Documentation/soc/intel: Add MP Initialization document

This patch provides documentation for MP initialization
option available in coreboot.

Change-Id: I055808e2ddf03663e1ec5d3d423054d1caa911cb
Signed-off-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31841
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

1 files changed, 56 insertions, 0 deletions

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+# Multiple Processor (MP) Initialization
+
+This section is intended to document the purpose of performing multiprocessor
+initialization and its possible ways in coreboot space.
+
+Entire CPU multiprocessor initialization can be divided into two parts
+1. BSP (Boot Strap Processor) Initialization
+2. AP (Application Processor) Initialization
+
+* [Multiple Processor Init](https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-vol-3a-part-1-manual.pdf) - section 8.4
+
+## Problem Statement
+
+1. coreboot is capable enough to handle multiprocessor initialization on
+IA platforms.
+
+2. With restricted CPU programming logic, there might be some cases where
+certain feature programming can't be open sourced at early development of SOC.
+
+Platform code might need to compromise on those closed source nature of CPU
+programming if we don't plan to provide an alternate interface which can be
+used by coreboot to get rid of such close sourced CPU programming.
+
+## Possible Solution Space
+
+Considering these facts, there are 3 possible solutions to perform MP
+initialization from coreboot + FSP space.
+
+1. coreboot to perform complete MP initialization by its own. This includes
+BSP and AP programming of CPU features mostly non-restricted one. Preferred
+Kconfig is USE_COREBOOT_NATIVE_MP_INIT. SoCs like SKL, KBL, APL are okay to
+make use of same Kconfig option for MP initialization.
+
+2. Alternatively, SoC users also can skip coreboot doing MP initialization
+and make use of FSP binary to perform same task. This can be achieved by using
+Kconfig name USE_INTEL_FSP_MP_INIT. As of 2019 all Google Chrome products are
+using coreboot native MP initialization mechanism and some IOTG platforms
+are using FSP MP Init solution as well.
+
+3. Final option is to let coreboot publish PPI (PEIM to PEIM Interface) to
+perform some restricted (closed source) CPU programming. In that case,
+coreboot will use its native MP init and additionally publish MP service PPI
+for FSP to consume. FSP will execute some CPU programming using same PPI
+service from its own context. One can use
+USE_INTEL_FSP_TO_CALL_COREBOOT_PUBLISH_MP_PPI Kconfig to perform this
+operation.
+
+For latest SoCs like CNL, WHL, ICL, etc, its recommended to make use of this
+option in order to perform SGX and C6DRAM enabling.
+
+Typically all platforms supported by FSP 2.1 specification will have
+external PPI service feature implemented.
+
+[References]
+- [PPI](../fsp/ppi/ppi.md)
+- [MP Service PPI](../fsp/ppi/mp_service_ppi.md)