Giant Locked drivers

Greetings,

It's no secret fiant-locked drivers' days are numbered. We've been more
sluggish about eliminating Giant than had been hoped. I plan in the coming
weeks to add a tunable 'debug.giant_drivers' which initially will be set to
enable/disable giant-locked drivers in the tree.

When set to 0, you get today's behavior. If set to 1, it will no longer
allow drivers that don't request MPSAFE interrupt handlers from registering
(the interrupt setup will return an error).

This will allow us to understand what is lost if we throw the switch, and
allow users to proactively test their systems to see if they are
affected or not (and if they are, if they want to live without the
functionality, or want to fund work in the area).

Comments?

Warner