Support N-key rollover

[TheButlah]

Hi, currently the KeyboardReport only supports reporting up to 6 key presses, not including modifier keys. I'd like to request that it be updated to support an arbitrary number of keypresses.

More can be read here: https://www.devever.net/~hl/usbnkro

[TheButlah]

I am happy to contribute if you can point me in the right direction!

[haata]

I have this working here: https://github.com/kiibohd/kiibohd-core/tree/main/kiibohd-usb/src

I'm traveling right now but I can provide some guidance in a couple days.

[mdevlamynck]

The scheme described at the end of the article that support both boot and nkro would be defined as:

#[gen_hid_descriptor(
    (collection = APPLICATION, usage_page = GENERIC_DESKTOP, usage = KEYBOARD) = {
        (usage_page = KEYBOARD, usage_min = 0xE0, usage_max = 0xE7) = {
            #[packed_bits 8]
            #[item_settings data,variable,absolute]
            modifier=input;
        };
        (usage_min = 0x00, usage_max = 0xFF) = {
            #[item_settings constant,variable,absolute]
            reserved=input;
        };
        (usage_page = LEDS, usage_min = 0x01, usage_max = 0x05) = {
            #[packed_bits 5]
            #[item_settings data,variable,absolute]
            leds=output;
        };
        (usage_min = 0x00, usage_max = 0xFF) = {
            #[item_settings constant,variable,absolute]
            keycodes=input;
        };
        (usage_page = KEYBOARD, usage_min = 0x00, usage_max = 0x65) = {
            #[packed_bits 101]
            #[item_settings data,variable,absolute]
            nkro_keycodes=input;
        };
    }
)]
pub struct KeyboardReport {
    pub modifier:     u8,
    pub reserved:     u8,
    pub leds:         u8,
    pub keycodes:     [u8; 6],
    pub nkro_keycodes: [u8; 13],
}

You might want more than [u8; 13] though to support the full range of values defined in the HID spec:

#[gen_hid_descriptor(
    (collection = APPLICATION, usage_page = GENERIC_DESKTOP, usage = KEYBOARD) = {
        (usage_min = 0xE0, usage_max = 0xE7) = {
            #[packed_bits 8]
            #[item_settings constant,variable,absolute]
            modifier=input;
        };
        (usage_min = 0x00, usage_max = 0xFF) = {
            #[item_settings constant,variable,absolute]
            reserved=input;
        };
        (usage_page = LEDS, usage_min = 0x01, usage_max = 0x05) = {
            #[packed_bits 5]
            #[item_settings data,variable,absolute]
            leds=output;
        };
        (usage_min = 0x00, usage_max = 0xFF) = {
            #[item_settings constant,variable,absolute]
            keycodes=input;
        };
        (usage_page = KEYBOARD, usage_min = 0x00, usage_max = 0xE7) = {
            #[packed_bits 232]
            #[item_settings data,variable,absolute]
            all_keycodes=input;
        };
    }
)]
pub struct KeyboardReport {
    pub modifier:     u8,
    pub reserved:     u8,
    pub leds:         u8,
    pub keycodes:     [u8; 6],
    pub all_keycodes: [u8; 29],
}

Here the modifiers are included in the all_keycodes field and modifier is configured as a constant to be ignored.