[PATCH net-next v2 3/3] net: wwan: mhi_wwan_ctrl: drive DTR/RTS via the IP_CTRL channel

[Peter Hunt]

Qualcomm/Sierra SDX55/SDX65 modems (e.g. Sierra EM9291) withhold
unsolicited AT result codes (URCs such as +CREG, and the +DMI OMA-DM/LwM2M
session indications) on an AT port until the host asserts DTR.
mhi_wwan_ctrl exposed the AT (DUN) ports but had no way to signal DTR, so
URCs never reached userspace.

Carry the host serial-control lines to the modem over the dedicated
IP_CTRL MHI channel, which this module now also binds. IP_CTRL uses a
separate mhi_driver with its own callbacks so the AT/QMI/MBIM data path is
untouched; the control-channel device for each MHI controller is tracked
in a small registry so an AT port drives the IP_CTRL channel of its own
modem (multiple modems are supported).

Mirror TTY semantics: an AT port raises DTR/RTS on open and drops them on
close, and the new wwan ->tiocmset op lets userspace change them via
TIOCMSET. This matches the TTY-based USB and legacy PCIe drivers for the
same modems, where DTR is asserted on open. Received device->host serial
state is not needed and is ignored; ->tiocmget is left unimplemented so
the wwan core reports the cached bits.

Signed-off-by: Peter Hunt <peter.hunt@xxxxxxxxxxxx>
---
v2: Use __le32 for mhi_dtr_ctrl_msg fields and cpu_to_le32() on
    assignment. Use guard(mutex) in mhi_wwan_ctrl_send_dtr. Return 0
    when IP_CTRL is not enumerated so TIOCMSET does not regress for
    devices without IP_CTRL.

 drivers/net/wwan/mhi_wwan_ctrl.c | 183 ++++++++++++++++++++++++++++++-
 1 file changed, 182 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wwan/mhi_wwan_ctrl.c b/drivers/net/wwan/mhi_wwan_ctrl.c
index a31d8540fbb8..95f4f0e6e4aa 100644
--- a/drivers/net/wwan/mhi_wwan_ctrl.c
+++ b/drivers/net/wwan/mhi_wwan_ctrl.c
@@ -1,8 +1,12 @@
 // SPDX-License-Identifier: GPL-2.0-only
 /* Copyright (c) 2021, Linaro Ltd <loic.poulain@xxxxxxxxxx> */
 #include <linux/kernel.h>
+#include <linux/list.h>
 #include <linux/mhi.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/slab.h>
+#include <linux/termios.h>
 #include <linux/wwan.h>
 
 /* MHI wwan flags */
@@ -14,6 +18,30 @@ enum mhi_wwan_flags {
 
 #define MHI_WWAN_MAX_MTU	0x8000
 
+/* IP_CTRL channel message that sets the modem's DTR/RTS control lines */
+struct mhi_dtr_ctrl_msg {
+	__le32 preamble;
+	__le32 msg_id;
+	__le32 dest_id;
+	__le32 size;
+	__le32 msg;
+} __packed;
+
+#define MHI_DTR_CTRL_MAGIC	0x4C525443	/* 'CTRL' */
+#define MHI_DTR_MSG_DTR		BIT(0)
+#define MHI_DTR_MSG_RTS		BIT(1)
+#define MHI_DTR_HOST_STATE	0x10
+
+/* Per-controller IP_CTRL channel, used to signal DTR/RTS to that modem */
+struct mhi_wwan_dtr {
+	struct mhi_controller *cntrl;
+	struct mhi_device *mhi_dev;
+	struct list_head node;
+};
+
+static LIST_HEAD(mhi_wwan_dtr_list);
+static DEFINE_MUTEX(mhi_wwan_dtr_lock);
+
 struct mhi_wwan_dev {
 	/* Lower level is a mhi dev, upper level is a wwan port */
 	struct mhi_device *mhi_dev;
@@ -23,6 +51,9 @@ struct mhi_wwan_dev {
 	unsigned long flags;
 	size_t mtu;
 
+	/* AT (DUN) ports raise DTR/RTS on open to receive unsolicited output */
+	bool is_at_port;
+
 	/* Protect against concurrent TX and TX-completion (bh) */
 	spinlock_t tx_lock;
 
@@ -103,6 +134,55 @@ static void mhi_wwan_ctrl_refill_work(struct work_struct *work)
 	}
 }
 
