kbd: replace vestigial spl calls with Giant assertions

[emaste]

Summary

The keyboard driver used to be protected via spl* interrupt priority
calls but as with the rest of the kernel was migrated to use the Giant
lock (mutex).  The spl calls are NOPs but may be confusing; remove them
and add assertions that Giant is held.

PR:             206680

[markj]

This seems ok. There is some conflation between the "bus topo" lock (which is Giant under the hood) and Giant: kbd_allocate() is called via DEVICE_ATTACH and is formally locked by the former, whereas cdev entry points are locked by Giant (via D_NEEDGIANT).

sys/dev/kbd/kbd.c

372

I think this function is unused, BTW. Maybe better to leave it though.

[emaste]

There is some conflation between the "bus topo" lock (which is Giant under the hood) and Giant

I will mention this in the commit message. I think it's fine, the assertion will either be a small item to address with bus topology locking work or will be removed if kbd is brought out from under Giant.

sys/dev/kbd/kbd.c

372

I am grepping through all commits now to confirm.

[emaste]

sys/dev/kbd/kbd.c

372

Indeed, the only commits referencing kbd_change_callback in the tree are:

R10:617b90803690a7ece8e76a49803385516bac380b (introduction)
R10:c959700c776fc1dd722aa4d265ccda49ee227fef (style)
R10:04551c6ce5981d985b31476094aa79186a874554 (referenced in diff context lines)
R10:bbecb73b0ebe929ece4f2409c8d990718655a52d (style)