GitLab

In order to quantify the overall time spent in the PSCI software
implementation, an initial collection of PMF instrumentation points
has been added.

Instrumentation has been added to the following code paths:

- Entry to PSCI SMC handler.  The timestamp is captured as early
  as possible during the runtime exception and stored in memory
  before entering the PSCI SMC handler.

- Exit from PSCI SMC handler.  The timestamp is captured after
  normal return from the PSCI SMC handler or if a low power state
  was requested it is captured in the bl31 warm boot path before
  return to normal world.

- Entry to low power state.  The timestamp is captured before entry
  to a low power state which implies either standby or power down.
  As these power states are mutually exclusive, only one timestamp
  is defined to describe both.  It is possible to differentiate between
  the two power states using the PSCI STAT interface.

- Exit from low power state.  The timestamp is captured after a standby
  or power up operation has completed.

To calculate the number of cycles spent running code in Trusted Firmware
one can perform the following calculation:

(exit_psci - enter_psci) - (exit_low_pwr - enter_low_pwr).

The resulting number of cycles can be converted to time given the
frequency of the counter.

Change-Id: Ie3b8f3d16409b6703747093b3a2d5c7429ad0166
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>

This patch moves the PSCI services and BL31 frameworks like context
management and per-cpu data into new library components `PSCI` and
`el3_runtime` respectively. This enables PSCI to be built independently from
BL31. A new `psci_lib.mk` makefile is introduced which adds the relevant
PSCI library sources and gets included by `bl31.mk`. Other changes which
are done as part of this patch are:

* The runtime services framework is now moved to the `common/` folder to
  enable reuse.
* The `asm_macros.S` and `assert_macros.S` helpers are moved to architecture
  specific folder.
* The `plat_psci_common.c` is moved from the `plat/common/aarch64/` folder
  to `plat/common` folder. The original file location now has a stub which
  just includes the file from new location to maintain platform compatibility.

Most of the changes wouldn't affect platform builds as they just involve
changes to the generic bl1.mk and bl31.mk makefiles.

NOTE: THE `plat_psci_common.c` FILE HAS MOVED LOCATION AND THE STUB FILE AT
THE ORIGINAL LOCATION IS NOW DEPRECATED. PLATFORMS SHOULD MODIFY THEIR
MAKEFILES TO INCLUDE THE FILE FROM THE NEW LOCATION.

Change-Id: I6bd87d5b59424995c6a65ef8076d4fda91ad5e86

This patch adds following optional PSCI STAT functions:

- PSCI_STAT_RESIDENCY: This call returns the amount of time spent
  in power_state in microseconds, by the node represented by the
  `target_cpu` and the highest level of `power_state`.

- PSCI_STAT_COUNT: This call returns the number of times a
  `power_state` has been used by the node represented by the
  `target_cpu` and the highest power level of `power_state`.

These APIs provides residency statistics for power states that has
been used by the platform. They are implemented according to v1.0
of the PSCI specification.

By default this optional feature is disabled in the PSCI
implementation. To enable it, set the boolean flag
`ENABLE_PSCI_STAT` to 1. This also sets `ENABLE_PMF` to 1.

Change-Id: Ie62e9d37d6d416ccb1813acd7f616d1ddd3e8aff

This patch adds a new optional platform hook `pwr_domain_pwr_down_wfi()` in
the plat_psci_ops structure. This hook allows the platform to perform platform
specific actions including the wfi invocation to enter powerdown. This hook
is invoked by both psci_do_cpu_off() and psci_cpu_suspend_start() functions.
The porting-guide.md is also updated for the same.

This patch also modifies the `psci_power_down_wfi()` function to invoke
`plat_panic_handler` incase of panic instead of the busy while loop.

