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

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

Hi Evan,

On Mon, Aug 10, 2026 at 02:25:03PM +0000, Evan Lawrence wrote:
> Synaptics touchpads with a pass-through port (PS/2 guest) default to
> encapsulating guest data in the touchpad's own packets, limiting the
> guest device (typically a TrackPoint) to the touchpad's low poll rate.
> Enabling transparent pass-through mode tells the touchpad to stop
> generating its own packets and relay the raw byte stream from the guest
> directly, letting the TrackPoint run at its full native speed while the
> touchpad is effectively disabled.
> 
> This reworks an earlier implementation by Miroslav Bendík from 2022
> that was never merged. It keeps the same hardware-mode approach but
> commits the mode flag only after a successful mode command, moves the
> reconnect handling into a generic psmouse hook instead of protocol-
> specific checks in psmouse-interrupt, and documents the interface.
> 
> The mode is exposed via a sysfs transparent_mode attribute that allows
> userspace to switch between full-rate TrackPoint and full-rate touchpad
> operation at runtime.
> 
> If the system warm-boots while transparent mode is active the touchpad
> stays in that mode and does not acknowledge command bytes, so
> ps2_command() hangs and the device fails to be detected.  Escape
> transparent mode by writing SETSCALE21 + SETSCALE11 directly to the wire
> with serio_write() before attempting a reset or probe, both during
> initial detection and on reconnect.

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)?

Thanks.

-- 
Dmitry