Finding available keyboards on linux - keyboard

Question

I want to bind to the hidraw of any plugged in keyboard. In order to do that I need to find all input devices that are actually keyboards.
I have done some research and want to share some of my findings to get some inputs around that subject.
I can get a list of usb devices using lsusb -v.
There I can find keyboards by interface class
Bus 001 Device 002: ID 17ef:6099 Lenovo Lenovo Traditional USB Keyboard
Device Descriptor:
...
idVendor 0x17ef Lenovo
idProduct 0x6099
...
Configuration Descriptor:
...
Interface Descriptor:
bLength 9
bDescriptorType 4
bInterfaceNumber 0
bAlternateSetting 0
bNumEndpoints 1
bInterfaceClass 3 Human Interface Device
bInterfaceSubClass 1 Boot Interface Subclass
bInterfaceProtocol 1 Keyboard
iInterface 0
that would be one way to locate keyboards and then use idVendor and idProduct to find the correlating device, but I felt like looking for another generic approach that is not limited to usb devices

Answer 1

By iterating through sys/class/input/inputN/capabilities/key I can get devices that have key events and check for the proper mapping.
The mapping correlates with the definition in /usr/include/linux/input-event-codes.h.
So in the above mentioned case key contains 1000000000007 ff9f207ac14057ff ff980000000007ff febeffdfffefffff fffffffffffffffe
the blocks are each 16 hex values so one block contains 8 bytes or 64 bits, with the least significant to the right. The first block only contains 13 hey values or 52 bits. A total of 52 + 64 + 64 + 64 = 244 bits.
start | end | bits
----- | --- | -------------------------------------------------------------------------------
243 | 192 | 0001 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0111
191 | 128 | 1111 1111 1001 1000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0111 1111 1111
1111 1111 1001 1111 0010 0000 0111 1010 1100 0001 0100 0000 0101 0111 1111 1111
127 | 64 | 1111 1110 1011 1110 1111 1111 1101 1111 1111 1111 1110 1111 1111 1111 1111 1111
63 | 0 | 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1110
This means that in my example all keys from bit 1 to bit 138 are defined but
0 ... KEY_RESERVED
84 ... (undefined)
101 ... KEY_LINEFEED
112 ... KEY_MACRO
118 ... KEY_KPPLUSMINUS
120 ... KEY_SCALE
and for the rest we have
140 ... KEY_CALC
142 ... KEY_SLEEP
150 ... KEY_WWW
152 ... KEY_COFFEE
158 ... KEY_BACK
159 ... KEY_FORWARD
161 ... KEY_EJECTCD
163 ... KEY_NEXTSONG
164 ... KEY_PLAYPAUSE
165 ... KEY_PREVIOUSSONG
166 ... KEY_STOPCD
173 ... KEY_REFRESH
176 ... KEY_EDIT
177 ... KEY_SCROLLUP
178 ... KEY_SCROLLDOWN
179 ... KEY_KPLEFTPAREN
180 ... KEY_KPRIGHTPAREN
183 ... KEY_F13
184 ... KEY_F14
185 ... KEY_F15
186 ... KEY_F16
187 ... KEY_F18
188 ... KEY_F19
189 ... KEY_F19
190 ... KEY_F20
191 ... KEY_F21
When I want to check if the keyboard supports the keys I need, like a-z would be
16 ... KEY_Q
...
25 ... KEY_P
30 ... KEY_A
...
38 ... KEY_L
44 ... KEY_Z
...
50 ... KEY_M
0111 1111 0000 0111 1111 1100 0011 1111 1111 0000 0000 0000 0000 => 7f07fc3ff0000
all that needs to be checked is if the last block validates fffffffffffffffe & 7f07fc3ff0000 == 7f07fc3ff0000
in bash i used this snippet to get the last part of each key capability, make sure it is interpreted as hex and test if the bitwise-and matches
ls -d1 /sys/class/input/input*/capabilities/key | while read dev; do KEY="$(cat $dev | rev | cut -d ' ' -f 1 | rev)"; [ $(( (0x$KEY & 0x7f07fc3ff0000) == 0x7f07fc3ff0000 )) == 1 ] && echo $dev ; done