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PSCI Power Domain Tree Structure
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================================
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Requirements
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------------
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#. A platform must export the ``plat_get_aff_count()`` and
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``plat_get_aff_state()`` APIs to enable the generic PSCI code to
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populate a tree that describes the hierarchy of power domains in the
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system. This approach is inflexible because a change to the topology
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requires a change in the code.
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It would be much simpler for the platform to describe its power domain tree
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in a data structure.
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#. The generic PSCI code generates MPIDRs in order to populate the power domain
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tree. It also uses an MPIDR to find a node in the tree. The assumption that
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a platform will use exactly the same MPIDRs as generated by the generic PSCI
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code is not scalable. The use of an MPIDR also restricts the number of
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levels in the power domain tree to four.
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Therefore, there is a need to decouple allocation of MPIDRs from the
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mechanism used to populate the power domain topology tree.
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#. The current arrangement of the power domain tree requires a binary search
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over the sibling nodes at a particular level to find a specified power
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domain node. During a power management operation, the tree is traversed from
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a 'start' to an 'end' power level. The binary search is required to find the
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node at each level. The natural way to perform this traversal is to
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start from a leaf node and follow the parent node pointer to reach the end
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level.
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Therefore, there is a need to define data structures that implement the tree in
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a way which facilitates such a traversal.
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#. The attributes of a core power domain differ from the attributes of power
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domains at higher levels. For example, only a core power domain can be identified
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using an MPIDR. There is no requirement to perform state coordination while
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performing a power management operation on the core power domain.
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Therefore, there is a need to implement the tree in a way which facilitates this
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distinction between a leaf and non-leaf node and any associated
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optimizations.
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--------------
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Design
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------
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Describing a power domain tree
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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To fulfill requirement 1., the existing platform APIs
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``plat_get_aff_count()`` and ``plat_get_aff_state()`` have been
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removed. A platform must define an array of unsigned chars such that:
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#. The first entry in the array specifies the number of power domains at the
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highest power level implemented in the platform. This caters for platforms
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where the power domain tree does not have a single root node, for example,
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the FVP has two cluster power domains at the highest level (1).
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#. Each subsequent entry corresponds to a power domain and contains the number
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of power domains that are its direct children.
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#. The size of the array minus the first entry will be equal to the number of
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non-leaf power domains.
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#. The value in each entry in the array is used to find the number of entries
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to consider at the next level. The sum of the values (number of children) of
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all the entries at a level specifies the number of entries in the array for
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the next level.
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The following example power domain topology tree will be used to describe the
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above text further. The leaf and non-leaf nodes in this tree have been numbered
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separately.
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::
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+-+
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|0|
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+-+
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/ \
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/ \
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/ \
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/ \
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/ \
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/ \
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/ \
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/ \
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/ \
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/ \
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+-+ +-+
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|1| |2|
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+-+ +-+
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/ \ / \
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/ \ / \
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/ \ / \
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/ \ / \
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+-+ +-+ +-+ +-+
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|3| |4| |5| |6|
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+-+ +-+ +-+ +-+
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+---+-----+ +----+----| +----+----+ +----+-----+-----+
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| | | | | | | | | | | | |
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| | | | | | | | | | | | |
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v v v v v v v v v v v v v
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+-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +--+ +--+ +--+
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|0| |1| |2| |3| |4| |5| |6| |7| |8| |9| |10| |11| |12|
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+-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +--+ +--+ +--+
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This tree is defined by the platform as the array described above as follows:
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.. code:: c
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#define PLAT_NUM_POWER_DOMAINS 20
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#define PLATFORM_CORE_COUNT 13
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#define PSCI_NUM_NON_CPU_PWR_DOMAINS \
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(PLAT_NUM_POWER_DOMAINS - PLATFORM_CORE_COUNT)
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unsigned char plat_power_domain_tree_desc[] = { 1, 2, 2, 2, 3, 3, 3, 4};
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Removing assumptions about MPIDRs used in a platform
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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To fulfill requirement 2., it is assumed that the platform assigns a
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unique number (core index) between ``0`` and ``PLAT_CORE_COUNT - 1`` to each core
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power domain. MPIDRs could be allocated in any manner and will not be used to
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populate the tree.
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``plat_core_pos_by_mpidr(mpidr)`` will return the core index for the core
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corresponding to the MPIDR. It will return an error (-1) if an MPIDR is passed
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which is not allocated or corresponds to an absent core. The semantics of this
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platform API have changed since it is required to validate the passed MPIDR. It
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has been made a mandatory API as a result.
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Another mandatory API, ``plat_my_core_pos()`` has been added to return the core
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index for the calling core. This API provides a more lightweight mechanism to get
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the index since there is no need to validate the MPIDR of the calling core.
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The platform should assign the core indices (as illustrated in the diagram above)
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such that, if the core nodes are numbered from left to right, then the index
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for a core domain will be the same as the index returned by
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``plat_core_pos_by_mpidr()`` or ``plat_my_core_pos()`` for that core. This
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relationship allows the core nodes to be allocated in a separate array
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(requirement 4.) during ``psci_setup()`` in such an order that the index of the
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core in the array is the same as the return value from these APIs.
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Dealing with holes in MPIDR allocation
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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For platforms where the number of allocated MPIDRs is equal to the number of
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core power domains, for example, Juno and FVPs, the logic to convert an MPIDR to
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a core index should remain unchanged. Both Juno and FVP use a simple collision
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proof hash function to do this.
