Total And Complete Wackyness

	I've got a stumper for y'all, all against Kernel 2.2.x:

	Assume you've got a removable media drive (/dev/rmbd)
that already has a kernel driver, and ``Widget X''. 
Here's Widget X's properties:

	*) Widget X physically looks like a piece of media for the
	   removate media block device (RMBD), but is _actually_
	   a bunch of electronics, memory, and some coils that rest
	   against the RMBD's read/write head.

	*) Does all communication through sectors 0-13 of the RMBD

	*) Has a device driver (call it /dev/widx) which
	   uses filp_open/block_read/block_write/filp_close/etc
	   to perform a communications protocol with 
	   Widget X via the RMBD.

	*) /dev/widx is a block device, and (while performing
	   it's wacky mechanations through the RMBD) looks
	   to userland like any another mkfs(8)able block device

    Got me so far? Good. Well, here comes the rub - since
/dev/widx is a different major number from /dev/rmbd, you
can get into a state where Widget X is in the drive, you're
happily using /dev/widx, and BAM - somebody (supermount,
autofs, whatever) reads/writes from /dev/rmbd - totally 
hosing your communications.

	So, what I would _like_ to do is:

	*) Change blk_dev[RMBD_MAJOR].request_fn to point to my
	   function that scans for the signature of a Widget X,
	   otherwise proceed as normally for /dev/rmbd

so that...

	*) Whenever Widget X is inserted into the RMBD, I override the 
	   file_operations and blk_dev[RMBD_MAJOR].request_fn 
	   properties of /dev/rmbd to point to the Widget X functions... 

but...

	*) Still be able to use the /dev/rmbd block device functionality
	   from the Widget X comminications protocol code... 

and...

	*) When the Widget X is removed, undo the override and restore
	   /dev/rmbd to its original functionality

with the condition that...

	*) I can't modify the /dev/rmbd kernel code, NOR include an entire
	   copy of the /dev/rmbd code into my driver (and all the nasty
	   state issues that would bring)

	Have I bitten off more than I can chew, or is protocol layering
over block devices REALLY possible?

-- 
Jason McMullan, Senior Linux Consultant, Linuxcare, Inc.
412.422.8077 tel, 415.701.0792 fax
[e-mail], http://www.linuxcare.com/
Linuxcare. Support for the revolution.