1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 ======================================== 3 ========================================
4 Manual parsing of HID report descriptors 4 Manual parsing of HID report descriptors
5 ======================================== 5 ========================================
6 6
7 Consider again the mouse HID report descriptor 7 Consider again the mouse HID report descriptor
8 introduced in Documentation/hid/hidintro.rst:: 8 introduced in Documentation/hid/hidintro.rst::
9 9
10 $ hexdump -C /sys/bus/hid/devices/0003\:093A 10 $ hexdump -C /sys/bus/hid/devices/0003\:093A\:2510.0002/report_descriptor
11 00000000 05 01 09 02 a1 01 09 01 a1 00 05 11 00000000 05 01 09 02 a1 01 09 01 a1 00 05 09 19 01 29 03 |..............).|
12 00000010 15 00 25 01 75 01 95 03 81 02 75 12 00000010 15 00 25 01 75 01 95 03 81 02 75 05 95 01 81 01 |..%.u.....u.....|
13 00000020 05 01 09 30 09 31 09 38 15 81 25 13 00000020 05 01 09 30 09 31 09 38 15 81 25 7f 75 08 95 03 |...0.1.8..%.u...|
14 00000030 81 06 c0 c0 14 00000030 81 06 c0 c0 |....|
15 00000034 15 00000034
16 16
17 and try to parse it by hand. 17 and try to parse it by hand.
18 18
19 Start with the first number, 0x05: it carries 19 Start with the first number, 0x05: it carries 2 bits for the
20 length of the item, 2 bits for the type of the 20 length of the item, 2 bits for the type of the item and 4 bits for the
21 function:: 21 function::
22 22
23 +----------+ 23 +----------+
24 | 00000101 | 24 | 00000101 |
25 +----------+ 25 +----------+
26 ^^ 26 ^^
27 ---- Length of data (see HID spec 6. 27 ---- Length of data (see HID spec 6.2.2.2)
28 ^^ 28 ^^
29 ------ Type of the item (see HID spec 29 ------ Type of the item (see HID spec 6.2.2.2, then jump to 6.2.2.7)
30 ^^^^ 30 ^^^^
31 --------- Function of the item (see HID sp 31 --------- Function of the item (see HID spec 6.2.2.7, then HUT Sec 3)
32 32
33 In our case, the length is 1 byte, the type is 33 In our case, the length is 1 byte, the type is ``Global`` and the
34 function is ``Usage Page``, thus for parsing t 34 function is ``Usage Page``, thus for parsing the value 0x01 in the second byte
35 we need to refer to HUT Sec 3. 35 we need to refer to HUT Sec 3.
36 36
37 The second number is the actual data, and its 37 The second number is the actual data, and its meaning can be found in
38 the HUT. We have a ``Usage Page``, thus we nee 38 the HUT. We have a ``Usage Page``, thus we need to refer to HUT
39 Sec. 3, "Usage Pages"; from there, one sees th 39 Sec. 3, "Usage Pages"; from there, one sees that ``0x01`` stands for
40 ``Generic Desktop Page``. 40 ``Generic Desktop Page``.
41 41
42 Moving now to the second two bytes, and follow 42 Moving now to the second two bytes, and following the same scheme,
43 ``0x09`` (i.e. ``00001001``) will be followed 43 ``0x09`` (i.e. ``00001001``) will be followed by one byte (``01``)
44 and is a ``Local`` item (``10``). Thus, the me 44 and is a ``Local`` item (``10``). Thus, the meaning of the remaining four bits
45 (``0000``) is given in the HID spec Sec. 6.2.2 45 (``0000``) is given in the HID spec Sec. 6.2.2.8 "Local Items", so that
46 we have a ``Usage``. From HUT, Sec. 4, "Generi 46 we have a ``Usage``. From HUT, Sec. 4, "Generic Desktop Page", we see that
47 0x02 stands for ``Mouse``. 47 0x02 stands for ``Mouse``.
48 48
49 The following numbers can be parsed in the sam 49 The following numbers can be parsed in the same way.
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