As we mentioned at the beginning of this chapter, what about using the ioctl interface for debug purposes? It can be used for this purpose. You can always insert a "debug" command into the switch-case block; it can be used to provide useful information to the user space application on the driver status, the values of key variables (health monitoring too), and more.
Not only that, but unless it's explicitly documented to the end user or customer, the precise commands that are used via the ioctl interface are unknown; thus, you are expected to document the interface while providing sufficient detail for other teams or the customer to make good use of them. This leads to an interesting point: you might choose to deliberately leave a certain ioctl command undocumented; it's now a "hidden" command that can be used by, say, field engineers to examine the device. (I leave doing this as an assignment to you.)