sda_cmd_alloc
cmdp = sda_cmd_alloc(slot, CMD_SWITCH_FUNC, arg, R1, NULL, KM_SLEEP);
cmdp = sda_cmd_alloc(slot, cmd, arg, rtype, NULL, KM_SLEEP);
cmdp = sda_cmd_alloc(slot, cmd, blkno << slot->s_bshift,
sda_cmd_t *sda_cmd_alloc(sda_slot_t *, sda_index_t, uint32_t, sda_rtype_t,