Re: [PATCH net-next v5 0/3] net: wwan: support DTR/RTS on AT ports via MHI IP_CTRL

[Manivannan Sadhasivam]

On Thu, Aug 20, 2026 at 08:49:24AM +1000, Peter Hunt wrote:
> Qualcomm/Sierra SDX55/SDX65 modems (e.g. EM9291) withhold unsolicited AT
> result codes until the host asserts DTR.  The in-tree mhi_wwan_ctrl driver
> exposed AT ports but never signalled DTR, so URCs never reached userspace.
> 
> Patch 1 extends the wwan core with an optional ->dtr_rts(port, mdmbits) port
> op.  The TIOCM bitmask state is tracked entirely in the wwan core; drivers
> receive the full bitmask so they can drive DTR and RTS independently.  The
> core raises DTR/RTS on first open of any AT port whose driver implements
> ->dtr_rts and drops them on last close, mirroring TTY semantics.  In the
> ioctl path mdmbits is re-read under data_lock inside ops_lock so the value
> passed always reflects the committed state and concurrent ioctls are
> correctly ordered.  wwan_remove_port() also de-asserts on hot-unplug.
> 
> Patch 2 enables the IP_CTRL MHI channel in the Sierra PCI table so that
> the IP_CTRL driver (patch 3) is actually bound on those controllers.
> 
> Patch 3 adds a second mhi_driver that binds the IP_CTRL channel and
> registers a ->dtr_rts op so that the wwan core's open/close DTR raise/drop
> and userspace TIOCMSET calls both reach the modem.  A single recycled sink
> buffer keeps the IP_CTRL DL ring live so the modem's transmit path does not
> stall.  The existing AT/QMI/MBIM data path is untouched.
> 
> v4: https://lore.kernel.org/netdev/20260816121705.858013-1-peter.hunt@xxxxxxxxxxxx/
> 
> v5:
> - Change ->dtr_rts from bool to unsigned int mdmbits (patch 1 + 3): DTR and
>   RTS are now driven independently; fixes RTS inversion when TIOCMBIS/BIC
>   changed RTS with DTR in the opposite state
> - Re-read mdmbits inside ops_lock in the ioctl path (patch 1): closes a
>   concurrent-ioctl ordering race where two racing TIOCM ioctls could leave
>   the modem line state inconsistent with TIOCMGET
> - Add ->dtr_rts(port, 0) call in wwan_remove_port() (patch 1): ensures
>   DTR/RTS are de-asserted when a port is removed while an fd is still open
> - Update kernel-doc to note ->dtr_rts is AT-only and describe mdmbits (patch 1)
> - Pre-queue RX sink buffer in mhi_wwan_dtr_probe(), requeue in dl_xfer_cb
>   (patch 3): prevents IP_CTRL DL ring from running empty and stalling the
>   modem MHI transmit path
> 
> Peter Hunt (3):
>   net: wwan: core: propagate modem control signals to port drivers
>   bus: mhi: host: pci_generic: enumerate IP_CTRL channel for Sierra
>     EM919x/EM929x

FYI, this patch got applied to MHI tree.

- Mani

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