[Solved] USB HID: Holding down a key

[__builtin]

Recently I've been trying to figure out how to simulate a key being held down via Rockbox's USB HID feature.

In firmware/usbstack/usb_hid.c where the function usb_hid_send() is defined, there's this piece of code:

Code: [Select]

    logf("Sending cmd 0x%x", id);
    length = report->buf_set(&buf[1], id) + 1;
    logf("length %u", length);

    if (!length)
        return;

    /* Key pressed */
    buf_dump(buf, length, "key press");
    usb_hid_queue(buf, length);

    if (report->is_key_released)
    {
        /* Key released */
        memset(&buf[1], 0, length-1);

        buf_dump(buf, length, "key release");
        usb_hid_queue(buf, length);
    }

Looking at that, it seems that the last if block is what sends the 'key up' event to the host.
My reasoning was that by removing this, you could simulate a key being held down.

Building on this, idea, I wrote a function identical to usb_hid_send(), but with that if statement removed:

Code: [Select]

void usb_hid_send_keydown(usage_page_t usage_page, int id)
{
    uint8_t length;
    static unsigned char buf[HID_BUF_SIZE_CMD] USB_DEVBSS_ATTR
        __attribute__((aligned(32)));
    usb_hid_report_t *report = NULL;

    for (uint8_t report_id = 1; report_id < REPORT_ID_COUNT; report_id++)
        if (usb_hid_reports[report_id].usage_page == usage_page)
        {
            report = &usb_hid_reports[report_id];
            buf[0] = report_id;
            break;
        }

    if (!report)
    {
        logf("Unsupported usage_page");
        return;
    }

    logf("Sending cmd 0x%x", id);
    length = report->buf_set(&buf[1], id) + 1;
    logf("length %u", length);

    if (!length)
        return;

    usb_hid_try_send_drv();
}
However, it doesn't work.

My question is this: what should I do to simulate a key being held down?

EDIT:As it turns, out, I was Doing It Wrong (TM). The HID driver's keyboard component can take up to 4 key codes at once, packed into a 4-byte int.