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