+/* Signal the modem's DTR/RTS lines over its own controller's IP_CTRL channel */
+static int mhi_wwan_ctrl_send_dtr(struct mhi_wwan_dev *mhiwwan, unsigned int mdmbits)
+{
+	struct mhi_controller *cntrl = mhiwwan->mhi_dev->mhi_cntrl;
+	struct mhi_device *ctrl_dev = NULL;
+	struct mhi_dtr_ctrl_msg *dtr_msg;
+	struct mhi_wwan_dtr *dtr;
+	u32 msg = 0;
+	int ret;
+
+	guard(mutex)(&mhi_wwan_dtr_lock);
+
+	list_for_each_entry(dtr, &mhi_wwan_dtr_list, node) {
+		if (dtr->cntrl == cntrl) {
+			ctrl_dev = dtr->mhi_dev;
+			break;
+		}
+	}
+	if (!ctrl_dev)
+		return 0; /* IP_CTRL not enumerated; best-effort, not an error */
+
+	dtr_msg = kzalloc_obj(*dtr_msg);
+	if (!dtr_msg)
+		return -ENOMEM;
+
+	if (mdmbits & TIOCM_DTR)
+		msg |= MHI_DTR_MSG_DTR;
+	if (mdmbits & TIOCM_RTS)
+		msg |= MHI_DTR_MSG_RTS;
+
+	dtr_msg->preamble = cpu_to_le32(MHI_DTR_CTRL_MAGIC);
+	dtr_msg->msg_id = cpu_to_le32(MHI_DTR_HOST_STATE);
+	dtr_msg->dest_id = cpu_to_le32(mhiwwan->mhi_dev->ul_chan_id);
+	dtr_msg->size = cpu_to_le32(sizeof(__le32));
+	dtr_msg->msg = cpu_to_le32(msg);
+
+	ret = mhi_queue_buf(ctrl_dev, DMA_TO_DEVICE, dtr_msg, sizeof(*dtr_msg),
+			    MHI_EOT);
+	if (ret)
+		kfree(dtr_msg);
+
+	return ret;
+}
+
+static int mhi_wwan_ctrl_tiocmset(struct wwan_port *port, unsigned int mdmbits)
+{
+	return mhi_wwan_ctrl_send_dtr(wwan_port_get_drvdata(port), mdmbits);
+}
+
 static int mhi_wwan_ctrl_start(struct wwan_port *port)
 {
 	struct mhi_wwan_dev *mhiwwan = wwan_port_get_drvdata(port);
@@ -122,6 +202,10 @@ static int mhi_wwan_ctrl_start(struct wwan_port *port)
 		mhi_wwan_ctrl_refill_work(&mhiwwan->rx_refill);
 	}
 
+	/* Raise DTR/RTS on open so the modem forwards unsolicited output */
+	if (mhiwwan->is_at_port)
+		mhi_wwan_ctrl_send_dtr(mhiwwan, TIOCM_DTR | TIOCM_RTS);
+
 	return 0;
 }
 
@@ -135,6 +219,10 @@ static void mhi_wwan_ctrl_stop(struct wwan_port *port)
 
 	cancel_work_sync(&mhiwwan->rx_refill);
 
+	/* Drop DTR/RTS on close */
+	if (mhiwwan->is_at_port)
+		mhi_wwan_ctrl_send_dtr(mhiwwan, 0);
+
 	mhi_unprepare_from_transfer(mhiwwan->mhi_dev);
 }
 
@@ -163,6 +251,7 @@ static const struct wwan_port_ops wwan_pops = {
 	.start = mhi_wwan_ctrl_start,
 	.stop = mhi_wwan_ctrl_stop,
 	.tx = mhi_wwan_ctrl_tx,
+	.tiocmset = mhi_wwan_ctrl_tiocmset,
 };
 
 static void mhi_ul_xfer_cb(struct mhi_device *mhi_dev,
@@ -223,6 +312,7 @@ static int mhi_wwan_ctrl_probe(struct mhi_device *mhi_dev,
 
 	mhiwwan->mhi_dev = mhi_dev;
 	mhiwwan->mtu = MHI_WWAN_MAX_MTU;
+	mhiwwan->is_at_port = (id->driver_data == WWAN_PORT_AT);
 	INIT_WORK(&mhiwwan->rx_refill, mhi_wwan_ctrl_refill_work);
 	spin_lock_init(&mhiwwan->tx_lock);
 	spin_lock_init(&mhiwwan->rx_lock);
@@ -255,6 +345,59 @@ static void mhi_wwan_ctrl_remove(struct mhi_device *mhi_dev)
 	kfree(mhiwwan);
 }
 
