PCD PEIM description

April 10, 2020 •

PCD PEIM produces PCD database to manage all dynamic PCD in PEI phase and install Pcd Ppi service.

This version PCD PEIM depends on the external PCD database binary file, not built in PCD data base.
There are two PCD PPIs as follows:


It is EDKII implementation which support Dynamic/DynamicEx Pcds.


It is defined by PI specification 1.2, Vol 3 which only support dynamicEx
type Pcd.

For dynamicEx type PCD, it is compatible between PCD_PPI and EFI_PEI_PCD_PPI.
PCD PEIM driver will produce above two PPIs at same time.

PCD database is generated as the separate binary image at build time. The binary image
will be intergrated into Firmware volume together with PCD driver.

  //                                                                            //
  //                      Introduction of PCD database                          //
  //                                                                            //

1, Introduction

PCD database hold all dynamic type PCD information. The structure of PEI PCD
database is generated by build tools according to dynamic PCD usage for
specified platform.

2, Dynamic Type PCD

Dynamic type PCD is used for the configuration/setting which value is determined dynamic. In contrast, the value of static type PCD (FeatureFlag, FixedPcd, PatchablePcd) is fixed in final generated FD image in build time.

2.1 The "dynamic" determination means one of below cases:

a) The PCD setting value is produced by someone driver and consumed by other driver in execution time.
b) The PCD setting value is set/get by user from FrontPage.
c) The PCD setting value is produced by platform OEM vendor in specified area.

2.2 According to module distribution way, dynamic PCD could be classfied as:

a) Dynamic:

  If module is released in source code and will be built with platform 
  DSC, the dynamic PCD used by this module can be accessed as:
  In building platform, build tools will translate PcdSampleDynamicPcd to
  pair of {Token Space Guid: Token Number} for this PCD. 

b) DynamicEx:

  If module is release as binary and will not pariticpate platform building,
  the dynamic PCD used by this module need be accessed as:
        PcdGetxxEx(gEfiMyTokenspaceGuid, PcdSampleDynamicPcd)
  Developer need explicity gives {Token Space Guid:Token Number} as parameter
  in writting source code.

2.3 According to PCD value's storage method, dynamic PCD could be classfied as:

a) Default Storage:

      - The PCD value is stored in PCD database maintained by PCD driver in boot 
        time memory.
      - This type is used for communication between PEIM/DXE driver, DXE/DXE 
        driver. But all set/get value will be losted after boot-time memory 
        is turn off.
      - [PcdsDynamicDefault] is used as section name for this type PCD in 
        platform DSC file. [PcdsDynamicExDefault] is used for dynamicEx type PCD.

b) Variable Storage:

      - The PCD value is stored in variable area. 
      - As default storage type, this type PCD could be used for PEI/DXE driver
        communication. But beside it, this type PCD could also be used to store 
        the value associate with a HII setting via variable interface.
      - In PEI phase, the PCD value could only be got but can not be set due 
        to variable area is readonly.
      - [PcdsDynamicHii] is used as section name for this type PCD in platform 
        DSC file. [PcdsDynamicExHii] is for dynamicEx type PCD.

c) OEM specificed storage area:

      - The PCD value is stored in OEM specified area which base address is 
        specified by a FixedAtBuild PCD setting - PcdVpdBaseAddress.
      - The area is read only for PEI and DXE phase.
      - [PcdsDynamicVpd] is used as section name for this type PCD in platform 
        DSC file. [PcdsDynamicExVpd] is for dynamicex type PCD.

2.4 When and how to use dynamic PCD

Module developer do not care the used PCD is dynamic or static when writting
source code/INF. Dynamic PCD and dynamic type is pointed by platform integrator
in platform DSC file. Please ref section 2.3 to get matching between dynamic
PCD type and section name in DSC file.

3, PCD database:

Although dynamic PCD could be in different storage type as above description,
but the basic information and default value for all dynamic PCD is hold
by PCD database maintained by PEI/DXE driver.