Fixes ARM-Software/tf-issues#375

Change-Id: Iba104469a1445ee8d59fb3a6fdd0a98e7f24dfa3

When a CPU is powered down using PSCI CPU OFF API, it disables its caches
and updates its `aff_info_state` to OFF. The corresponding cache line is
invalidated by the CPU so that the update will be observed by other CPUs
running with caches enabled. There is a possibility that another CPU
which has been trying to turn ON this CPU via PSCI CPU ON API,
has already seen the update to `aff_info_state` and proceeds to update
the state to ON_PENDING prior to the cache invalidation. This may result
in the update of the state to ON_PENDING being discarded.

This patch fixes this issue by making sure that the update of `aff_info_state`
to ON_PENDING sticks by reading back the value after the cache flush and
retrying it if not updated. The patch also adds a dsbish() to
`psci_do_cpu_off()` to ensure ordering of the update to `aff_info_state`
prior to cache line invalidation.

Fixes ARM-software/tf-issues#349

Change-Id: I225de99957fe89871f8c57bcfc243956e805dcca

This patch fixes an issue in the PSCI framework where the affinity info
state of a core was being set to OFF even when the SPD had denied the
CPU_OFF request. Now, the state remains set to ON instead.

Fixes ARM-software/tf-issues#323

Change-Id: Ia9042aa41fae574eaa07fd2ce3f50cf8cae1b6fc

This patch reworks the PSCI generic implementation to conform to ARM
Trusted Firmware coding guidelines as described here:
https://github.com/ARM-software/arm-trusted-firmware/wiki

This patch also reviews the use of signed data types within PSCI
Generic code and replaces them with their unsigned counterparts wherever
they are not appropriate. The PSCI_INVALID_DATA macro which was defined
to -1 is now replaced with PSCI_INVALID_PWR_LVL macro which is defined
to PLAT_MAX_PWR_LVL + 1.

Change-Id: Iaea422d0e46fc314e0b173c2b4c16e0d56b2515a

This commit does the switch to the new PSCI framework implementation replacing
the existing files in PSCI folder with the ones in PSCI1.0 folder. The
corresponding makefiles are modified as required for the new implementation.
The platform.h header file is also is switched to the new one
as required by the new frameworks. The build flag ENABLE_PLAT_COMPAT defaults
to 1 to enable compatibility layer which let the existing platform ports to
continue to build and run with minimal changes.

The default weak implementation of platform_get_core_pos() is now removed from
platform_helpers.S and is provided by the compatibility layer.

Note: The Secure Payloads and their dispatchers still use the old platform
and framework APIs and hence it is expected that the ENABLE_PLAT_COMPAT build
flag will remain enabled in subsequent patch. The compatibility for SPDs using
the older APIs on platforms migrated to the new APIs will be added in the
following patch.

Change-Id: I18c51b3a085b564aa05fdd98d11c9f3335712719

The state-id field in the power-state parameter of a CPU_SUSPEND call can be
used to describe composite power states specific to a platform. The current PSCI
implementation does not interpret the state-id field. It relies on the target
power level and the state type fields in the power-state parameter to perform
state coordination and power management operations. The framework introduced
in this patch allows the PSCI implementation to intepret generic global states
like RUN, RETENTION or OFF from the State-ID to make global state coordination
decisions and reduce the complexity of platform ports. It adds support to
involve the platform in state coordination which facilitates the use of
composite power states and improves the support for entering standby states
at multiple power domains.

The patch also includes support for extended state-id format for the power
state parameter as specified by PSCIv1.0.

The PSCI implementation now defines a generic representation of the power-state
parameter. It depends on the platform port to convert the power-state parameter
(possibly encoding a composite power state) passed in a CPU_SUSPEND call to this
representation via the `validate_power_state()` plat_psci_ops handler. It is an
array where each index corresponds to a power level. Each entry contains the
local power state the power domain at that power level could enter.

