* Always use polled mode. The intr approach did not work for many
controllers and required the hw.acpi.ec.event_driven workaround.
* Only use an edge (not level) triggered GPE handler
* Add sc->ec_mtx for locking operations to a single EC. There were
many race conditions earlier between an SCI event and EcRead/Write.
* Use 1 ms as the global lock timeout
* Only acquire global lock if _GLK != 0
* Update EcWaitEvent to use an incremental backoff delay in its
poll loop. Wait 50 ms max instead of 10. Most ECs respond
in < 5 us (50 us when heavily loaded). However, some time out
occasionally even with a 10 ms timeout. For delays past 1 ms, use
msleep instead of DELAY to give SCI interrupts a chance to occur.
* Add EcCommand to send a command and wait for the appropriate event.
* The hw.acpi.ec.event_driven tunable is no longer applicable and
has been removed.
Ideas from: Linux
1f04e8f501