As the whole EFI BIOS boot path is divided into PEI/DXE phase, the PCD database
also is divided into Pei/Dxe database maintaied by PcdPeim/PcdDxe driver separatly.
To make PcdPeim's driver image smaller, PEI PCD database only hold all dynamic
PCD information used in PEI phase or use in both PEI/DXE phase. And DXE PCD
database contains all PCDs used in PEI/DXE phase in memory.

 Build tool will generate PCD database into the separate binary file for 
 PEI/DXE PCD driver according to dynamic PCD section in platform DSC file. 

3.1 PcdPeim and PcdDxe

   PEI PCD database is maintained by PcdPeim driver run from flash. PcdPeim driver
   build guid hob in temporary memory and copy the binary data base from flash 
   to temporary memory for PEI PCD database. 
   DXE PCD database is maintained by PcdDxe driver.At entry point of PcdDxe driver,
   a new PCD database is allocated in boot-time memory which including all
   PEI PCD and DXE PCD entry.
   Pcd driver should run as early as possible before any other driver access
   dynamic PCD's value. PEI/DXE "Apriori File" mechanism make it possible by
   making PcdPeim/PcdDxe as first dispatching driver in PEI/DXE phase.

3.2 Token space Guid/Token number, Platform token, Local token number

        Dynamic PCD
       +-----------+               +---------+
       |TokenSpace |               |Platform |
       |   Guid    |  build tool   | Token   |
       |    +      +-------------->| Number  |
       |  Token    |               +---------+`._
       |  Number   |                             `.
       +-----------+                               `.  +------+
                                                     `-|Local |
                                                       |Token |
                            DynamicEx PCD            ,-|Number|
                            +-----------+         ,-'  +------+
                            |TokenSpace |      ,-'
                            |   Guid    |  _,-'
                            |    +      +.'
                            |  Token    |
                            |  Number   |

3.2.1 Pair of Token space guid + Token number

     Any type PCD is identified by pair of "TokenSpaceGuid + TokeNumber". But it
     is not easy maintained by PCD driver, and hashed token number will make 
     searching slowly. 

3.2.2 Platform Token Number

"Platform token number" concept is introduced for mapping to a pair of
"TokenSpaceGuid + TokenNumber". The platform token number is generated by
build tool in autogen.h and all of them are continual in a platform scope
started from 1.(0 meaning invalid internal token number)
With auto-generated "platform token number", PcdGet(PcdSampleDynamicPcd)
in source code is translated to LibPcdGet(_PCD_TOKEN_PcdSampleDynamicPcd)
in autogen.h.
Notes: The mapping between pair of "tokenspace guid + token number" and
"internal token number" need build tool establish, so "platform token number"
mechanism is not suitable for binary module which use DynamicEx type PCD.
To access a dynamicEx type PCD, pair of "token space guid/token number" all need
to be specificed for PcdSet/PcdGet accessing macro.

 Platform Token Number is started from 1, and inceased continuous. From whole 
 platform scope, there are two zones: PEI Zone and DXE Zone
               |                      Platform Token Number
     PEI Zone: |            1                 ~  PEI_LOCAL_TOKEN_NUMBER

3.2.3 Local Token Number

To fast searching a PCD entry in PCD database, PCD driver translate
platform token number to local token number via a mapping table.
For binary DynamicEx type PCD, there is a another mapping table to translate
"token space guid + token number" to local token number directly.
Local token number is identifier for all internal interface in PCD PEI/DXE

     A local token number is a 32-bit value in following meaning:
      32 ------------- 28 ---------- 24 -------- 0
       | PCD type mask  | Datum Type  |  Offset  |
       PCd type mask: indicate Pcd type from following macro:
       Datum Type   : indicate PCD vaue type from following macro:
       Offset      : indicate the related offset of PCD value in PCD database array.
    Based on local token number, PCD driver could fast determine PCD type, value
    type and get PCD entry from PCD database.

