The syntax for devmap_unload(9F) is as follows:
int devmap_unload(devmap_cookie_t handle, offset_t offset, size_t len);
devmap_unload() invalidates the mapping translations for the pages of the mapping specified by handle, offset, and len. By invalidating the mapping translations for these pages, the device driver is telling the system to intercept accesses to these pages of the mapping. Furthermore, the system must notify the device driver the next time that these mapping pages are accessed by calling the devmap_access(9E) entry point.
For both functions, requests affect the entire page that contains the offset and all pages up to and including the entire page that contains the last byte, as indicated by offset + len. The device driver must ensure that for each page of device memory being mapped, only one process has valid translations at any one time.
Both functions return zero if successful. If, however, an error occurred in validating or invalidating the mapping translations, that error is returned to the device driver. The device driver must return this error to the system.