Hi guys,
I finally started implementing code for ABS_MT_TOUCH_MAJOR to try to work
with better touch shapes (in libinput). The result is... discouraging.
libinput use physical dimensions everywhere possible, e.g. touchpad software
features are X mm large, etc. but for ABS_MT_TOUCH_MAJOR it's almost
impossible (ABS_MT_TOUCH_MINOR has the same issues, but is omitted for
brevity).
ABS_MT_MAJOR is documented as, paraphrased, the longer axis of the ellipsis,
in applicable surface units at the current rotation. So at a rotation of 0
(== vertical), ABS_MT_MAJOR corresponds to y units. This is... suboptimal,
to get to the physical lengths we have to do sin/cos on every event. That
gets more entertaining on devices with uneven x/y resolutions. AFAICT no
driver sets a specific resolution for major.
What we have in the kernel is a mix of touch_major min-max ranges
(largely independent of the x/y axes) with some orientation value that may
or may not help.
bcm5974 driver:
ABS_MT_TOUCH_MAJOR max is hardcoded as 2048, the actual value is sent is
double whatever the hardware gives us. There is no resolution for x/y but
libinput has a hwdb entry for those touchpads to get at the physical
dimensions. Without that, conversion would be impossible.
Either way, I wonder what data the device provides and whether sin/cos
gets us to the real values, esp. as this device has uneven x/y
resolutions.
Orientation is -16384..16384, forwarded as-is from the hardware but with a
large fuzz of ~3200.
magic trackpad:
driver just sends four times whatever the hardware gives us.
ABS_MT_TOUCH_MAJOR max is hardcoded to 1020 but we have x/y resolution
(hardcoded in the driver). Doing the conversion means the largest
detectable touch is 22mm. But the values are generally below what is
actually in contact [1].
Orientation is hardcoded to -31..32, forwarded as is, seems to be largely
accurate.
hid-mt:
maps HID_DG_WIDTH to ABS_MT_TOUCH_MAJOR. The x/y resolution is set
depending on whether the HID field has the unit attribute set.
The HID spec for HID_DG_WIDTH says "Unit are assumed to match x's units",
but hid-mt takes whatever the larger axis is as major. So alternatively it
may send width as major, or height. At least orientation is only 0 or 1,
so this could be reversed in userspace. That matches the kernel
documentation:
ABS_MT_TOUCH_MAJOR := max(X, Y)
ABS_MT_TOUCH_MINOR := min(X, Y)
ABS_MT_ORIENTATION := bool(X > Y)
This cannot reliably work for uneven x/y resolutions though because the
max() takes device units.
wacom:
ABS_MT_TOUCH_MAJOR max matches x max, otherwise see hid-mt logic
hid-ntrig:
looks like the same as wacom
rmi4:
ABS_MT_TOUCH_MAJOR max is hardcoded to 0xf. x/y has resolution of 12 or 20
in the devices I've seen so far. orientation is only 0 or 1, uses same
logic as hid-mt. Orientation is 0/1.
Based on a recording in [2], the highest value is 25, but firmware palm
detection kicks in at 13 already. Even at the lower resolution of 12,
the touch major max value of 25 would be a 2mm touch. That's a very small
palm.
cyapa:
ABS_MT_TOUCH_MAJOR max is hardcoded to 255, orientation is -127..127, data
is forwarded as-is from the hardware. The max is 255 because it only gets
one byte in the protocol.
I don't have a device to read the x/y ranges, so I can't verify
what a 255 would correspond to. The ones I've seen are 1280x960 with
uneven x/y resolutions, so 255 would correspond to ca 1/4 of the touchpad.
That seems sensible.
elantech:
ABS_MT_TOUCH_MAJOR max is set up as 15 ("finger width") times something
called max_width. In one recording I have it ends up at 2445 (x/y max
are 3260/2282 [3]) but it may swap x and y during initialization already,
so we have nothing to go on. It doesn't report orientation and
reports major as max of (x, y), so we can't guess how to get back to
physical coordinates.
atmel:
missing ABS_MT_TOUCH_MAJOR, forwards the 'area' as major (that's
protocol-correct). ABS_MT_TOUCH_MAJOR max is 0xff, but x/y have a
resolution of 20. So that would leave us with a maximum touch size of ~12
mm. That doesn't seem correct either.
There are a few other drivers I didn't check but it comes down to: for the
purpose of actually determining touch size, ABS_MT_TOUCH_MAJOR is largely
useless. I'll need per-device flags and hwdb entries to make this even
remotely useful and even then I'll have to rely on specific driver
behaviour. So the question is, is there anything we can do about this?
Cheers,
Peter
[1] ok, I don't have a 'normalized' finger but guesstimating the contact
surface and comparing the data - the major is smaller by a few mm than the
physical contact.
[2] https://bugs.freedesktop.org/show_bug.cgi?id=100243, the highest
[3] https://github.com/whot/evemu-devices/blob/master/touchpads/ETPS2%20Elantech%20Touchpad.Samsung-700Z3A.events
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