The meaning of the local power state values is platform defined, and may vary
between levels in a single platform. The PSCI implementation constrains the
values only so that it can classify the state as RUN, RETENTION or OFF as
required by the specification:
   * zero means RUN
   * all OFF state values at all levels must be higher than all RETENTION
     state values at all levels
   * the platform provides PLAT_MAX_RET_STATE and PLAT_MAX_OFF_STATE values
     to the framework

The platform also must define the macros PLAT_MAX_RET_STATE and
PLAT_MAX_OFF_STATE which lets the PSCI implementation find out which power
domains have been requested to enter a retention or power down state. The PSCI
implementation does not interpret the local power states defined by the
platform. The only constraint is that the PLAT_MAX_RET_STATE <
PLAT_MAX_OFF_STATE.

For a power domain tree, the generic implementation maintains an array of local
power states. These are the states requested for each power domain by all the
cores contained within the domain. During a request to place multiple power
domains in a low power state, the platform is passed an array of requested
power-states for each power domain through the plat_get_target_pwr_state()
API. It coordinates amongst these states to determine a target local power
state for the power domain. A default weak implementation of this API is
provided in the platform layer which returns the minimum of the requested
power-states back to the PSCI state coordination.

Finally, the plat_psci_ops power management handlers are passed the target
local power states for each affected power domain using the generic
representation described above. The platform executes operations specific to
these target states.

The platform power management handler for placing a power domain in a standby
state (plat_pm_ops_t.pwr_domain_standby()) is now only used as a fast path for
placing a core power domain into a standby or retention state should now be
used to only place the core power domain in a standby or retention state.

The extended state-id power state format can be enabled by setting the
build flag PSCI_EXTENDED_STATE_ID=1 and it is disabled by default.

Change-Id: I9d4123d97e179529802c1f589baaa4101759d80c

This patch removes the assumption in the current PSCI implementation that MPIDR
based affinity levels map directly to levels in a power domain tree. This
enables PSCI generic code to support complex power domain topologies as
envisaged by PSCIv1.0 specification. The platform interface for querying
the power domain topology has been changed such that:

1. The generic PSCI code does not generate MPIDRs and use them to query the
   platform about the number of power domains at a particular power level. The
   platform now provides a description of the power domain tree on the SoC
   through a data structure. The existing platform APIs to provide the same
   information have been removed.

2. The linear indices returned by plat_core_pos_by_mpidr() and
   plat_my_core_pos() are used to retrieve core power domain nodes from the
   power domain tree. Power domains above the core level are accessed using a
   'parent' field in the tree node descriptors.

The platform describes the power domain tree in an array of 'unsigned
char's. The first entry in the array specifies the number of power domains at
the highest power level implemented in the system. Each susbsequent entry
corresponds to a power domain and contains the number of power domains that are
its direct children. This array is exported to the generic PSCI implementation
via the new `plat_get_power_domain_tree_desc()` platform API.

The PSCI generic code uses this array to populate its internal power domain tree
using the Breadth First Search like algorithm. The tree is split into two
arrays:

1. An array that contains all the core power domain nodes

2. An array that contains all the other power domain nodes

A separate array for core nodes allows certain core specific optimisations to
be implemented e.g. remove the bakery lock, re-use per-cpu data framework for
storing some information.

Entries in the core power domain array are allocated such that the
array index of the domain is equal to the linear index returned by
plat_core_pos_by_mpidr() and plat_my_core_pos() for the MPIDR
corresponding to that domain. This relationship is key to be able to use
an MPIDR to find the corresponding core power domain node, traverse to higher
power domain nodes and index into arrays that contain core specific
information.

An introductory document has been added to briefly describe the new interface.

Change-Id: I4b444719e8e927ba391cae48a23558308447da13

As per Section 4.2.2. in the PSCI specification, the term "affinity"
is used in the context of describing the hierarchical arrangement
of cores. This often, but not always, maps directly to the processor
power domain topology of the system. The current PSCI implementation
assumes that this is always the case i.e. MPIDR based levels of
affinity always map to levels in a power domain topology tree.

This patch is the first in a series of patches which remove this
assumption. It removes all occurences of the terms "affinity
instances and levels" when used to describe the power domain
topology. Only the terminology is changed in this patch. Subsequent
patches will implement functional changes to remove the above
mentioned assumption.

