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CHROMIUM: Input: synaptics - process finger (<=3) transitions
Synaptics image sensor touchpads track 5 fingers, but only report 2. This patch attempts to deal with some idiosyncrasies of these touchpads: * When there are 3 or more fingers, only two are reported. * The touchpad tracks the 5 fingers in slot through slot. * It always reports the lowest and highest valid slots in SGM and AGM packets, respectively. * The number of fingers is only reported in the SGM packet. However, the number of fingers can change either before or after an AGM packet. * Thus, if an SGM reports a different number of fingers than the last SGM, it is impossible to tell whether the intervening AGM corresponds to the old number of fingers or the new number of fingers. * For example, when going from 2->3 fingers, it is not possible to tell whether tell AGM contains slot (old 2nd finger) or slot (new 3rd finger). * When fingers are added one at at time, from 1->2->3, it is possible to track which slots are contained in the SGM and AGM packets: 1 finger: SGM = slot, no AGM 2 fingers: SGM = slot, AGM = slot 3 fingers: SGM = slot, AGM = slot * It is also possible to track which slot is contained in the SGM when 1 of 2 fingers is removed. This is because the touchpad sends a special (0,0,0) AGM packet whenever all fingers are removed except slot: Last AGM == (0,0,0): SGM contains slot Else: SGM contains slot * However, once there are 3 fingers, if exactly 1 finger is removed, it is impossible to tell which 2 slots are contained in SGM and AGM. The (SGM,AGM) could be (0,1), (0,2), or (1,2). There is no way to know. * Similarly, if two fingers are simultaneously removed (3->1), then it is only possible to know if SGM still contains slot. * Since it is not possible to reliably track which slot is being reported, we invalidate the tracking_id every time the number of fingers changes until this ambiguity is resolved when: a) All fingers are removed. b) 4 or 5 fingers are touched, generates an AGM-CONTACT packet. c) All fingers are removed except slot. In this special case, the ambiguity is resolved since by the (0,0,0) AGM packet. Behavior of the driver: When 2 or more fingers are present on the touchpad, the kernel reports up to two MT-B slots containing the position data for two of the fingers reported by the touchpad. If the identity of a finger cannot be tracked when the number-of-fingers changes, the corresponding MT-B slot will be invalidated (track_id set to -1), and a new track_id will be assigned in a subsequent input event report. The driver always reports the total number of fingers using one of the EV_KEY/BTN_TOOL_*TAP events. This could differ from the number of valid MT-B slots for two reasons: a) There are more than 2 fingers on the pad. b) During ambiguous number-of-fingers transitions, the correct track_id for one or both of the slots cannot be determined, so the slots are invalidated. Thus, this is a hybrid singletouch/MT-B scheme. Userspace can detect this behavior by noting that the driver supports more EV_KEY/BTN_TOOL_*TAP events than its maximum EV_ABS/ABS_MT_SLOT. Signed-off-by: Daniel Kurtz <firstname.lastname@example.org> Acked-by: Chase Douglas <email@example.com> Change-Id: I26021fbb30a93e9713ac68be96194364c81c909d BUG=chromium-os:19094 TEST=builds clean; evtest to confirm raw events confirm motion, 2f-tap, 2f-scroll, using xf86-input-cmt Reviewed-on: http://gerrit.chromium.org/gerrit/4258 Reviewed-by: Andrew de los Reyes <firstname.lastname@example.org> Reviewed-by: Daniel Kurtz <email@example.com> Tested-by: Daniel Kurtz <firstname.lastname@example.org>
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