net: wwan: core: propagate modem control signals to port drivers

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 TIOCM output bitmask so they can
assert or de-assert DTR and RTS independently. As in tty_tiocmset(), the
wwan core keeps only the output bits (DTR, RTS, OUT1, OUT2, LOOP) of the
TIOCMSET/TIOCMBIS/TIOCMBIC value in port->at_data.mdmbits, and calls
->dtr_rts when DTR or RTS actually 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 if HUPCL is set in the port's termios. This follows TTY
open/close semantics, although the termios baud rate is not used, and
means individual drivers do not need to implement this themselves. As on
a TTY, HUPCL defaults to on for AT ports, and userspace can clear it
with TCSETS to keep DTR asserted across close, for example so that
closing the port does not end a call on a modem set to AT&D1/AT&D2.
TCSETS and TCSETS2 now only commit the new termios when the copy from
userspace succeeds, so a faulting call cannot silently clear HUPCL.

->dtr_rts must be called under ops_lock, which serialises port ops and
protects against port removal (removal clears port->ops under it). So
that updates to the TIOCM state and the ->dtr_rts calls are serialised
together, the termios/TIOCM state now also lives under ops_lock and the
separate data_lock is removed. The AT/QCDM ioctl handler only holds
ops_lock while it reads or updates that state, never across a
user-memory copy, and takes it interruptibly since a driver's blocking
write holds it while it waits. Ioctls that do not touch that state, such
as TCFLSH, TIOCEXCL and the FIONREAD fallback, do not take it.

wwan_remove_port() also drops DTR/RTS before ->stop() when a port is
removed while still open, regardless of HUPCL since the device is going
away. The raise and drop paths go through two small helpers, which do
the AT port and ->dtr_rts checks and pass the driver the resulting
bitmask.

Signed-off-by: Peter Hunt <peter.hunt@opengear.com>
Reviewed-by: Loic Poulain <loic.poulain@oss.qualcomm.com>
Link: https://patch.msgid.link/20261013183500.261229-2-peter.hunt@opengear.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions