On OMAP4+ devices, GIC register context is lost when MPUSS hits
the OSWR(Open Switch Retention). On the CPU wakeup path, ROM code
gets executed and one of the steps in it is to restore the
saved context of the GIC. The ROM Code GIC distributor restoration
is split in two parts: CPU specific register done by each CPU and
common register done by only one CPU.

Below is the abstract flow.

...............................................................
- MPUSS in OSWR state.
- CPU0 wakes up on the event(interrupt) and start executing ROM code.

[..]

- CPU0 executes "GIC Restoration:"

[...]

- CPU0 swicthes to non-secure mode and jumps to OS resume code.

[...]

- CPU0 is online in OS
- CPU0 enables the GIC distributor. GICD.Enable Non-secure = 1
- CPU0 wakes up CPU1 with clock-domain force wakeup method.
- CPU0 continues it's execution.
[..]

- CPU1 wakes up and start executing ROM code.

[..]

- CPU1 executes "GIC Restoration:"

[..]

- CPU1 swicthes to non-secure mode and jumps to OS resume code.

[...]

- CPU1 is online in OS and start executing.
[...]   -

GIC Restoration: /* Common routine for HS and GP devices */
{
       if (GICD != 1)  { /* This will be true in OSWR state */
               if (GIC_SAR_BACKUP_STATE == SAVED)
                       - CPU restores GIC distributor
               else
                       - reconfigure GIC distributor to boot values.

               GICD.Enable secure = 1
       }

       if (GIC_SAR_BACKUP_STATE == SAVED)
               - CPU restore its GIC CPU interface registers if saved.
       else
               - reconfigure its GIC CPU interface registers to boot
                       values.
}
...............................................................

So as mentioned in the flow, GICD != 1 condition decides how
the GIC registers are handled in ROM code wakeup path from
OSWR. As evident from the flow, ROM code relies on the entire
GICD register value and not specific register bits.

The assumption was valid till CortexA9 r1pX version since there
was only one banked bit to control secure and non-secure GICD.
Secure view which ROM code sees:
       bit 0 == Enable Non-secure
Non-secure view which HLOS sees:
       bit 0 == Enable secure

But GICD register has changed between CortexA9 r1pX and r2pX.
On r2pX GICD register is composed of 2 bits.
Secure view which ROM code sees:
       bit 1 == Enable Non-secure
       bit 0 == Enable secure
Non-secure view which HLOS sees:
       bit 0 == Enable Non-secure

Hence on OMAP4460(r2pX) devices, if you go through the
above flow again during CPU1 wakeup, GICD == 3 and hence
ROM code fails to understand the real wakeup power state
and reconfigures GIC distributor to boot values. This is
nasty since you loose the entire interrupt controller
context in a live system.

The ROM code fix done on next OMAP4 device (OMAP4470 - r2px) is to
check "GICD.Enable secure != 1" for GIC restoration in OSWR wakeup path.

Since ROM code can't be fixed on OMAP4460 devices, a work around
needs to be implemented. As evident from the flow, as long as
CPU1 sees GICD == 1 in it's wakeup path from OSWR, the issue
won't happen. Below is the flow with the work-around.

...............................................................
- MPUSS in OSWR state.
- CPU0 wakes up on the event(interrupt) and start executing ROM code.

[..]

- CPU0 executes "GIC Restoration:"

[..]

- CPU0 swicthes to non-secure mode and jumps to OS resume code.

[..]

- CPU0 is online in OS.
- CPU0 does GICD.Enable Non-secure = 0
- CPU0 wakes up CPU1 with clock domain force wakeup method.
- CPU0 waits for GICD.Enable Non-secure = 1
- CPU0 coninues it's execution.
[..]

- CPU1 wakes up and start executing ROM code.

[..]

- CPU1 executes "GIC Restoration:"

[..]

- CPU1 swicthes to non-secure mode and jumps to OS resume code.

[..]

- CPU1 is online in OS
- CPU1 does GICD.Enable Non-secure = 1
- CPU1 start executing
[...]
...............................................................

With this procedure, the GIC configuration done between the
CPU0 wakeup and CPU1 wakeup will not be lost but during this
short windows, the CPU0 will not receive interrupts.

The BUG is applicable to only OMAP4460(r2pX) devices.
OMAP4470 (also r2pX) is not affected by this bug because
ROM code has been fixed.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>