How to debug USB HID scancode-keycode translation in Linux - linux

Question

I have recently converted a 122-key terminal keyboard to USB as a configfs USB gadget (the keyboard portion is using HID codes), it works quite well in except that I cannot seem to get my Archlinux installation to recognize certain HID codes (specifically at the moment F13-F24) and translate them into event codes that I can use in X.
HID codes received properly
I am able to see the HID being received by the computer through /dev/hidraw2 (the output below is when pressing F24, HID code 0x73 based on: https://gist.github.com/MightyPork/6da26e382a7ad91b5496ee55fdc73db2.
sudo hexdump -C /dev/hidraw2
00000000 00 00 00 00 00 00 00 00 00 00 73 00 00 00 00 00 |..........s.....|
HID codes not translated to events
However, as soon as I try showkey -s, evtest, or xev I get absolutely nothing, it is as if I am not pressing a key. All of the "normal" HID codes work (in fact this message is being typed on the keyboard right now) so it must be something between the receipt of the HID code and its translation.
Interestingly, my evtest capabilities do not list the event codes for the F13-F24 (see below) but I have seen these capabilities listed in other people's output (eg https://unix.stackexchange.com/questions/130656/how-to-get-all-my-keys-to-send-keycodes).
Input driver version is 1.0.1
Input device ID: bus 0x3 vendor 0x1d6b product 0x104 version 0x101
Input device name: "CPMH 122 Gadget"
Supported events:
Event type 0 (EV_SYN)
Event type 1 (EV_KEY)
Event code 1 (KEY_ESC)
Event code 2 (KEY_1)
Event code 3 (KEY_2)
Event code 4 (KEY_3)
Event code 5 (KEY_4)
Event code 6 (KEY_5)
Event code 7 (KEY_6)
Event code 8 (KEY_7)
Event code 9 (KEY_8)
Event code 10 (KEY_9)
Event code 11 (KEY_0)
Event code 12 (KEY_MINUS)
Event code 13 (KEY_EQUAL)
Event code 14 (KEY_BACKSPACE)
Event code 15 (KEY_TAB)
Event code 16 (KEY_Q)
Event code 17 (KEY_W)
Event code 18 (KEY_E)
Event code 19 (KEY_R)
Event code 20 (KEY_T)
Event code 21 (KEY_Y)
Event code 22 (KEY_U)
Event code 23 (KEY_I)
Event code 24 (KEY_O)
Event code 25 (KEY_P)
Event code 26 (KEY_LEFTBRACE)
Event code 27 (KEY_RIGHTBRACE)
Event code 28 (KEY_ENTER)
Event code 29 (KEY_LEFTCTRL)
Event code 30 (KEY_A)
Event code 31 (KEY_S)
Event code 32 (KEY_D)
Event code 33 (KEY_F)
Event code 34 (KEY_G)
Event code 35 (KEY_H)
Event code 36 (KEY_J)
Event code 37 (KEY_K)
Event code 38 (KEY_L)
Event code 39 (KEY_SEMICOLON)
Event code 40 (KEY_APOSTROPHE)
Event code 41 (KEY_GRAVE)
Event code 42 (KEY_LEFTSHIFT)
Event code 43 (KEY_BACKSLASH)
Event code 44 (KEY_Z)
Event code 45 (KEY_X)
Event code 46 (KEY_C)
Event code 47 (KEY_V)
Event code 48 (KEY_B)
Event code 49 (KEY_N)
Event code 50 (KEY_M)
Event code 51 (KEY_COMMA)
Event code 52 (KEY_DOT)
Event code 53 (KEY_SLASH)
Event code 54 (KEY_RIGHTSHIFT)
Event code 55 (KEY_KPASTERISK)
Event code 56 (KEY_LEFTALT)
Event code 57 (KEY_SPACE)
Event code 58 (KEY_CAPSLOCK)
Event code 59 (KEY_F1)
Event code 60 (KEY_F2)
Event code 61 (KEY_F3)
Event code 62 (KEY_F4)
Event code 63 (KEY_F5)
Event code 64 (KEY_F6)
Event code 65 (KEY_F7)
Event code 66 (KEY_F8)
Event code 67 (KEY_F9)
Event code 68 (KEY_F10)
Event code 69 (KEY_NUMLOCK)
Event code 70 (KEY_SCROLLLOCK)
Event code 71 (KEY_KP7)
Event code 72 (KEY_KP8)
Event code 73 (KEY_KP9)
Event code 74 (KEY_KPMINUS)
Event code 75 (KEY_KP4)
Event code 76 (KEY_KP5)
Event code 77 (KEY_KP6)
Event code 78 (KEY_KPPLUS)
Event code 79 (KEY_KP1)
Event code 80 (KEY_KP2)
Event code 81 (KEY_KP3)
Event code 82 (KEY_KP0)
Event code 83 (KEY_KPDOT)
Event code 86 (KEY_102ND)
Event code 87 (KEY_F11)
Event code 88 (KEY_F12)
Event code 96 (KEY_KPENTER)
Event code 97 (KEY_RIGHTCTRL)
Event code 98 (KEY_KPSLASH)
Event code 99 (KEY_SYSRQ)
Event code 100 (KEY_RIGHTALT)
Event code 102 (KEY_HOME)
Event code 103 (KEY_UP)
Event code 104 (KEY_PAGEUP)
Event code 105 (KEY_LEFT)
Event code 106 (KEY_RIGHT)
Event code 107 (KEY_END)
Event code 108 (KEY_DOWN)
Event code 109 (KEY_PAGEDOWN)
Event code 110 (KEY_INSERT)
Event code 111 (KEY_DELETE)
Event code 119 (KEY_PAUSE)
Event code 125 (KEY_LEFTMETA)
Event code 126 (KEY_RIGHTMETA)
Event code 127 (KEY_COMPOSE)
Event type 4 (EV_MSC)
Event code 4 (MSC_SCAN)
Event type 17 (EV_LED)
Event code 0 (LED_NUML) state 1
Event code 1 (LED_CAPSL) state 0
Event code 2 (LED_SCROLLL) state 0
Event code 3 (LED_COMPOSE) state 0
Event code 4 (LED_KANA) state 0
Key repeat handling:
Repeat type 20 (EV_REP)
Repeat code 0 (REP_DELAY)
Value 250
Repeat code 1 (REP_PERIOD)
Value 33
Is it the driver?
Based on the difference between my evtest EV_KEY capabilities and others I have seen I thought maybe it is that my driver can't map the USB HID codes to the appropriate event codes.
Using hwinfo I can tell that my keyboard is using the hid-generic driver. Looking at the source for hid-input.c (http://elixir.free-electrons.com/linux/v3.5.2/source/drivers/hid/hid-input.c) I can see that the mapping is indeed there (the HID code 0x73 maps to 194 which maps to the KEY_F24 event, at least according to what I found here: https://github.com/wayland-project/libinput/blob/master/include/linux/input-event-codes.h). So I am led from what I have read online to believe that the problem lies somewhere between the hid-generic and libinput.
I'm not sure where to go from here, with nothing coming out of evtest I am not sure what to map the keys to. Is it just that the capability for those keys isn't defined? Is there anyway to specify the capabilities of my keyboard to evdev?

