a/include/linux/wwan.h b/include/linux/wwan.h
index 1e0e2cb53579..57406139304e 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 updates the modem control lines to match
+ *           @mdmbits (a TIOCM_* bitmask). Only called for WWAN_PORT_AT ports.
  *
  * 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, unsigned int mdmbits);
 };
 
 /** struct wwan_port_caps - The WWAN port capbilities
-- 
2.43.0