ACPI / EC: Fix several GPE handling issues by deploying ACPI_GPE_DISPATCH_RAW_HANDLER mode.

This patch switches EC driver into ACPI_GPE_DISPATCH_RAW_HANDLER mode where
the GPE lock is not held for acpi_ec_gpe_handler() and the ACPICA internal
GPE enabling/disabling/clearing operations are bypassed so that further
improvements are possible with the GPE APIs.

There are 2 strong reasons for deploying raw GPE handler mode in the EC
driver:
1. Some hardware logics can control their interrupts via their own
   registers, so their interrupts can be disabled/enabled/acknowledged
   without using the super IRQ controller provided functions. While there
   is no mean (EC commands) for the EC driver to achieve this.
2. During suspending, the EC driver is still working for a while to
   complete the platform firmware provided functionailities using ec_poll()
   after all GPEs are disabled (see acpi_ec_block_transactions()), which
   means the EC driver will drive the EC GPE out of the GPE core's control.

Without deploying the raw GPE handler mode, we can see many races between
the EC driver and the GPE core due to the above restrictions:
1. There is a race condition due to ACPICA internal GPE
   disabling/clearing/enabling logics in acpi_ev_gpe_dispatch():
     Orignally EC GPE is disabled (EN=0), cleared (STS=0) before invoking a
     GPE handler and re-enabled (EN=1) after invoking a GPE handler in
     acpi_ev_gpe_dispatch(). When re-enabling appears, GPE may be flagged
     (STS=1).
       =================================================================
       (event pending A)
       =================================================================
       acpi_ev_gpe_dispatch()    ec_poll()
         EN=0
         STS=0
         acpi_ec_gpe_handler()
       *****************************************************************
       (event handling A)
           Lock(EC)
           advance_transaction()
             EC_SC read
       =================================================================
       (event pending B)
       =================================================================
             EC_SC handled
           Unlock(EC)
       *****************************************************************
       *****************************************************************
       (event handling B)
                                   Lock(EC)
                                   advance_transaction()
                                     EC_SC read
       =================================================================
       (event pending C)
       =================================================================
                                     EC_SC handled
                                   Unlock(EC)
       *****************************************************************
           EN=1
   This race condition is the root cause of different issues on different
   silicon variations.
   A. Silicon variation A:
      On some platforms, GPE will be triggered due to "writing 1 to EN when
      STS=1". This is because both EN and STS lines are wired to the GPE
      trigger line.
      1. Issue 1:
         We can see no-op acpi_ec_gpe_handler() invoked on such platforms.
         This is because:
         a. event pending B: An event can arrive after ACPICA's GPE
            clearing performed in acpi_ev_gpe_dispatch(), this event may
            fail to be detected by EC_SC read that is performed before its
            arrival;
         b. event handling B: The event can be handled in ec_poll() because
            EC lock is released after acpi_ec_gpe_handler() invocation;
         c. There is no code in ec_poll() to clear STS but the GPE can
            still be triggered by the EN=1 write performed in
            acpi_ev_finish_gpe(), this leads to a no-op EC GPE handler
            invocation;
         d. As no-op GPE handler invocations are counted by the EC driver
            to trigger the command storming conditions, the wrong no-op
            GPE handler invocations thus can easily trigger wrong command
            storming conditions.
         Note 1:
         If we removed GPE disabling/enabling code from
         acpi_ev_gpe_dispatch(), we could still see no-op GPE handlers
         triggered by the event arriving after the GPE clearing and before
         the GPE handling on both silicon variation A and B. This can only
         occur if the CPU is very slow (timing slice between STS=0 write
         and EC_SC read should be short enough before hardware sets another
         GPE indication). Thus this is very rare and is not what we need to
         fix.
   B. Silicon variation B:
      On other platforms, GPE may not be triggered due to "writing 1 to EN
      when STS=1". This is because only STS line is wired to the GPE
      trigger line.
      2. Issue 2:
         We can see GPE loss on such platforms. This is because:
         a. event pending B vs. event handling A: An event can arrive after
            ACPICA's GPE handling performed in acpi_ev_gpe_dispatch(), or
            event pending C vs. event handling B: An event can arrive after
            Linux's GPE handling performed in ec_poll(),
            these events may fail to be detected by EC_SC read that is
