Re: [PATCH net v2] net: wwan: mhi_wwan_ctrl: send a DTR drop before it can be coalesced

[Loic Poulain]

On Tue, Oct 27, 2026 at 6:11 PM Peter Hunt <peter.hunt@xxxxxxxxxxxx> wrote:
>
> DTR/RTS changes are sent from a work item that only sends the latest
> state, so a drop followed quickly by a raise, for example TIOCMBIC then
> TIOCMBIS or a quick close and reopen, can reach the modem as just the
> raise. A drop matters on its own, a modem set to AT&D2 hangs up on it.
>
> Flush the work when DTR goes low so the drop is sent before ->dtr_rts()
> returns, and flush it in stop() so any pending state is sent before the
> channel is stopped. A drop that hits a full UL ring is still retried
> with whatever state is latest by then.
>
> Fixes: d509edf90bc6 ("net: wwan: mhi_wwan_ctrl: drive DTR/RTS via the IP_CTRL channel")
> Suggested-by: Loic Poulain <loic.poulain@xxxxxxxxxxxxxxxx>
> Signed-off-by: Peter Hunt <peter.hunt@xxxxxxxxxxxx>

Reviewed-by: Loic Poulain <loic.poulain@xxxxxxxxxxxxxxxx>

> ---
> v2: Only handle the DTR drop, by flushing the work when DTR goes low,
>     and flush it in stop() as well (Loic Poulain). Drop the port list
>     and resend on a late IP_CTRL bind, which is only reachable by
>     unbinding and rebinding mhi_wwan_dtr through sysfs.
> v1: https://lore.kernel.org/all/20261023174113.2388353-1-peter.hunt@xxxxxxxxxxxx/
>
> Tested on a Sierra EM9291 (CM8100): back-to-back DTR drop+raise and
> close+reopen now always send the drop, bursts of 10,000 DTR changes,
> HUPCL, teardown and lock stress tests.
>
>  drivers/net/wwan/mhi_wwan_ctrl.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/drivers/net/wwan/mhi_wwan_ctrl.c b/drivers/net/wwan/mhi_wwan_ctrl.c
> index e3c207588956..772319927867 100644
> --- a/drivers/net/wwan/mhi_wwan_ctrl.c
> +++ b/drivers/net/wwan/mhi_wwan_ctrl.c
> @@ -209,9 +209,17 @@ static void mhi_wwan_ctrl_dtr_work(struct work_struct *work)
>  static void mhi_wwan_ctrl_dtr_rts(struct wwan_port *port, unsigned int mdmbits)
>  {
>         struct mhi_wwan_dev *mhiwwan = wwan_port_get_drvdata(port);
> +       bool drop = (READ_ONCE(mhiwwan->dtr_mdmbits) & TIOCM_DTR) &&
> +                   !(mdmbits & TIOCM_DTR);
>
>         WRITE_ONCE(mhiwwan->dtr_mdmbits, mdmbits);
>         schedule_delayed_work(&mhiwwan->dtr_work, 0);
> +
> +       /* A DTR drop matters on its own, e.g. to hang up a modem set to AT&D2,
> +        * so send it now rather than let a following raise coalesce it away
> +        */
> +       if (drop)
> +               flush_delayed_work(&mhiwwan->dtr_work);
>  }
>
>  static int mhi_wwan_ctrl_start(struct wwan_port *port)
> @@ -246,6 +254,9 @@ static void mhi_wwan_ctrl_stop(struct wwan_port *port)
>
>         cancel_work_sync(&mhiwwan->rx_refill);
>
> +       /* Send any pending DTR/RTS state before the channel stops */
> +       flush_delayed_work(&mhiwwan->dtr_work);
> +
>         mhi_unprepare_from_transfer(mhiwwan->mhi_dev);
>  }
>
> --
> 2.43.0
>