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55 lines
2.9 KiB
Text
55 lines
2.9 KiB
Text
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/**
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@page touchpad_jumping_cursor Touchpad jumping cursor bugs
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A common bug encountered on touchpads is a cursor jump when alternating
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between fingers on a multi-touch-capable touchpad. For example, after moving
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the cursor a user may use a second finger in the software button area to
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physically click the touchpad. Upon setting the finger down, the cursor
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exhibits a jump towards the bottom left or right, depending on the finger
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position.
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When libinput detects a cursor jump it prints a bug warning to the log with
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the text <b>"Touch jump detected and discarded."</b> and a link to this page.
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In most cases, this is a bug in the kernel driver and to libinput it appears
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that the touch point moves from its previous position. The pointer jump can
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usually be seen in the evemu-record output for the device:
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<pre>
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E: 249.206319 0000 0000 0000 # ------------ SYN_REPORT (0) ----------
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E: 249.218008 0003 0035 3764 # EV_ABS / ABS_MT_POSITION_X 3764
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E: 249.218008 0003 0036 2221 # EV_ABS / ABS_MT_POSITION_Y 2221
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E: 249.218008 0003 003a 0065 # EV_ABS / ABS_MT_PRESSURE 65
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E: 249.218008 0003 0000 3764 # EV_ABS / ABS_X 3764
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E: 249.218008 0003 0001 2216 # EV_ABS / ABS_Y 2216
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E: 249.218008 0003 0018 0065 # EV_ABS / ABS_PRESSURE 65
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E: 249.218008 0000 0000 0000 # ------------ SYN_REPORT (0) ----------
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E: 249.230881 0003 0035 3752 # EV_ABS / ABS_MT_POSITION_X 3752
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E: 249.230881 0003 003a 0046 # EV_ABS / ABS_MT_PRESSURE 46
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E: 249.230881 0003 0000 3758 # EV_ABS / ABS_X 3758
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E: 249.230881 0003 0018 0046 # EV_ABS / ABS_PRESSURE 46
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E: 249.230881 0000 0000 0000 # ------------ SYN_REPORT (0) ----------
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E: 249.242648 0003 0035 1640 # EV_ABS / ABS_MT_POSITION_X 1640
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E: 249.242648 0003 0036 4681 # EV_ABS / ABS_MT_POSITION_Y 4681
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E: 249.242648 0003 003a 0025 # EV_ABS / ABS_MT_PRESSURE 25
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E: 249.242648 0003 0000 1640 # EV_ABS / ABS_X 1640
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E: 249.242648 0003 0001 4681 # EV_ABS / ABS_Y 4681
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E: 249.242648 0003 0018 0025 # EV_ABS / ABS_PRESSURE 25
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E: 249.242648 0000 0000 0000 # ------------ SYN_REPORT (0) ----------
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E: 249.254568 0003 0035 1648 # EV_ABS / ABS_MT_POSITION_X 1648
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E: 249.254568 0003 003a 0027 # EV_ABS / ABS_MT_PRESSURE 27
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E: 249.254568 0003 0000 1644 # EV_ABS / ABS_X 1644
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E: 249.254568 0003 0018 0027 # EV_ABS / ABS_PRESSURE 27
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</pre>
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In this recording, the pointer jumps from its position 3752/2216 to
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1640/4681 within a single frame. On this particular touchpad, this would
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represent a physical move of almost 50mm. libinput detects some of these
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jumps and discards the movement but otherwise continues as usual. However,
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the bug should be fixed at the kernel level.
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When you encounter the warning in the log, please generate an evemu
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recording of your touchpad and file a bug. See @ref reporting_bugs for more
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details.
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*/
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