50 lines
1.9 KiB
ReStructuredText
50 lines
1.9 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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========================================
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Manual parsing of HID report descriptors
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========================================
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Consider again the mouse HID report descriptor
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introduced in Documentation/hid/hidintro.rst::
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$ hexdump -C /sys/bus/hid/devices/0003\:093A\:2510.0002/report_descriptor
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00000000 05 01 09 02 a1 01 09 01 a1 00 05 09 19 01 29 03 |..............).|
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00000010 15 00 25 01 75 01 95 03 81 02 75 05 95 01 81 01 |..%.u.....u.....|
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00000020 05 01 09 30 09 31 09 38 15 81 25 7f 75 08 95 03 |...0.1.8..%.u...|
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00000030 81 06 c0 c0 |....|
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00000034
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and try to parse it by hand.
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Start with the first number, 0x05: it carries 2 bits for the
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length of the item, 2 bits for the type of the item and 4 bits for the
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function::
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+----------+
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| 00000101 |
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+----------+
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^^
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---- Length of data (see HID spec 6.2.2.2)
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^^
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------ Type of the item (see HID spec 6.2.2.2, then jump to 6.2.2.7)
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^^^^
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--------- Function of the item (see HID spec 6.2.2.7, then HUT Sec 3)
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In our case, the length is 1 byte, the type is ``Global`` and the
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function is ``Usage Page``, thus for parsing the value 0x01 in the second byte
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we need to refer to HUT Sec 3.
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The second number is the actual data, and its meaning can be found in
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the HUT. We have a ``Usage Page``, thus we need to refer to HUT
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Sec. 3, "Usage Pages"; from there, one sees that ``0x01`` stands for
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``Generic Desktop Page``.
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Moving now to the second two bytes, and following the same scheme,
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``0x09`` (i.e. ``00001001``) will be followed by one byte (``01``)
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and is a ``Local`` item (``10``). Thus, the meaning of the remaining four bits
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(``0000``) is given in the HID spec Sec. 6.2.2.8 "Local Items", so that
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we have a ``Usage``. From HUT, Sec. 4, "Generic Desktop Page", we see that
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0x02 stands for ``Mouse``.
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The following numbers can be parsed in the same way.
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