Re: [PATCH] HID: logitech-hidpp: do not overwrite the device's hi-res wheel mode

[johannes . goede]

Hi,

On 23-Sep-26 20:11, Roman Stingler wrote:
> hi_res_scroll_enable() unconditionally puts HID++ 2.0 devices supporting
> the HiRes Wheel feature (0x2121) into high-resolution mode on every
> connect event.
> 
> On at least the MX Master 4 that mode is persistent state in the device.
> With hid-logitech-hidpp unloaded, a mode set from userspace survives
> switching the mouse off and on again. Writing it at connect therefore
> destroys a setting the user configured, and does so on every probe --
> cold boot, receiver replug or module reload -- so userspace cannot
> reliably keep it either: it gets no indication that the mode it set has
> been changed underneath it.
> 
> This became visible when Bolt receivers gained support in 7.3. Before
> that these devices were driven by hid-generic, hid-logitech-hidpp never
> bound to them, and nothing in the kernel wrote the setting.
> 
> 0x2121 exposes getWheelMode alongside setWheelMode. Read the current
> mode and scale vertical_wheel_counter.wheel_multiplier to match rather
> than forcing high resolution: a device left in high-resolution mode
> still gets its multiplier, and one the user configured for low
> resolution is left alone.
> 
> Note this changes behaviour for devices sitting at a low-resolution
> factory default -- the kernel will no longer switch those to
> high-resolution scrolling.
> 
> Link: https://lore.kernel.org/all/20260920094508.39682-1-roman.stingler@xxxxxxxxx/
> Signed-off-by: Roman Stingler <roman.stingler@xxxxxxxxx>

I assume the low-res mode factory default is because not all OS-es
can handle hires mode (I guess mostly much older Os-es cannot).

This regresses (new factory default mice) to no longer support
hi-res wheel mode at all under Linux, even the non Bolt ones even
though this is only a Bolt issue.

So in so far as this is a solution at all, it really should be
limited to Bolt devices only. Although I wonder what switching
Bolt devices to the hidpp driver wins us after this patch?
Do they still gain any meaningful functionality from the switch
to the hidpp driver?

Also I wonder if there is not a non-persistent equivalent of
setWheelMode? That would be ideal.

Do we know what Windows (with Logitech drivers) does here?

I cannot believe that Windows will not use hi-res wheel mode
when the Logitech drivers are installed.

Regards,

Hans