HKBD_IN_BUF_SIZE
#define HKBD_IN_BUF_FULL ((HKBD_IN_BUF_SIZE / 2) - 1) /* scancodes */
uint32_t sc_input[HKBD_IN_BUF_SIZE]; /* input buffer */
tail = (sc->sc_inputtail + 1) % HKBD_IN_BUF_SIZE;
head = (head + 1) % HKBD_IN_BUF_SIZE;