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The psci implementation does not track target affinity level requests
specified during cpu_suspend calls correctly as per the following
example.

1. cpu0.cluster0 calls cpu_suspend with the target affinity level as 0
2. Only the cpu0.cluster0 is powered down while cluster0 remains
   powered up
3. cpu1.cluster0 calls cpu_off to power itself down to highest
   possible affinity level
4. cluster0 will be powered off even though cpu0.cluster0 does not
   allow cluster shutdown

This patch introduces reference counts at affinity levels > 0 to track
the number of cpus which want an affinity instance at level X to
remain powered up. This instance can be turned off only if its
reference count is 0. Cpus still undergo the normal state transitions
(ON, OFF, ON_PENDING, SUSPEND) but the higher levels can only be
either ON or OFF depending upon their reference count.

The above issue is thus fixed as follows:

1. cluster0's reference count is incremented by two when cpu0 and cpu1
   are initially powered on.

2. cpu0.cluster0 calls cpu_suspend with the target affinity level as
   0. This does not affect the cluster0 reference count.

3. Only the cpu0.cluster0 is powered down while cluster0 remains
   powered up as it has a non-zero reference count.

4. cpu1.cluster0 call cpu_off to power itself down to highest possible
   affinity level. This decrements the cluster0 reference count.

5. cluster0 is still not powered off since its reference count will at
   least be 1 due to the restriction placed by cpu0.

Change-Id: I433dfe82b946f5f6985b1602c2de87800504f7a9

Change-Id: Ic7fb61aabae1d515b9e6baf3dd003807ff42da60

This patch performs a major rework of the psci generic implementation
to achieve the following:

1. replace recursion with iteration where possible to aid code
   readability e.g. affinity instance states are changed iteratively
   instead of recursively.

2. acquire pointers to affinity instance nodes at the beginning of a
   psci operation. All subsequent actions use these pointers instead
   of calling psci_get_aff_map_node() repeatedly e.g. management of
   locks has been abstracted under functions which use these pointers
   to ensure correct ordering. Helper functions have been added to
   create these abstractions.

3. assertions have been added to cpu level handlers to ensure correct
   state transition

4. the affinity level extents specified to various functions have the
   same meaning i.e. start level is always less than the end level.

Change-Id: If0508c3a7b20ea3ddda2a66128429382afc3dfc8

This patch:

1. removes a duplicate assertion to check that the only error
   condition that can be returned while turning a cpu off is
   PSCI_E_DENIED. Having this assertion after calling
   psci_afflvl_off() is sufficient.

2. corrects some incorrect usage of 'its' vs 'it is'

3. removes some unwanted white spaces

Change-Id: Icf014e269b54f5be5ce0b9fbe6b41258e4ebf403

- Add instructions for contributing to ARM Trusted Firmware.

- Update copyright text in all files to acknowledge contributors.

Change-Id: I9311aac81b00c6c167d2f8c889aea403b84450e5