+/* IP_CTRL channel driver, bound separately so the data-port path is untouched */
+static void mhi_wwan_dtr_ul_xfer_cb(struct mhi_device *mhi_dev,
+				    struct mhi_result *mhi_result)
+{
+	/* MHI core has done with the buffer, release it */
+	kfree(mhi_result->buf_addr);
+}
+
+static void mhi_wwan_dtr_dl_xfer_cb(struct mhi_device *mhi_dev,
+				    struct mhi_result *mhi_result)
+{
+	/* Modem serial state is not needed, drop it */
+}
+
+static int mhi_wwan_dtr_probe(struct mhi_device *mhi_dev,
+			      const struct mhi_device_id *id)
+{
+	struct mhi_wwan_dtr *dtr;
+	int ret;
+
+	dtr = kzalloc_obj(*dtr);
+	if (!dtr)
+		return -ENOMEM;
+
+	ret = mhi_prepare_for_transfer(mhi_dev);
+	if (ret) {
+		kfree(dtr);
+		return ret;
+	}
+
+	dtr->cntrl = mhi_dev->mhi_cntrl;
+	dtr->mhi_dev = mhi_dev;
+	dev_set_drvdata(&mhi_dev->dev, dtr);
+
+	mutex_lock(&mhi_wwan_dtr_lock);
+	list_add(&dtr->node, &mhi_wwan_dtr_list);
+	mutex_unlock(&mhi_wwan_dtr_lock);
+
+	return 0;
+}
+
+static void mhi_wwan_dtr_remove(struct mhi_device *mhi_dev)
+{
+	struct mhi_wwan_dtr *dtr = dev_get_drvdata(&mhi_dev->dev);
+
+	mutex_lock(&mhi_wwan_dtr_lock);
+	list_del(&dtr->node);
+	mutex_unlock(&mhi_wwan_dtr_lock);
+
+	mhi_unprepare_from_transfer(mhi_dev);
+	kfree(dtr);
+}
+
 static const struct mhi_device_id mhi_wwan_ctrl_match_table[] = {
 	{ .chan = "DUN", .driver_data = WWAN_PORT_AT },
 	{ .chan = "DUN2", .driver_data = WWAN_PORT_AT },
@@ -278,7 +421,45 @@ static struct mhi_driver mhi_wwan_ctrl_driver = {
 	},
 };
 
-module_mhi_driver(mhi_wwan_ctrl_driver);
+static const struct mhi_device_id mhi_wwan_dtr_match_table[] = {
+	{ .chan = "IP_CTRL" },
+	{},
+};
+MODULE_DEVICE_TABLE(mhi, mhi_wwan_dtr_match_table);
+
+static struct mhi_driver mhi_wwan_dtr_driver = {
+	.id_table = mhi_wwan_dtr_match_table,
+	.remove = mhi_wwan_dtr_remove,
+	.probe = mhi_wwan_dtr_probe,
+	.ul_xfer_cb = mhi_wwan_dtr_ul_xfer_cb,
+	.dl_xfer_cb = mhi_wwan_dtr_dl_xfer_cb,
+	.driver = {
+		.name = "mhi_wwan_dtr",
+	},
+};
+
+static int __init mhi_wwan_ctrl_init(void)
+{
+	int ret;
+
+	ret = mhi_driver_register(&mhi_wwan_dtr_driver);
+	if (ret)
+		return ret;
+
+	ret = mhi_driver_register(&mhi_wwan_ctrl_driver);
+	if (ret)
+		mhi_driver_unregister(&mhi_wwan_dtr_driver);
+
+	return ret;
+}
+module_init(mhi_wwan_ctrl_init);
+
+static void __exit mhi_wwan_ctrl_exit(void)
+{
+	mhi_driver_unregister(&mhi_wwan_ctrl_driver);
+	mhi_driver_unregister(&mhi_wwan_dtr_driver);
+}
+module_exit(mhi_wwan_ctrl_exit);
 
 MODULE_LICENSE("GPL v2");
 MODULE_DESCRIPTION("MHI WWAN CTRL Driver");
-- 
2.43.0