MultiSkuDistinctionPei -- Multi-SKU Distinction PEI Module

Overview

This PEIM detects the platform SKU (Stock Keeping Unit / hardware variant) by
reading SKU configuration from CMOS and/or a GUID-extended HOB installed by
an earlier PEIM (likely from the CrystalRidge chipset initialization). It then
installs a PPI via EFI_PEI_SERVICES to advertise the SKU identity so that
downstream PEIMs can adapt their behavior.

Key Insight: The module has two code paths:

• A "HOB-provisioned" path where a previous PEIM (CrystalRidge) writes SKU
  data into a GUID HOB with sentinel 0x55
• A "default" path where SKU PPI is installed with a notification mechanism
  for delayed SKU provisioning

Module Information

GUIDs Used

Segment Layout

Function Map

Call Flow

_ModuleEntryPoint (0xffe6b414)

  +-- (*PeiServices)->AllocatePages()   -- Allocate boot buffer

  +-- SkuInit() (0xffe6b545)
     +-- FindMatchingHob() (0xffe6b6ed)
           +-- GetHobList() (0xffe6b63a)
           |     +-- GetPeiServices() (0xffe6b722)
           |     |     +-- ReadIdtr() (0xffe6b780)
           |     +-- (*PeiServices)->GetHobList()
           +-- FindHobByType() (0xffe6b6a8)
           +-- IsGuidMatch() (0xffe6b5d6)
                 +-- ReadUnaligned64() (0xffe6b754) [x4]

  +-- [sentinel == 0x55] --> (*PeiServices)->InstallPpi (active path)
  +-- [sentinel != 0x55] --> InstallSkuPpi() (0xffe6b488)

        +-- DebugPrint()
     +-- GetDebugInstance() (0xffe6b4cc)
     |     +-- GetPeiServices()
     |     +-- (*PeiServices)->LocatePpi()
     +-- GetSkuFromCmos() (0xffe6b587) -- check debug mask

        +-- SkuInit()
        +-- (*PeiServices)->NotifyPpi() -- install default PPI

Execution Flow Details

Phase 1: Module Entry

1. Allocate buffer: PeiServices->AllocatePages (EfiBootServicesData, 52, &Buffer)
2. Write revision data: The allocated buffer receives platform revision data
   and a sentinel byte (0x55) at offset 16.
3. SkuInit: Reads the CrystalRidge SKU HOB. If found, 3 SKU identifier
   bytes (at HOB offsets 48, 49, 50) are copied into a local buffer.
4. Decision: If SkuBuffer[24] == 0x55 (sentinel present):
   • Install the active SKU PPI descriptor
   • This path means CrystalRidge-provisioned SKU data is already available
   • Otherwise: call InstallSkuPpi() for the default path

Phase 2: InstallSkuPpi (Default Path)

1. Prints "InstallSkuPpi is called\n" via debug print
2. Tries SkuInit() again (defensive)
3. If SkuBuffer[24] != 0x55:
   • Installs the "default" PPI notification via PeiServices->NotifyPpi()
   • This causes EntryPointStub(0xffe6b57b) to fire when the PPI is installed
   • EntryPointStub re-enters InstallSkuPpi() recursively

SKU Detection: GetSkuFromCmos

Hardware-level SKU detection with CMOS fallback:

1. CMOS register 0x4A: Read via I/O ports 0x70/0x71
   • NMI bit (0x80) is preserved from original CMOS index
2. If value <= 3: Return directly (small SKU IDs 0-3)
   • value 0x00 -> return 0 (SKU not found)
   • value 0x01 -> return 1 (SKU found)
3. If value == 0: Fallback to MMIO at 0xFEDAF0490
   • Mask bit 1, OR with 1
   • result = (MMIO & 2) | 1
   • If result is 0x01, SKU is valid
4. If value == 0xFF: Return 0 (SKU invalid)
5. Otherwise: Return EFI error codes

Library Dependencies (internal implementations)

Hardware Addresses Referenced

Notes

• The module has no imported functions -- all dependencies are linked
  statically as internal implementations.
• The PEI Services Table Pointer is obtained via the IDT pointer trick:
  the PEI Services pointer is stored 4 bytes before the IDT base address.
• GUID comparison uses unaligned 64-bit reads instead of CompareGuid(),
  which is a common pattern in EDK2 BaseLib implementations where full GUID
  comparison is replaced with 2 x 64-bit integer comparisons.
• The module is part of the Lenovo CrystalRidge platform initialization,
  providing SKU distinction so that subsequent PEIMs (especially those in
  the CrystalRidge PEI phase) can select the correct configuration for
  the detected hardware variant.
• 32 functions renamed in total (was 18 unnamed + 1 entry point).