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

[Loic Poulain]

On Thu, Aug 6, 2026 at 5:54 PM Peter Hunt <peter.hunt@xxxxxxxxxxxx> wrote:
>
> 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;

Do we really need that? the wwan core already has this knowledge and
so will only forward/support tiocm ops to AT ports. Also this info can
be retrieved from the wwan_port type.

> +
>         /* 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);

Why not handling this enablement directly in the wwan core?

> +
>         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
>