[PATCH net v2] net: wwan: mhi_wwan_ctrl: send a DTR drop before it can be coalesced

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

DTR/RTS changes are sent from a work item that only sends the latest
state, so a drop followed quickly by a raise, for example TIOCMBIC then
TIOCMBIS or a quick close and reopen, can reach the modem as just the
raise. A drop matters on its own, a modem set to AT&D2 hangs up on it.

Flush the work when DTR goes low so the drop is sent before ->dtr_rts()
returns, and flush it in stop() so any pending state is sent before the
channel is stopped. A drop that hits a full UL ring is still retried
with whatever state is latest by then.

Fixes: d509edf90bc6 ("net: wwan: mhi_wwan_ctrl: drive DTR/RTS via the IP_CTRL channel")
Suggested-by: Loic Poulain <loic.poulain@oss.qualcomm.com>
Signed-off-by: Peter Hunt <peter.hunt@opengear.com>
---
v2: Only handle the DTR drop, by flushing the work when DTR goes low,
    and flush it in stop() as well (Loic Poulain). Drop the port list
    and resend on a late IP_CTRL bind, which is only reachable by
    unbinding and rebinding mhi_wwan_dtr through sysfs.
v1: https://lore.kernel.org/all/20261023174113.2388353-1-peter.hunt@opengear.com/

Tested on a Sierra EM9291 (CM8100): back-to-back DTR drop+raise and
close+reopen now always send the drop, bursts of 10,000 DTR changes,
HUPCL, teardown and lock stress tests.

 drivers/net/wwan/mhi_wwan_ctrl.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/net/wwan/mhi_wwan_ctrl.c b/drivers/net/wwan/mhi_wwan_ctrl.c
index e3c207588956..772319927867 100644
--- a/drivers/net/wwan/mhi_wwan_ctrl.c
+++ b/drivers/net/wwan/mhi_wwan_ctrl.c
@@ -209,9 +209,17 @@ static void mhi_wwan_ctrl_dtr_work(struct work_struct *work)
 static void mhi_wwan_ctrl_dtr_rts(struct wwan_port *port, unsigned int mdmbits)
 {
 	struct mhi_wwan_dev *mhiwwan = wwan_port_get_drvdata(port);
+	bool drop = (READ_ONCE(mhiwwan->dtr_mdmbits) & TIOCM_DTR) &&
+		    !(mdmbits & TIOCM_DTR);
 
 	WRITE_ONCE(mhiwwan->dtr_mdmbits, mdmbits);
 	schedule_delayed_work(&mhiwwan->dtr_work, 0);
+
+	/* A DTR drop matters on its own, e.g. to hang up a modem set to AT&D2,
+	 * so send it now rather than let a following raise coalesce it away
+	 */
+	if (drop)
+		flush_delayed_work(&mhiwwan->dtr_work);
 }
 
 static int mhi_wwan_ctrl_start(struct wwan_port *port)
@@ -246,6 +254,9 @@ static void mhi_wwan_ctrl_stop(struct wwan_port *port)
 
 	cancel_work_sync(&mhiwwan->rx_refill);
 
+	/* Send any pending DTR/RTS state before the channel stops */
+	flush_delayed_work(&mhiwwan->dtr_work);
+
 	mhi_unprepare_from_transfer(mhiwwan->mhi_dev);
 }
 
-- 
2.43.0