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diff --git a/Documentation/hid/hidreport-parsing.rst b/Documentation/hid/hidreport-parsing.rst new file mode 100644 index 0000000000..1d3c17f29f --- /dev/null +++ b/Documentation/hid/hidreport-parsing.rst @@ -0,0 +1,49 @@ +.. 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. |