            performed before their arrival;
         b. The GPE cannot be triggered by EN=1 write performed in
            acpi_ev_finish_gpe();
         c. If no polling mechanism is implemented in the driver for the
            pending event (for example, SCI_EVT), this event is lost due to
            no GPE being triggered.
         Note 2:
         On most platforms, there might be another rule that GPE may not be
         triggered due to "writing 1 to STS when STS=1 and EN=1".
         Then on silicon variation B, an even worse case is if the issue 2
         event loss happens, further events may never trigger GPE again on
         such platforms due to being blocked by the current STS=1. Unless
         someone clears STS, all events have to be polled.
2. There is a race condition due to lacking in GPE status checking in EC
   driver:
     Originally, GPE status is checked in ACPICA core but not checked in
     the GPE handler. Thus since the status checking and handling is not
     locked, it can be interrupted by another handling path.
       =================================================================
       (event pending A)
       =================================================================
       acpi_ev_gpe_detect()        ec_poll()
         if (EN==1 && STS==1)
       *****************************************************************
       (event handling A)
                                     Lock(EC)
                                     advance_transaction()
                                       EC_SC read
                                       EC_SC handled
                                     Unlock(EC)
       *****************************************************************
         acpi_ev_gpe_dispatch()
           EN=0
           STS=0
           acpi_ec_gpe_handler()
       *****************************************************************
       (event handling B)
             Lock(EC)
             advance_transaction()
               EC_SC read
             Unlock(EC)
       *****************************************************************
      3. Issue 3:
         We can see no-op acpi_ec_gpe_handler() invoked on both silicon
         variation A and B. This is because:
         a. event pending A: An event can arrive to trigger an EC GPE and
            ACPICA checks it and is about to invoke the EC GPE handler;
         b. event handling A: The event can be handled in ec_poll() because
            EC lock is not held after the GPE status checking;
         c. event handling B: Then when the EC GPE handler is invoked, it
            becomes a no-op GPE handler invocation.
         d. As no-op GPE handler invocations are counted by the EC driver
            to trigger the command storming conditions, the wrong no-op
            GPE handler invocations thus can easily trigger wrong command
            storming conditions.
      Note 3:
      This no-op GPE handler invocation is rare because the time between
      the IRQ arrival and the acpi_ec_gpe_handler() invocation is less than
      the timeout value waited in ec_poll(). So most of the no-op GPE
      handler invocations are caused by the reason described in issue 1.
3. There is a race condition due to ACPICA internal GPE clearing logic in
   acpi_enable_gpe():
     During runtime, acpi_enable_gpe() can be invoked by the EC storming
     prevention code. When it is invoked, GPE may be flagged (STS=1).
       =================================================================
       (event pending A)
       =================================================================
       acpi_ev_gpe_dispatch()    acpi_ec_transaction()
         EN=0
         STS=0
         acpi_ec_gpe_handler()
       *****************************************************************
       (event handling A)
           Lock(EC)
           advance_transaction()
             EC_SC read
             EC_SC handled
           Unlock(EC)
       *****************************************************************
         EN=1 ?
                                   Lock(EC)
                                   Unlock(EC)
       =================================================================
       (event pending B)
       =================================================================
                                   acpi_enable_gpe()
                                     STS=0
                                     EN=1
    4. Issue 4:
       We can see GPE loss on both silicon variation A and B platforms.
       This is because:
       a. event pending B: An event can arrive right before ACPICA's GPE
          clearing performed in acpi_enable_gpe();
       b. If the GPE is cleared when GPE is disabled, then EN=1 write in
          acpi_enable_gpe() cannot trigger this GPE;
       c. If no polling mechanism is implemented in the driver for this
          event (for example, SCI_EVT), this event is lost due to no GPE
          being triggered.
       Note 4:
       Currently we don't have this issue, but after we switch the EC
       driver into ACPI_GPE_DISPATCH_RAW_HANDLER mode, we need to take care
       of handling this because the EN=1 write in acpi_ev_gpe_dispatch()
       will be abandoned.

