drv_hztousec() converts into microseconds the time expressed by hertz, which is in system clock ticks.
The kernel variable lbolt, whose value should be retrieved by calling ddi_get_lbolt(9F), is the length of time the system has been up since boot and is expressed in clock ticks. Drivers often use the value of lbolt before and after an I/O request
to measure the amount of time it took the device to process the request. drv_hztousec() can be used by the driver to convert the reading from clock ticks to a known unit of time.