net: wwan: mhi_wwan_ctrl: drive DTR/RTS via the IP_CTRL channel

Qualcomm/Sierra SDX55/SDX65 modems 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).

The IP_CTRL channel has to be declared in the controller's channel config
for this driver to bind. For the Sierra EM919x/EM929x this is done by
commit 83c29a55b89e ("bus: mhi: host: pci_generic: Add IP_CTRL channel
for Sierra EM919x/EM929x") in the MHI tree. On controllers that do not
declare IP_CTRL the driver does not bind and ->dtr_rts is a no-op.

The wwan core, with "net: wwan: core: propagate modem control signals to
port drivers", raises DTR/RTS on first open and drops them on last close
(if HUPCL is set) for any AT port whose driver implements ->dtr_rts, so
no open/close handling is needed here. The new ->dtr_rts op lets
userspace assert or de-assert them via TIOCMSET/TIOCMBIC/TIOCMBIS.
Received device->host serial state messages are silently discarded; a
single recycled sink buffer keeps the IP_CTRL DL ring live so the
modem's transmit path does not stall.

->dtr_rts is void, following the tty_port_operations model it is
modelled on. It records the requested state and schedules a per-port
work item that sends the latest state to the modem, so a burst of
changes is coalesced into one message and cannot fill the IP_CTRL UL
ring. If a send fails because the ring is full or memory is short, the
work retries shortly with whatever state is latest by then, so the modem
ends up with the final state that was requested. The DL sink buffer is
requeued from a work item, which retries a transient -ENOMEM so the DL
ring does not stay empty, and which remove() disables before tearing
down the channel.

Signed-off-by: Peter Hunt <peter.hunt@opengear.com>
Reviewed-by: Loic Poulain <loic.poulain@oss.qualcomm.com>
Link: https://patch.msgid.link/20261013183500.261229-3-peter.hunt@opengear.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions