software-trackpoint input driver

[Peter Hutterer]

On Wed, Aug 21, 2013 at 11:38:38PM -0500, Andrew Deason wrote:
> Hi,
> 
> I have a USB trackpoint device I would like to use with X on Linux (the
> trackpoint is the nub mouse thing on some keyboards, usually known for
> laptop keyboards). All current input drivers seem to interpret movement
> data for it incorrectly, and I'd like to fix that. But I'm unfamiliar
> with xorg development, so I'm not sure where such code "should" go.

what product is this? do you have a link to a description or a photo? also,
how common is this?

> So anyway, for reference, the device looks like this:
> 
> # evtest /dev/input/event17
> Input driver version is 1.0.1
> Input device ID: bus 0x3 vendor 0x6cb product 0x9 version 0x15
> Input device name: "Synaptics Inc. Composite TouchPad / TrackPoint (Stick)"
> Supported events:
>   Event type 0 (EV_SYN)
>   Event type 1 (EV_KEY)
>     Event code 272 (BTN_LEFT)
>     Event code 273 (BTN_RIGHT)
>     Event code 274 (BTN_MIDDLE)
>   Event type 2 (EV_REL)
>     Event code 0 (REL_X)
>     Event code 1 (REL_Y)
>   Event type 3 (EV_ABS)
>     Event code 24 (ABS_PRESSURE)
>       Value      0
>       Min        0
>       Max      127
> 
> The information that Linux gives us about this device is something like
> the pressure readings for the x (REL_X), y (REL_Y), and z (ABS_PRESSURE)
> axes [1]. Most trackpoint devices I have encountered before internally
> process these pressure readings and just give the OS relative pointer
> movements, but this one seems to rely on an OS driver performing that
> calculation [2]. Right now by default this gets handled by evdev, which
> just interprets the x and y coordinates as relative mouse movements, and
> ignores the pressure. I would like something to interpret that pressure,
> as it results in a much more familiar and intuitive trackpoint feel (how
> all other trackpoint devices feel).
> 
> As mentioned, I'm unfamiliar with xorg dev, so I'm not exactly sure
> where support for this should go. There is an existing driver for
> 'synaptics', but from what I can tell it is focused on touchpads, not
> trackpoints. There is also another driver for pointing sticks
> specifically: <https://github.com/hiikezoe/xf86-input-pointing-stick>,
> but currently it does not support devices like this.
> 
> Modifying evdev to interpret the ABS_PRESSURE axis seems easy enough
> (that's what I'm doing right now, so I can use it), but I'm not sure if
> it's frowned upon to put such device-specific stuff in evdev? I could
> add support to xf86-input-pointing-stick, or even add an entirely new
> driver just for this, but evdev has other features I want for this, like
> emulated wheel scrolling; duplicating code for those features in other
> driver seems not great.
> 
> So my intuition says to just add support for evdev. Or the Linux kernel
> driver could do this kind of calculation itself, to make the device
> appear more like other trackpoint devices, like the PS/2 ones; then the
> evdev X driver would do the right thing. Am I on the right track, here?

adding a new xorg driver is generally a bad idea, we're trying to reduce the
number to the sane minimum that's really needed to get the various feature
sets. you can try to get this into the kernel, but I'm not sure that'd be
beyond what the kernel is happy to do.

anyway. evdev could support this but it would require a custom pointer
acceleration method similar to what synaptics has. and have that enabled for
this device only (i.e. optional, with an entry in the quicks.conf to apply
it for this device). not a big fan of this either, tbh, because chances are
this will break at random time because we don't have any upstream developers
testing it.

Cheers,
   Peter


> [1] Actually, there are two modes of operation, via USB alternate
> interfaces. One interface provides regular HID mouse relative
> coordinates, and that works fine, but does not allow the user to change
> the speed/sensitivity/etc that is used to generate the coordinates. The
> other "advanced" interface provides the information described above, and
> that is currently what the synaptics_usb driver in Linux always uses.
> 
> [2] Aside from the fact that we get a separate 'pressure' reading at
> all, which suggests this, this has been confirmed by myself and others
> by sniffing USB traffic on a Windows machine with the
> IBM/Lenovo-provided driver. Changing sensitivity settings for the
> trackpoint results in no USB traffic, so this clearly doesn't work like
> e.g. PS/2 trackpoint devices.
> 
> -- 
> Andrew Deason
> adeason at dson.org
>