Device drivers do not usually print messages, except for unexpected errors such as data corruption. Instead, the driver entry points should return error codes so that the application can determine how to handle the error. Use the cmn_err(9F) function to write messages to a system log that can then be displayed on the console.
The format string specifier interpreted by cmn_err(9F) is similar to the printf(3C) format string specifier, with the addition of the format %b, which prints bit fields. The first character of the format string can have a special meaning. Calls to cmn_err(9F) also specify the message level, which indicates the severity label to be printed. See the cmn_err(9F) man page for more details.
The level CE_PANIC has the side effect of crashing the system. This level should be used only if the system is in such an unstable state that to continue would cause more problems. The level can also be used to get a system core dump when debugging. CE_PANIC should not be used in production device drivers.