Invalidating Caches When a Block Device Changes

Hello,

I'm writing a block device driver for a device who's data might change
without the kernel performing any specific writes.  I'm looking for a
way to inform any upstream caches that the data it holds is no longer
valid and must be discarded.

I've found APIs for flushing caches *to* the block device, but none
that makes sense to call within the context of the block device driver
itself when it has learned the underlying device itself has changed.

Can anyone point me to the proper APIs?