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It is possible that on some platforms, the allocation of MPIDRs is not
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contiguous or certain cores have been disabled. This essentially means that the
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MPIDRs have been sparsely allocated, that is, the size of the range of MPIDRs
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used by the platform is not equal to the number of core power domains.
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The platform could adopt one of the following approaches to deal with this
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scenario:
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#. Implement more complex logic to convert a valid MPIDR to a core index while
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maintaining the relationship described earlier. This means that the power
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domain tree descriptor will not describe any core power domains which are
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disabled or absent. Entries will not be allocated in the tree for these
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domains.
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#. Treat unallocated MPIDRs and disabled cores as absent but still describe them
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in the power domain descriptor, that is, the number of core nodes described
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is equal to the size of the range of MPIDRs allocated. This approach will
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lead to memory wastage since entries will be allocated in the tree but will
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allow use of a simpler logic to convert an MPIDR to a core index.
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Traversing through and distinguishing between core and non-core power domains
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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To fulfill requirement 3 and 4, separate data structures have been defined
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to represent leaf and non-leaf power domain nodes in the tree.
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.. code:: c
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/*******************************************************************************
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* The following two data structures implement the power domain tree. The tree
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* is used to track the state of all the nodes i.e. power domain instances
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* described by the platform. The tree consists of nodes that describe CPU power
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* domains i.e. leaf nodes and all other power domains which are parents of a
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* CPU power domain i.e. non-leaf nodes.
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******************************************************************************/
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typedef struct non_cpu_pwr_domain_node {
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/*
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* Index of the first CPU power domain node level 0 which has this node
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* as its parent.
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*/
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unsigned int cpu_start_idx;
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/*
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* Number of CPU power domains which are siblings of the domain indexed
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* by 'cpu_start_idx' i.e. all the domains in the range 'cpu_start_idx
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* -> cpu_start_idx + ncpus' have this node as their parent.
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*/
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unsigned int ncpus;
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/* Index of the parent power domain node */
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unsigned int parent_node;
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-----
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} non_cpu_pd_node_t;
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typedef struct cpu_pwr_domain_node {
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u_register_t mpidr;
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/* Index of the parent power domain node */
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unsigned int parent_node;
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-----
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} cpu_pd_node_t;
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The power domain tree is implemented as a combination of the following data
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structures.
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.. code:: c
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non_cpu_pd_node_t psci_non_cpu_pd_nodes[PSCI_NUM_NON_CPU_PWR_DOMAINS];
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cpu_pd_node_t psci_cpu_pd_nodes[PLATFORM_CORE_COUNT];
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Populating the power domain tree
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The ``populate_power_domain_tree()`` function in ``psci_setup.c`` implements the
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algorithm to parse the power domain descriptor exported by the platform to
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populate the two arrays. It is essentially a breadth-first-search. The nodes for
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each level starting from the root are laid out one after another in the
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``psci_non_cpu_pd_nodes`` and ``psci_cpu_pd_nodes`` arrays as follows:
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::
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psci_non_cpu_pd_nodes -> [[Level 3 nodes][Level 2 nodes][Level 1 nodes]]
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psci_cpu_pd_nodes -> [Level 0 nodes]
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For the example power domain tree illustrated above, the ``psci_cpu_pd_nodes``
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will be populated as follows. The value in each entry is the index of the parent
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node. Other fields have been ignored for simplicity.
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::
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+-------------+ ^
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CPU0 | 3 | |
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+-------------+ |
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CPU1 | 3 | |
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+-------------+ |
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CPU2 | 3 | |
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+-------------+ |
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CPU3 | 4 | |
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+-------------+ |
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CPU4 | 4 | |
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+-------------+ |
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CPU5 | 4 | | PLATFORM_CORE_COUNT
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+-------------+ |
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CPU6 | 5 | |
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+-------------+ |
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CPU7 | 5 | |
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+-------------+ |
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CPU8 | 5 | |
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+-------------+ |
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CPU9 | 6 | |
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+-------------+ |
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CPU10 | 6 | |
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+-------------+ |
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CPU11 | 6 | |
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+-------------+ |
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CPU12 | 6 | v
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+-------------+
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The ``psci_non_cpu_pd_nodes`` array will be populated as follows. The value in
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each entry is the index of the parent node.
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::
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+-------------+ ^
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PD0 | -1 | |
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+-------------+ |
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PD1 | 0 | |
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+-------------+ |
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PD2 | 0 | |
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+-------------+ |
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PD3 | 1 | | PLAT_NUM_POWER_DOMAINS -
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+-------------+ | PLATFORM_CORE_COUNT
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PD4 | 1 | |
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+-------------+ |
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PD5 | 2 | |
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+-------------+ |
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PD6 | 2 | |
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+-------------+ v
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Each core can find its node in the ``psci_cpu_pd_nodes`` array using the
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``plat_my_core_pos()`` function. When a core is turned on, the normal world
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provides an MPIDR. The ``plat_core_pos_by_mpidr()`` function is used to validate
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the MPIDR before using it to find the corresponding core node. The non-core power
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domain nodes do not need to be identified.
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--------------
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*Copyright (c) 2017-2018, Arm Limited and Contributors. All rights reserved.*
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