3.3 PCD Database binary file

   PCD Database binary file will be created at build time as the standalone binary image. 
   To understand the binary image layout, PCD Database C structure is still generated 
   as comments by build tools in PCD driver's autogen.h/
   autogen.c file. In generated C structure, following information is stored:
   - ExMapTable: This table is used translate a binary dynamicex type PCD's 
                 "tokenguid + token" to local token number.
   - LocalTokenNumberTable:
                 This table stores all local token number in array, use "Internal
                 token number" as array index to get PCD entry's offset fastly.
   - SizeTable:  This table stores the size information for all PCD entry.
   - GuidTable:  This table stores guid value for DynamicEx's token space,
                 HII type PCD's variable GUID.
   - SkuIdTable: TBD
   - SystemSkuId: TBD
   - PCD value structure:  
                 Every PCD has a value record in PCD database. For different
                 datum type PCD has different record structure which will be 
                 introduced in 3.3.1
   In a PCD database structure, there are two major area: Init and UnInit. 
   Init area is use stored above PCD internal structure such as ExMapTable, 
   LocalTokenNumberTable etc and the (default) value of PCD which has default 
   value specified in platform DSC file.
   Unint area is used stored the value of PCD which has no default value in
   platform DSC file, the value of NULL, 0 specified in platform DSC file can
   be seemed as "no default value".

3.3.1 Simple Sample PCD Database C Structure

A general sample of PCD database structue is as follows:

     typedef struct _PCD_DATABASE {
       typedef struct _PCD_DATABASE_INIT {
         //===== Following is PCD database internal maintain structures
         UINT32            LocalTokenNumberTable[PEI_LOCAL_TOKEN_NUMBER_TABLE_SIZE];
         GUID              GuidTable[PEI_GUID_TABLE_SIZE];
         SIZE_INFO         SizeTable[PEI_SIZE_TABLE_SIZE];
         UINT8             SkuIdTable[PEI_SKUID_TABLE_SIZE];
         SKU_ID            SystemSkuId;
         //===== Following is value structure for PCD with default value
       } Init;
       typedef struct _PCD_DATABSE_UNINIT {
         //==== Following is value structure for PCD without default value
       } UnInit;

3.3.2 PCD value structure in PCD database C structure

The value's structure is generated by build tool in PCD database C structure.
The PCDs in different datum type has different value structure. UINT8/UINT16/UINT32/UINT64 datum type PCD

       The C structure for these datum type PCD is just a UINT8/UINT16/UINT32/UINT64
       data member in PCD database, For example:
       UINT16  PcdHardwareErrorRecordLevel_d3705011_bc19_4af7_be16_f68030378c15_VariableDefault_0;
       Above structure is generated by build tool, the member name is "PcdCName_Guidvalue"
       Member type is UINT16 according to PcdHardwareErrorRecordLevel declaration
       in DEC file. VOID* datum type PCD

       The value of VOID* datum type PCD is a UINT8/UINT16 array in PCD database. VOID* - string type

         If the default value for VOID* datum type PCD like L"xxx", the PCD is 
         used for unicode string, and C structure of this datum type PCD is 
         UINT16 string array in PCD database, for example:
         UINT16 StringTable[29];
         The number of 29 in above sample is max size of a unicode string.
         If the default value for VOID* datum type PCD like "xxx", the PCD is
         used for ascii string, and C structure of this datum type PCD is 
         UINT8 string array in PCD database, for example:
         UINT8 StringTable[20];
         The number of 20 in above sample is max size of a ascii string.
  VOID* - byte array

         If the default value of VOID* datum type PCD like {'0x29', '0x01', '0xf2'}
         the PCD is used for byte array. The generated structrue is same as 
         above ascii string table,
         UINT8 StringTable[13];
         The number of 13 in above sample is max size of byte array.

3.3.3 Some utility structures in PCD Database GuidTable

       GuidTable array is used to store all related GUID value in PCD database:
         - Variable GUID for HII type PCD
         - Token space GUID for dynamicex type PCD 

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