Re: [PATCH v5] Input: synaptics - add pass-through mode for TrackPoint

[Evan Lawrence <development@laserology.net>]

Hi Dmitry,

On Thu, Aug 14, 2026 at 07:13:00PM +0000, Dmitry Torokhov wrote:
> I have not looked at the implementation yet, just a high-level question:
>
> What devices need this? Can they work in SMbus/RMI4 mode instead (which
> should alleviate bandwidth concerns)?

The target devices are ThinkPad T14/P14s Gen 1/2-class Synaptics pass-through touchpads. The primary test hardware is a ThinkPad T14 Gen 1 (AMD), whose touchpad reports firmware id LEN2064 and whose TrackPoint is an Elan PS/2 device connected through the touchpad's pass-through port.

On this device SMBus/RMI4 is not a viable alternative. Even with psmouse.synaptics_intertouch=1 forced and the touchpad re-probed, it stays in PS/2 mode (no RMI4 device is created). The touchpad's own bandwidth isn't problem; it reports at 75-85 Hz in both modes. The bottleneck is specifically the TrackPoint: behind the pass-through port it is serviced at only 35-40 Hz, but with transparent mode enabled it reaches 75-110 Hz. The exact peak rate may vary by model, but the improvement should be consistently positive.

The transparent pass-through mode addresses exactly that PS/2 pass-through bottleneck by relaying the guest's raw byte stream directly instead of encapsulating it.

I'll follow up with a v6 that also reduces the mode-switch latency by avoiding the ACK-waiting ps2_command() calls on the escape sequence; I will send it as soon as I have tested it.

Thanks, Evan