Change-Id: Iee162f051b228828310610c5a320ff9d31009b4e

This patch optimizes the invocation of the platform power management hooks for
ON, OFF and SUSPEND such that they are called only for the highest affinity
level which will be powered off/on. Earlier, the hooks were being invoked for
all the intermediate levels as well.

This patch requires that the platforms migrate to the new semantics of the PM
hooks.  It also removes the `state` parameter from the pm hooks as the `afflvl`
parameter now indicates the highest affinity level for which power management
operations are required.

Change-Id: I57c87931d8a2723aeade14acc710e5b78ac41732

This patch creates a copy of the existing PSCI files and related psci.h and
platform.h header files in a new `PSCI1.0` directory. The changes for the
new PSCI power domain topology and extended state-ID frameworks will be
added incrementally to these files. This incremental approach will
aid in review and in understanding the changes better. Once all the
changes have been introduced, these files will replace the existing PSCI
files.

Change-Id: Ibb8a52e265daa4204e34829ed050bddd7e3316ff

This patch implements conditional checks in psci_smc_handler() to verify
that the psci function invoked by the caller is supported by the platform
or SPD implementation. The level of support is saved in the 'psci_caps'
variable. This check allows the PSCI implementation to return an error
early.

As a result of the above verification, the checks performed within the psci
handlers for the pm hooks are now removed and replaced with assertions.

Change-Id: I9b5b646a01d8566dc28c4d77dd3aa54e9bf3981a

This patch adds support to return SUCCESS if a pending interrupt is
detected during a CPU_SUSPEND call to a power down state. The check
is performed as late as possible without losing the ability to return
to the caller. This reduces the overhead incurred by a CPU in
undergoing a complete power cycle when a wakeup interrupt is already
pending.

Fixes ARM-Software/tf-issues#102

Change-Id: I1aff04a74b704a2f529734428030d1d10750fd4b

This patch allows the platform to validate the power_state and
entrypoint information from the normal world early on in PSCI
calls so that we can return the error safely. New optional
pm_ops hooks `validate_power_state` and `validate_ns_entrypoint`
are introduced to do this.

As a result of these changes, all the other pm_ops handlers except
the PSCI_ON handler are expected to be successful. Also, the PSCI
implementation will now assert if a PSCI API is invoked without the
corresponding pm_ops handler being registered by the platform.

NOTE : PLATFORM PORTS WILL BREAK ON MERGE OF THIS COMMIT. The
pm hooks have 2 additional optional callbacks and the return type
of the other hooks have changed.

Fixes ARM-Software/tf-issues#229

Change-Id: I036bc0cff2349187c7b8b687b9ee0620aa7e24dc

This patch removes the non-secure entry point information being passed
to the platform pm_ops which is not needed. Also, it removes the `mpidr`
parameter for  platform pm hooks which are meant to do power management
operations only on the current cpu.

NOTE: PLATFORM PORTS MUST BE UPDATED AFTER MERGING THIS COMMIT.

Change-Id: If632376a990b7f3b355f910e78771884bf6b12e7

This patch implements the following cleanups in PSCI generic code:

1. It reworks the affinity level specific handlers in the PSCI implementation
   such that.

   a. Usage of the 'rc' local variable is restricted to only where it is
      absolutely needed

   b. 'plat_state' local variable is defined only when a direct invocation of
      plat_get_phys_state() does not suffice.

   c. If a platform handler is not registered then the level specific handler
      returns early.

2. It limits the use of the mpidr_aff_map_nodes_t typedef to declaration of
   arrays of the type instead of using it in function prototypes as well.

3. It removes dangling declarations of __psci_cpu_off() and
   __psci_cpu_suspend(). The definitions of these functions were removed in
   earlier patches.

Change-Id: I51e851967c148be9c2eeda3a3c41878f7b4d6978

This patch adds APIs to find, save and retrieve the highest affinity level which
will enter or exit from the physical OFF state during a PSCI power management
operation. The level is stored in per-cpu data.

