Re: [PATCH] net: wwan: t7xx: Add delay between MD and SAP suspend

[Loic Poulain]

On Wed, May 27, 2026 at 8:15 AM Jose Ignacio Tornos Martinez
<jtornosm@xxxxxxxxxx> wrote:
>
> SAP (Service Access Point) suspend occasionally times out with error
> -110 (ETIMEDOUT), followed by modem port errors and complete modem
> failure requiring a system reboot to recover.
>
> Error symptoms:
>   mtk_t7xx 0000:72:00.0: [PM] SAP suspend error: -110
>   mtk_t7xx 0000:72:00.0: can't suspend (...returned -110)
>   mtk_t7xx 0000:07:00.0: Failed to send skb: -22
>   mtk_t7xx 0000:07:00.0: Write error on MBIM port, -22
>
> The modem firmware needs time after receiving the MD (modem) suspend
> request to complete internal operations before it is ready to accept
> the SAP suspend request. Without this delay, if runtime PM attempts
> to suspend while the firmware is busy, the SAP suspend command times
> out, leaving the modem in an unrecoverable state.
>
> Root cause and userspace interaction:
> ModemManager 1.24+ includes changes that reduce the likelihood of this
> issue by ensuring the modem is in a low-power state before the kernel
> attempts runtime suspend. However, the kernel driver should not depend
> on specific userspace behavior or ModemManager versions. Older versions
> (1.20-1.22) are still widely deployed, and the kernel should be robust
> regardless of userspace implementation details.
>
> There appears to be no hardware status register or other mechanism
> available to query whether the firmware is ready for SAP suspend.
> A delay between the two suspend requests is the most reliable solution
> found through testing.
>
> Add a 50ms delay between MD suspend and SAP suspend. This gives the
> firmware adequate time to complete internal operations without adding
> significant latency to the suspend path. This makes the driver robust
> across all ModemManager versions and system conditions.
>
> Testing: 96+ hours of continuous operation with ModemManager 1.20.2
> and Fibocom FM350-GL modem. Zero SAP suspend timeouts observed across
> 2000+ successful suspend/resume cycles. Previously failed within
> 24 hours with 100% reproducibility.
>
> Signed-off-by: Jose Ignacio Tornos Martinez <jtornosm@xxxxxxxxxx>

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

> ---
>  drivers/net/wwan/t7xx/t7xx_pci.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/net/wwan/t7xx/t7xx_pci.c b/drivers/net/wwan/t7xx/t7xx_pci.c
> index eb137e078423..46613bba1f41 100644
> --- a/drivers/net/wwan/t7xx/t7xx_pci.c
> +++ b/drivers/net/wwan/t7xx/t7xx_pci.c
> @@ -447,6 +447,9 @@ static int __t7xx_pci_pm_suspend(struct pci_dev *pdev)
>                 goto abort_suspend;
>         }
>
> +       /* Delay to prevent SAP suspend timeout */
> +       msleep(50);
> +
>         ret = t7xx_send_pm_request(t7xx_dev, H2D_CH_SUSPEND_REQ_AP);
>         if (ret) {
>                 t7xx_send_pm_request(t7xx_dev, H2D_CH_RESUME_REQ);
> --
> 2.54.0
>