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Hi all,

Could you please take a look at the following patches to improve topology implementation on `feature-twincpu` branch? https://review.trustedfirmware.org/c/trusted-firmware-m/+/1542 ~ https://review.trustedfirmware.org/c/trusted-firmware-m/+/1549 and https://review.trustedfirmware.org/c/trusted-firmware-m/+/1603/

As you know, we are bringing up TF-M on multi-core topology platform. Currently, preprocessor directives TFM_MULTI_CORE_TOPOLOGY are used to comment/uncomment the functionalities here and there to fit different scenarios. It can work but makes code difficult to be understood or maintained, in both single Armv8-M and multi-core topologies. The above patches try to eliminate multi-core directives from common sequence and functions shared by single Armv8-M and multi-core topologies. It is expected to not only improve the code readability, but also protect one topology from being affected by changes to another.

Although those changes are for twincpu feature branch, I'd like to ask for review and comment since they also impact the single Armv8-M scenario. Merging feature-twincpu branch back to master would be more smooth if we can figure out and fix issues now on `feature-twincpu`.

I'd like to summarize the common basic ideas of the topology implementation. 1. If single Armv8-M and multi-core topology both call the same API but require different implementations a. If that API can be classified to a specific functionality/module, separate the implementations into topology exclusive files. Take https://review.trustedfirmware.org/c/trusted-firmware-m/+/1542 as an example. Add multi-core specific NS Client ID implementations in tfm_multi_core.c. Thus multi-core topology doesn't rely on the single Armv8-M implementations in tfm_nspm.c. Then all the multi-core directives can be removed from tfm_nspm.c and tfm_nspm.c can be clearly excluded from multi-core topology build.

b. If that API don't belong to a specific functionality/module, extract it out and organize the implementations in topology abstraction file. For example, in https://review.trustedfirmware.org/c/trusted-firmware-m/+/1545, tfm_core_topology_set_pendsv_priority() is defined to wrap the PendSV settings in diverse topologies. The implementations are selected according to current topology in tfm_core_topology.h. It help maintain a more clear and uniform sequence by removing multi-core directives from tfm_core.c.

2. In common sequence, if an API is called in single Armv8-M but not used in multi-core topology, add an empty function for this API in multi-core topology in the header file and vice versa. For example, secure core in multi-core topology doesn't need to configure_ns_code(). As show in https://review.trustedfirmware.org/c/trusted-firmware-m/+/1543, instead of placing several multi-core directives in tfm_core.c, move configure_ns_code() implementation to tfm_nspm.c and define an empty function in header file tfm_nspm.h in multi-core topology for configure_ns_code().

Any comment or suggestion is welcome. Please kindly let me know if the above changes may cause troubles in single Armv8-M scenario. Thank you.

Best regards, Hu Ziji