It then reworks the PSCI implementation to perform cache maintenance only
when the handler for the highest affinity level to enter/exit the OFF state is
called.

For example. during a CPU_SUSPEND operation, state management is done prior to
calling the affinity level specific handlers. The highest affinity level which
will be turned off is determined using the psci_find_max_phys_off_afflvl()
API. This level is saved using the psci_set_max_phys_off_afflvl() API. In the
code that does generic handling for each level, prior to performing cache
maintenance it is first determined if the current affinity level matches the
value returned by psci_get_max_phys_off_afflvl(). Cache maintenance is done if
the values match.

This change allows the last CPU in a cluster to perform cache maintenance
independently. Earlier, cache maintenance was started in the level 0 handler and
finished in the level 1 handler. This change in approach will facilitate
implementation of tf-issues#98.

Change-Id: I57233f0a27b3ddd6ddca6deb6a88b234525b0ae6

This patch pulls out state management from the affinity level specific handlers
into the top level functions specific to the operation
i.e. psci_afflvl_suspend(), psci_afflvl_on() etc.

In the power down path this patch will allow an affinity instance at level X to
determine the state that an affinity instance at level X+1 will enter before the
level specific handlers are called. This will be useful to determine whether a
CPU is the last in the cluster during a suspend/off request and so on.

Similarly, in the power up path this patch will allow an affinity instance at
level X to determine the state that an affinity instance at level X+1 has
emerged from, even after the level specific handlers have been called. This will
be useful in determining whether a CPU is the first in the cluster during a
on/resume request and so on.

As before, while powering down, state is updated before the level specific
handlers are invoked so that they can perform actions based upon their target
state. While powering up, state is updated after the level specific handlers have
been invoked so that they can perform actions based upon the state they emerged
from.

Change-Id: I40fe64cb61bb096c66f88f6d493a1931243cfd37

This patch removes the allocation of memory for coherent stacks, associated
accessor function and some dead code which called the accessor function. It also
updates the porting guide to remove the concept and the motivation behind using
stacks allocated in coherent memory.

Fixes ARM-software/tf-issues#198

Change-Id: I00ff9a04f693a03df3627ba39727e3497263fc38

This patch uses stacks allocated in normal memory to enable the MMU early in the
warm boot path thus removing the dependency on stacks allocated in coherent
memory. Necessary cache and stack maintenance is performed when a cpu is being
powered down and up. This avoids any coherency issues that can arise from
reading speculatively fetched stale stack memory from another CPUs cache. These
changes affect the warm boot path in both BL3-1 and BL3-2.

The EL3 system registers responsible for preserving the MMU state are not saved
and restored any longer. Static values are used to program these system
registers when a cpu is powered on or resumed from suspend.

Change-Id: I8357e2eb5eb6c5f448492c5094b82b8927603784

Many of the interfaces internal to PSCI pass the current CPU
MPIDR_EL1 value from function to function. This is not required,
and with inline access to the system registers is less efficient
than requiring the code to read that register whenever required.

This patch remove the mpidr parameter from the affected interfaces
and reduces code in FVP BL3-1 size by 160 bytes.

Change-Id: I16120a7c6944de37232016d7e109976540775602

Consolidate all BL3-1 CPU context initialization for cold boot, PSCI
and SPDs into two functions:
*  The first uses entry_point_info to initialize the relevant
   cpu_context for first entry into a lower exception level on a CPU
*  The second populates the EL1 and EL2 system registers as needed
   from the cpu_context to ensure correct entry into the lower EL

This patch alters the way that BL3-1 determines which exception level
is used when first entering EL1 or EL2 during cold boot - this is now
fully determined by the SPSR value in the entry_point_info for BL3-3,
as set up by the platform code in BL2 (or otherwise provided to BL3-1).

In the situation that EL1 (or svc mode) is selected for a processor
that supports EL2, the context management code will now configure all
essential EL2 register state to ensure correct execution of EL1. This
allows the platform code to run non-secure EL1 payloads directly
without requiring a small EL2 stub or OS loader.