There might be more race issues for the current GPE handler usages. This is
because the EC IRQ's enabling/disabling/checking/clearing/handling
operations should be locked by a single lock that is under the EC driver's
control to achieve the serialization. Which means we need to invoke GPE
APIs with EC driver's lock held and all ACPICA internal GPE operations
related to the GPE handler should be abandoned. Invoking GPE APIs inside of
the EC driver lock and bypassing ACPICA internal GPE operations requires
the ACPI_GPE_DISPATCH_RAW_HANDLER mode where the same lock used by the APIs
are released prior than invoking the handlers. Otherwise, we can see dead
locks due to circular locking dependencies (see Reference below).

This patch then switches the EC driver into the
ACPI_GPE_DISPATCH_RAW_HANDLER mode so that it can perform correct GPE
operations using the GPE APIs:
1. Bypasses EN modifications performed in acpi_ev_gpe_dispatch() by
   using acpi_install_gpe_raw_handler() and invoking all GPE APIs with EC
   spin lock held. This can fix issue 1 as it makes a non frequent GPE
   enabling/disabling environment.
2. Bypasses STS clearing performed in acpi_enable_gpe() by replacing
   acpi_enable_gpe()/acpi_disable_gpe() with acpi_set_gpe(). This can fix
   issue 4. And this can also help to fix issue 1 as it makes a no sudden
   GPE clearing environment when GPE is frequently enabled/disabled.
3. Ensures STS acknowledged before handling by invoking acpi_clear_gpe()
   in advance_transaction(). This can finally fix issue 1 even in a
   frequent GPE enabling/disabling environment. And this can also finally
   fix issue 3 when issue 2 is fixed.
   Note 3:
   GPE clearing is edge triggered W1C, which means we can clear it right
   before handling it. Since all EC GPE indications are handled in
   advance_transaction() by previous commits, we can now move GPE clearing
   into it to implement the correct GPE clearing.
   Note 4:
   We can use acpi_set_gpe() which is not shared GPE safer instead of
   acpi_enable_gpe()/acpi_disable_gpe() because EC GPE is not shared by
   other hardware, which is mentioned in the ACPI specification 5.0, 12.6
   Interrupt Model: "OSPM driver treats this as an edge event (the EC SCI
   cannot be shared)". So we can stop using shared GPE safer APIs
   acpi_enable_gpe()/acpi_disable_gpe() in the EC driver. Otherwise
   cleanups need to be made in acpi_ev_enable_gpe() to bypass the GPE
   clearing logic before keeping acpi_enable_gpe().
This patch also invokes advance_transaction() when GPE is re-enabled in the
task context which:
1. Ensures EN=1 can trigger GPE by checking and handling EC status register
   right after EN=1 writes. This can fix issue 2.

After applying this patch, without frequent GPE enablings considered:
       =================================================================
       (event pending A)
       =================================================================
       acpi_ec_gpe_handler()     ec_poll()
       *****************************************************************
       (event handling A)
         Lock(EC)
           advance_transaction()
             if STS==1
               STS=0
             EC_SC read
       =================================================================
       (event pending B)
       =================================================================
             EC_SC handled
         Unlock(EC)
       *****************************************************************
       *****************************************************************
       (event handling B)
                                   Lock(EC)
                                     advance_transaction()
                                       if STS==1
                                         STS=0
                                       EC_SC read
       =================================================================
       (event pending C)
       =================================================================
                                       EC_SC handled
                                   Unlock(EC)
       *****************************************************************
The event pending for issue 1 (event pending B) can arrive as a next GPE
due to the previous IRQ context STS=0 write. And if it is handled by
ec_poll() (event handling B), as it is also acknowledged by ec_poll(), the
event pending for issue 2 (event pending C) can properly arrive as a next
GPE after the task context STS=0 write. So no GPE will be lost and never
triggered due to GPE clearing performed in the wrong position. And since
all GPE handling is performed after a locked GPE status checking, we can
hardly see no-op GPE handler invocations due to issue 1 and 3. We may still
see no-op GPE handler invocations due to "Note 1", but as it is inevitable,
it needn't be fixed.

After applying this patch, with frequent GPE enablings considered:
       =================================================================
       (event pending A)
       =================================================================
       acpi_ec_gpe_handler()     acpi_ec_transaction()
       *****************************************************************
       (event handling A)
         Lock(EC)
           advance_transaction()
             if STS==1
               STS=0
             EC_SC read
       =================================================================
       (event pending B)
       =================================================================
             EC_SC handled
         Unlock(EC)
       *****************************************************************
       *****************************************************************
       (event handling B)
                                   Lock(EC)
                                     EN=1
                                     if STS==1
                                       advance_transaction()
                                         if STS==1
                                           STS=0
                                         EC_SC read
       =================================================================
       (event pending C)
       =================================================================
                                         EC_SC handled
                                   Unlock(EC)
       *****************************************************************
The event pending for issue 2 can be manually handled by
advance_transaction(). And after the STS=0 write performed in the manual
triggered advance_transaction(), GPE can always arrive. So no GPE will be
lost due to frequent GPE disabling/enabling performed in the driver like
issue 4.
Note 5:
It's ideally when EN=1 write occurred, an IRQ thread should be woken up to
handle the GPE when the GPE was raised. But this requires the IRQ thread to
contain the poller code for all EC GPE indications, while currently some of
the indications are handled in the user tasks. It then is very hard for the
code to determine whether a user task should be invoked or the poller work
item should be scheduled. So we have to invoke advance_transaction()
directly now and it leaves us such a restriction for the GPE re-enabling:
it must be performed in the task context to avoid starving the GPEs.

As a conclusion: we can see the EC GPE is always handled in serial after
deploying the raw GPE handler mode:
  Lock(EC)
  if (STS==1)
    STS=0
  EC_SC read
  EC_SC handled
  Unlock(EC)
The EC driver specific lock is responsible to make the EC GPE handling
processes serialized so that EC can handle its GPE from both IRQ and task
contexts and the next IRQ can be ensured to arrive after this process.

Note 6:
We have many EC_FLAGS_MSI qurik users in the current driver. They all seem
to be suffering from unexpected GPE triggering source lost. And they are
false root caused to a timing issue. Since EC communication protocol has
already flow control defined, timing shouldn't be the root cause, while
this fix might be fixing the root cause of the old bugs.

Link: https://lkml.org/lkml/2014/11/4/974
Link: https://lkml.org/lkml/2014/11/18/316
Link: https://www.spinics.net/lists/linux-acpi/msg54340.html
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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