Re: [PATCH] thunderbolt: Reset downstream port after failed link restore

[Mika Westerberg]

Hi,

On Sat, Aug 29, 2026 at 04:57:05PM -0700, Amaan Lalani wrote:
> A directly connected Thunderbolt device may fail to restore
> its link after a runtime suspend, therefore leaving the connection
> unusable.

How does it show up? Can you share more details, like full dmesg with
thunderbolt.dyndbg=+p in the command line?

> Reset the downstream port when the link restoration fails. This drives
> SBTX low, causing the partner to observe a USB4 disconnect and allowing
> the Type-C/PD firmware to renegotiate the connection. For dual-mode
> devices, this may allow the connection to fall back to native USB3.x.
> 
> Tested on a Microsoft Surface Pro 11 with a UGreen Thunderbolt
> 4 external NVMe enclosure, where the device successfully reconnects as a
> USB3 device when the Thunderbolt 4 link fails to recover.
> 
> Signed-off-by: Amaan Lalani <zlzzm2014@xxxxxxxxxxx>
> ---
>  drivers/thunderbolt/switch.c | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/thunderbolt/switch.c b/drivers/thunderbolt/switch.c
> index 404c0693df50..7b88aa75a07a 100644
> --- a/drivers/thunderbolt/switch.c
> +++ b/drivers/thunderbolt/switch.c
> @@ -3600,7 +3600,27 @@ int tb_switch_resume(struct tb_switch *sw, bool runtime)
>  
>  		if (tb_wait_for_port(port, true) <= 0) {
>  			tb_port_warn(port,
> -				     "lost during suspend, disconnecting\n");
> +				    "lost during suspend, disconnecting\n");
> +			/*
> +			 * If a directly connected USB4/thunderbolt device did not restore
> +			 * its link after a runtime suspend, assert a downstream
> +			 * port reset. This drives SBTX low and makes the partner
> +			 * observe a real USB4 disconnect. A dual-mode device and
> +			 * the Type-C/PD firmware will then try to renegotiate
> +			 * the connection in a native USB 3.x mode.
> +			 *
> +			 * Restrict this to the host router's downstream port.
> +			 * Resetting an intermediate router would not cause the Type-C
> +			 * to be renegotiated.
> +			 */
> +			if (runtime && !tb_route(sw) &&
> +			    tb_switch_is_usb4(sw) && port->cap_usb4) {

Yea I'm not entirely sure we want to add hacks like this to the driver to
be honest.

The link should re-negotiate as USB4 so something is wrong there. Have you
checked if you have the latest firmwares on both sides?

> +				err = tb_port_reset(port);
> +				if (err)
> +					tb_port_warn(port,
> +						     "failed to reset port for USB3 fallback: %d\n",
> +						     err);
> +			}
>  			if (tb_port_has_remote(port))
>  				tb_sw_set_unplugged(port->remote->sw);
>  			else if (port->xdomain)
> -- 
> 2.55.0