Change-Id: If9fbb2417e82d2226e47568203d5a369f39d3b0f

The current code does not always use data and instruction
barriers as required by the architecture and frequently uses
barriers excessively due to their inclusion in all of the
write_*() helper functions.

Barriers should be used explicitly in assembler or C code
when modifying processor state that requires the barriers in
order to enable review of correctness of the code.

This patch removes the barriers from the helper functions and
introduces them as necessary elsewhere in the code.

PORTING NOTE: check any port of Trusted Firmware for use of
system register helper functions for reliance on the previous
barrier behaviour and add explicit barriers as necessary.

Fixes ARM-software/tf-issues#92

Change-Id: Ie63e187404ff10e0bdcb39292dd9066cb84c53bf

Reduce the number of header files included from other header
files as much as possible without splitting the files. Use forward
declarations where possible. This allows removal of some unnecessary
"#ifndef __ASSEMBLY__" statements.

Also, review the .c and .S files for which header files really need
including and reorder the #include statements alphabetically.

Fixes ARM-software/tf-issues#31

Change-Id: Iec92fb976334c77453e010b60bcf56f3be72bd3e

Add tag names to all unnamed structs in header files. This
allows forward declaration of structs, which is necessary to
reduce header file nesting (to be implemented in a subsequent
commit).

Also change the typedef names across the codebase to use the _t
suffix to be more conformant with the Linux coding style. The
coding style actually prefers us not to use typedefs at all but
this is considered a step too far for Trusted Firmware.

Also change the IO framework structs defintions to use typedef'd
structs to be consistent with the rest of the codebase.

Change-Id: I722b2c86fc0d92e4da3b15e5cab20373dd26786f

Make codebase consistent in its use of #include "" syntax for
user includes and #include <> syntax for system includes.

Fixes ARM-software/tf-issues#65

Change-Id: If2f7c4885173b1fd05ac2cde5f1c8a07000c7a33

Each ARM Trusted Firmware image should know in which EL it is running
and it should use the corresponding register directly instead of reading
currentEL and knowing which asm register to read/write

Change-Id: Ief35630190b6f07c8fbb7ba6cb20db308f002945

This patch implements ARM Standard Service as a runtime service and adds
support for call count, UID and revision information SMCs. The existing
PSCI implementation is subsumed by the Standard Service calls and all
PSCI calls are therefore dispatched by the Standard Service to the PSCI
handler.

At present, PSCI is the only specification under Standard Service. Thus
call count returns the number of PSCI calls implemented. As this is the
initial implementation, a revision number of 0.1 is returned for call
revision.

Fixes ARM-software/tf-issues#62

Change-Id: I6d4273f72ad6502636efa0f872e288b191a64bc1

At present SPD power management hooks and BL3-2 entry are implemented
using weak references. This would have the handlers bound and registered
with the core framework at build time, but leaves them dangling if a
service fails to initialize at runtime.

This patch replaces implementation by requiring runtime handlers to
register power management and deferred initialization hooks with the
core framework at runtime. The runtime services are to register the
hooks only as the last step, after having all states successfully
initialized.

Change-Id: Ibe788a2a381ef39aec1d4af5ba02376e67269782

This patch implements a set of handlers in the SPD which are called by
the PSCI runtime service upon receiving a power management
operation. These handlers in turn pass control to the Secure Payload
image if required before returning control to PSCI. This ensures that
the Secure Payload has complete visibility of all power transitions in
the system and can prepare accordingly.

Change-Id: I2d1dba5629b7cf2d53999d39fe807dfcf3f62fe2

This patch creates a 'services' directory and moves the PSCI under
it. Other runtime services e.g. the Secure Payload Dispatcher service
will be placed under the same directory in the future.

Also fixes issue ARM-software/tf-issues#12

Change-Id: I187f83dcb660b728f82155d91882e961d2255068

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