the system into config_deactivate(dev): deactivate dev and all of
its descendants. Block all interrupts while calling each device's
activation hook, ca_activate. Now it is possible to simplify or
to delete several device-activation hooks throughout the system.
Do not deactivate a driver while detaching it! If the driver was
already deactivated (because of accidental/emergency removal), let
the driver cope with the knowledge that DVF_ACTIVE has been cleared.
Otherwise, let the driver access the underlying hardware (so that
it can flush caches, restore original register settings, et cetera)
until it exits its device-detachment hook.
Let multiple readers and writers simultaneously access the system's
device_t list, alldevs, from either interrupt or thread context:
postpone changing alldevs linkages and freeing autoconf device
structures until a garbage-collection phase that runs after all
readers & writers have left the list.
Give device iterators (deviter(9)) a consistent view of alldevs no
matter whether device_t's are added and deleted during iteration:
keep a global alldevs generation number. When an iterator enters
alldevs, record the current generation number in the iterator and
increase the global number. When a device_t is created, label it
with the current global generation number. When a device_t is
deleted, add a second label, the current global generation number.
During iteration, compare a device_t's added- and deleted-generation
with the iterator's generation and skip a device_t that was deleted
before the iterator entered the list or added after the iterator
entered the list.
The alldevs generation number is never 0. The garbage collector
reaps device_t's whose delete-generation number is non-zero.
Make alldevs private to sys/kern/subr_autoconf.c. Use deviter(9)
to access it.
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