Answer 1

Found the problem. After looking through the code for usbhid I realized that it was what was assigning the available event codes discovered by evtest. To do this, it reads through the HID descriptor. As it turned out I had used a generic HID descriptor and the Logical Maximum and Usage Maximum were cutting off the higher numbered HID codes. I used https://github.com/DIGImend/hidrd to get an editable version of my binary descriptor and then changed the Usage Maximum and Logical Maximum to the highest HID code I use. Reconverted it to binary format, uploaded and started the keyboard. Now evtest recognizes all keys.
Here is my original spec:
Usage Page (Desktop), ; Generic desktop controls (01h)
Usage (Keyboard), ; Keyboard (06h, application collection)
Collection (Application),
Usage Page (Keyboard), ; Keyboard/keypad (07h)
Usage Minimum (KB Leftcontrol), ; Keyboard left control (E0h, dynamic value)
Usage Maximum (KB Right GUI), ; Keyboard right GUI (E7h, dynamic value)
Logical Minimum (0),
Logical Maximum (1),
Report Size (1),
Report Count (8),
Input (Variable),
Report Count (1),
Report Size (8),
Input (Constant, Variable),
Report Count (5),
Report Size (1),
Usage Page (LED), ; LEDs (08h)
Usage Minimum (01h),
Usage Maximum (05h),
Output (Variable),
Report Count (1),
Report Size (3),
Output (Constant, Variable),
Report Count (6),
Report Size (8),
Logical Minimum (0),
Logical Maximum (101),
Usage Page (Keyboard), ; Keyboard/keypad (07h)
Usage Minimum (None), ; No event (00h, selector)
Usage Maximum (KB Application), ; Keyboard Application (65h, selector)
Input,
End Collection
And my updated spec:
Usage Page (Desktop), ; Generic desktop controls (01h)
Usage (Keyboard), ; Keyboard (06h, application collection)
Collection (Application),
Usage Page (Keyboard), ; Keyboard/keypad (07h)
Usage Minimum (KB Leftcontrol), ; Keyboard left control (E0h, dynamic value)
Usage Maximum (KB Right GUI), ; Keyboard right GUI (E7h, dynamic value)
Logical Minimum (0),
Logical Maximum (1),
Report Size (1),
Report Count (8),
Input (Variable),
Report Count (1),
Report Size (8),
Input (Constant, Variable),
Report Count (5),
Report Size (1),
Usage Page (LED), ; LEDs (08h)
Usage Minimum (01h),
Usage Maximum (05h),
Output (Variable),
Report Count (1),
Report Size (3),
Output (Constant, Variable),
Report Count (6),
Report Size (8),
Logical Minimum (0),
Logical Maximum (115),
Usage Page (Keyboard), ; Keyboard/keypad (07h)
Usage Minimum (None), ; No event (00h, selector)
Usage Maximum (KB F24), ; Keyboard F24 (73h, selector)
Input,
End Collection