ACPI / EC: Fix issues related to the event handling.

ACPI specification doesn't define the SCI_EVT clearing timing for the EC firmware. There could be 4 possible positions the firmware may use to clear the SCI_EVT indication:
STATUS: After indicating SCI_EVT to the host via the status register
(EC_SC), the target can clear SCI_EVT at any time.
QUERY: After seeing the query request (QR_EC) written to the command
register (EC_CMD) by the host and having prepared the responding event value in the data register (EC_DATA), the target can safely clear SCI_EVT.
EVENT: After seeing the event response read from the data register
(EC_DATA) by the host, the target can clear SCI_EVT.
METHOD: After the event has been handled via the host _Qxx evaluation,
the target can clear SCI_EVT.

The old EC driver re-checked SCI_EVT using the 3rd position, while during the race fixes, the new EC driver was switched to re-check SCI_EVT using the 1st position. Using the earliest position may help to reduce issues when the other races are not fully fixed and there is an ACPI specification statement around an expected behavior:
The query value of zero is reserved for a spurious query result and indicates "no outstanding events."
This statement was the motivation of this choice.

But there are platforms do not follow this definition, since there is also a coverity issue left in the new EC driver, the unmatched SCI_EVT clearing timing finally was trapped by this coverity issue, and users can see that the new EC driver stops processing further SCI_EVTs. The new EC driver then implemented EC_FLAGS_QUERY_HANDSHAKE quirk as a workaround for such platforms.
Now more and more reports can be seen for this SCI_EVT clearing timing unmatched problem, we need to consider a better approach to fix all of them instead of adding new EC_FLAGS_QUERY_HANDSHAKE quirk users.

This patchset thus finally implements 3 of 4 modes to handle the SCI_EVT clearing timing variations (1 was proven by 94411 not compliant to the Windows behavior). And according to the tests, chooses the next earlier position (the 2rd position) as the default behavior.

This patchset also fixes the coverity problem so that even though the default behavior still cannot match the Windows behavior, it is possible for the new EC driver to proceed to handle further SCI_EVT indications.

Related bug reports are as follows:

Lv Zheng (5):
ACPI / EC: Cleanup transaction state transition.
ACPI / EC: Convert event handling work queue into loop style. ACPI / EC: Add event clearing variation support.
ACPI / EC: Fix EC_FLAGS_QUERY_HANDSHAKE platforms using new event
clearing timing.
ACPI / EC: Fix a code coverity issue when QR_EC transactions are
failed.

 drivers/acpi/ec.c       |  217 +++++++++++++++++++++++++++++++++++++++++------
 drivers/acpi/internal.h |    1 +
 2 files changed, 191 insertions(+), 27 deletions(-)

1.7.10