Virtual keyboard using UHID

Question

I am trying to achieve what the QMK firmware does for keyboards but using software. So, I would like to turn off keys, substitute some keys for others, layers and etc.

The first thing that appeared for me when searching was this example. The problem is, running the test it only works when the terminal that is executing the binary has focus, and I would like to overpass the windowing system.

Also the code presents this array that I don't have any idea how to fill.

static unsigned char rdesc[] = {
    0x05, 0x01, /* USAGE_PAGE (Generic Desktop) */
    0x09, 0x02, /* USAGE (Mouse) */
    0xa1, 0x01, /* COLLECTION (Application) */
    0x09, 0x01,     /* USAGE (Pointer) */
    0xa1, 0x00,     /* COLLECTION (Physical) */
    0x85, 0x01,         /* REPORT_ID (1) */
    0x05, 0x09,         /* USAGE_PAGE (Button) */
    0x19, 0x01,         /* USAGE_MINIMUM (Button 1) */
    0x29, 0x03,         /* USAGE_MAXIMUM (Button 3) */
    0x15, 0x00,         /* LOGICAL_MINIMUM (0) */
    0x25, 0x01,         /* LOGICAL_MAXIMUM (1) */
    0x95, 0x03,         /* REPORT_COUNT (3) */
    0x75, 0x01,         /* REPORT_SIZE (1) */
    0x81, 0x02,         /* INPUT (Data,Var,Abs) */
    0x95, 0x01,         /* REPORT_COUNT (1) */
    0x75, 0x05,         /* REPORT_SIZE (5) */
    0x81, 0x01,         /* INPUT (Cnst,Var,Abs) */
    0x05, 0x01,         /* USAGE_PAGE (Generic Desktop) */
    0x09, 0x30,         /* USAGE (X) */
    0x09, 0x31,         /* USAGE (Y) */
    0x09, 0x38,         /* USAGE (WHEEL) */
    0x15, 0x81,         /* LOGICAL_MINIMUM (-127) */
    0x25, 0x7f,         /* LOGICAL_MAXIMUM (127) */
    0x75, 0x08,         /* REPORT_SIZE (8) */
    0x95, 0x03,         /* REPORT_COUNT (3) */
    0x81, 0x06,         /* INPUT (Data,Var,Rel) */
    0xc0,           /* END_COLLECTION */
    0xc0,       /* END_COLLECTION */
    0x05, 0x01, /* USAGE_PAGE (Generic Desktop) */
    0x09, 0x06, /* USAGE (Keyboard) */
    0xa1, 0x01, /* COLLECTION (Application) */
    0x85, 0x02,     /* REPORT_ID (2) */
    0x05, 0x08,     /* USAGE_PAGE (Led) */
    0x19, 0x01,     /* USAGE_MINIMUM (1) */
    0x29, 0x03,     /* USAGE_MAXIMUM (3) */
    0x15, 0x00,     /* LOGICAL_MINIMUM (0) */
    0x25, 0x01,     /* LOGICAL_MAXIMUM (1) */
    0x95, 0x03,     /* REPORT_COUNT (3) */
    0x75, 0x01,     /* REPORT_SIZE (1) */
    0x91, 0x02,     /* Output (Data,Var,Abs) */
    0x95, 0x01,     /* REPORT_COUNT (1) */
    0x75, 0x05,     /* REPORT_SIZE (5) */
    0x91, 0x01,     /* Output (Cnst,Var,Abs) */
    0xc0,       /* END_COLLECTION */
};

I could not find any more documentation on the matter. At this time, I am not even sure if it is possible to do what I want. I have seen things like KMonad that I could not get to work.

At the end, these are my questions:

• How to fill the rdesc array to keyboards?
• How to overpass the windowing system?
• Where can I find documentation about UHID?

Answer 1

Was trying to solve this myself, and kept running across this post. I think I found it, the rdisc variable here is the HID Report Descriptor. That kernel doc page states:

The format of HID report descriptors is described by two documents, available from the USB Implementers Forum HID web page...

and then the kernel doc links specifically to the HID USB Device Class Definition and to the HID Usage Tables.

That should take care of your first question. For your third question there's this documentation, which is sparse, because UHID is basically just the glue for a userspace HID driver, and you'll get more out of the regular HID documentation. Sorry, I don't really know enough to provide an answer to your second question.