HID: Add introduction about HID for non-kernel programmers

Add an introduction about HID meant for the casual programmer
that is trying either to fix his device or to understand
what is going wrong.

Signed-off-by: Marco Morandini <marco.morandini@polimi.it>
Co-authored-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

1 files changed, 49 insertions, 0 deletions

diff --git a/Documentation/hid/hidreport-parsing.rst b/Documentation/hid/hidreport-parsing.rst
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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.