Re: [RFC] linux-input alps - loosing precision

["dmitry.torokhov@gmail.com" <dmitry.torokhov@gmail.com>]

Hi Pali,

On Fri, Jan 17, 2020 at 10:42:06AM +0100, Pali Rohár wrote:
> On Thursday 09 January 2020 14:26:16 Leutwein Tobias (BEG/ENC1) wrote:
> > Hello,
> > 
> > At the file
> > https://github.com/torvalds/linux/blob/master/drivers/input/mouse/alps.c
> > I've seen that values coming from the trackpoint/trackstick are divided at some devices, which results in a loss of precision.
> > 
> > As I was not lucky with the behavior of the trackpoint of my computer I've made a fork of libinput https://gitlab.freedesktop.org/leutwe/libinput/tree/master/
> > where I changed  src/filter-trackpoint.c . With this change, the values from lib evdev are multiplied by a factor (dpToFac_au16 []). The array element used from dpToFac_au16 [] is also the value received by evdev.
> > 
> > At higher forces I use a factor much higher than 1, so the undivided value from the device would be the best for me.
> > 
> > In order not to change the behavior for other users, it might be possible to change the divider at runtime via the sys- file system - like it is at trackpoint.c and take the currently used divider as default value.
> > 
> > Positions at source code were I see the division:
> > 
> > alps_process_trackstick_packet_v3()
> > /*
> > * The x and y values tend to be quite large, and when used
> > * alone the trackstick is difficult to use. Scale them down
> > * to compensate.
> > */
> > x /= 8;
> > y /= 8;
> > 
> > alps_process_packet_v6()
> > /* Divide 4 since trackpoint's speed is too fast */ input_report_rel(dev2, REL_X, (char)x / 4); input_report_rel(dev2, REL_Y, -((char)y / 4));
> 
> Hello Dmitry!
> 
> This problem is about fact that alps.c for some ALPS devices already
> truncate precision of trackpoint movement.
> 
> Tobias is unhappy with this fact and would like to get all bits of
> movement information, not just some high.
> 
> But existing userspace application expects that for these devices kernel
> already truncate precision and process truncated data from kernel.
> 
> Now question is: Are we able to send to userspace input data without
> doing truncation and without breaking existing applications?
> 
> It looks like that for such thing we need some userspace <--> kernel API
> which disable truncating of precision.
> 
> Have you been solving similar issue for other other drivers or in other
> area of input code?

Unfortunately I do not think we can automatically "recover" the lost
precision without help of libinput, which would need to tweak the
trackpoint [sysfs] property letting kernel know that it should send
original data.

I'm adding Peter to hear his take on this.

Thanks.

-- 
Dmitry