Re: [PATCH net] net: ipa: fix stalled modem TX queue after runtime resume

[Simon Horman]

On Sat, Aug 15, 2026 at 06:03:02AM +0200, Jorijn van der Graaf wrote:
> ipa_start_xmit() unconditionally stops the TX queue before calling
> pm_runtime_get(), relying on the wake scheduled by runtime resume
> (ipa_modem_wake_queue_work()) to restart it once power is ACTIVE.
> But that work is queued from within the runtime resume callback,
> before the device's power state reaches RPM_ACTIVE, so it can run
> while the device is still RPM_RESUMING.  The wake is then consumed
> too early: the transmit it restarts stops the queue again,
> pm_runtime_get() returns -EINPROGRESS without arranging any future
> wake (deferred_resume exists only for RPM_SUSPENDING), and after the
> resume completes nothing is left to wake the queue.  Transmit stalls
> permanently: packets pile up in the qdisc behind the stopped queue,
> the device runtime-suspends, and since the netdev registers no
> ndo_tx_timeout the watchdog never fires.  Observed on SM7635
> (Fairphone 6) as the cellular data path going permanently deaf
> within hours, RX included, since nothing resumes the suspended
> endpoints.
> 
> Close the window by making the wake work wait for the resume to
> complete (pm_runtime_get_sync()) before waking the queue.  Every
> queue stop is then guaranteed a later wake that happens while power
> is ACTIVE; a transmit racing a new suspend/resume cycle re-schedules
> the work.  If the device could not be resumed, wake the queue anyway
> so pending packets are dropped by the transmit path rather than
> stranded.
> 
> The STARTED power flag used to narrow this window: a wake running
> before the transmit path's stop suppressed that stop, but only once,
> as the flag was cleared by the first stop it absorbed.  Removing the
> flag made a single transmit during an in-flight resume sufficient to
> strand the queue, which is the form observed.
> 
> With an accelerated reproducer (autosuspend delay shortened to 5 ms,
> ~20 packets/s of TX), an unpatched kernel stalled three times in
> 230 s / 4380 packets; with this patch the same test ran 3601 s /
> 70298 packets without a stall.
> 
> Fixes: 688de12f080f ("net: ipa: kill the STARTED IPA power flag")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Jorijn van der Graaf <jorijnvdgraaf@xxxxxxxxxxxxx>
> ---
> 
> Runtime testing was done on a v7.1.2-based device kernel carrying
> this same change, on a drivers/net/ipa/ipa_modem.c otherwise identical
> to this tree's; the patch as posted was build-tested on net at the
> base commit.
> 
>  drivers/net/ipa/ipa_modem.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)

Reviewed-by: Simon Horman <horms@xxxxxxxxxx>