[PATCH v1 1/9] ShellPkg: Add AcpiView validators readme

["Chris Jones" <christopher.jones@arm.com>]

Bugzilla: 3773 (https://bugzilla.tianocore.org/show_bug.cgi?id=3773)

Add readme to explain the function and use of the new AcpiView validator
framework.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
---
 ShellPkg/Library/UefiShellAcpiViewCommandLib/Validators/Readme.md | 113 ++++++++++++++++++++
 1 file changed, 113 insertions(+)

diff --git a/ShellPkg/Library/UefiShellAcpiViewCommandLib/Validators/Readme.md b/ShellPkg/Library/UefiShellAcpiViewCommandLib/Validators/Readme.md
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+# Validators Framework
+
+The validators framework allows for simple and easy post-parsing validation to
+take place. This framework gives AcpiView the ability to perform more complex
+validations by allowing validators access to ACPI data from all parsed tables
+at the same time.
+
+This framework is intended to be both generic and extensible to allow anyone to
+create their own validators with minimal changes needed to parsers and the
+AcpiView internal application code.
+
+## Running ACPI Validators
+
+The ACPI standard validator is always run after the parsing stage in AcpiView.
+Additional validators can also be run by using the command line flag
+`-r ValidatorId`, more information on this flag can be found in the AcpiView
+help string (`acpiview -?`).
+
+## Cross-table Validations
+
+The validators framework provides the ability to perform more complex
+validations, on ACPI data, than single field validation. By storing the data
+during the parsing stage it can later be used to perform validations using data
+from multiple tables. This is especially useful with ACPI fields of variable
+length as the storage and validation is handled dynamically at runtime.
+
+# ACPI Data Store
+
+The ACPI data store allows for any type of ACPI data to be stored and later
+accessed via the relevant methods in `AcpiDataStore.h`.
+
+`StoreAcpiMetaData(ListType, NodeType, Ptr, Length)`:
+   Called during the parsing stage to store relevant ACPI meta data as needed
+   for validation later. Each call stores one "packet" of meta data, of the
+   given length, in a linked list of the given ListType. NodeType can be used
+   to store nodes of different types within the same linked list.
+
+`GetMetaDataListHead(Type, Node)`:
+   Called during the validation stage to retrieve ACPI meta data stored in the
+   ACPI data store. The head of a linked list is returned that contains all
+   stored meta data of the given type.
+
+   Note: `GetMetaDataListHead()` returns the *head* of a linked list of meta
+   data. This head node is created upon initialisation and does not contain
+   any meta data itself, therefore
+   `(META_DATA_NODE*)GetFirstNode (&ListHead->Link);` should be used before
+   trying to dereference any meta data.
+
+## Example usage
+
+An example parser storing an ACPI table in the data store.
+```C
+// Parser.c
+
+// Our example table signature to store. This could be any data type as it will
+// be casted to VOID* in the data store.
+UINT32 Signature = SIGNATURE_32('A', 'B', 'C', 'D');
+
+// Store our table signature into our ACPI data store.
+Status = StoreAcpiMetaData (
+           MetaDataExampleSig,
+           MetaDataExampleNodeSig,
+           &Signature,
+           sizeof (Siganture)
+           );
+if (EFI_ERROR (Status)) {
+   return Status;
+}
+```
+
+An example validator retrieving and using meta data from the ACPI data store:
+```C
+// Validator.c
+
+META_DATA_NODE *ListHead;
+META_DATA_NODE *Node;
+
+// Retrieve a list of installed tables from the data store.
+Status = GetMetaDataListHead (MetaDataInstalledTables, &ListHead);
+if (EFI_ERROR (Status)) {
+   return Status;
+}
+
+// The first node is just a list head and contains no data, get the first
+// "real" node in the list.
+Node = (META_DATA_NODE*)GetFirstNode (&ListHead->Link);
+
+// Iterate through nodes and perform some validations with the data.
+while (!IsNull (&ListHead->Link, &Node->Link)) {
+   // ...
+}
+```
+
+## Structure
+
+The ACPI data store structure is an array containing linked lists of related
+META_DATA_NODE's. These nodes are grouped according to their META_DATA_TYPE
+with each META_DATA_TYPE pertaining to one linked list of META_DATA_NODEs.
+
+Each linked list of meta nodes is implemented via the standard doubly linked
+list implementation in MdePkg. This allows any of the standard traversal,
+query and accessing functions located in `MdePkg/Library/BaseLib/LinkedList.c`
+to be used on the linked list returned from calls to `GetMetaDataListHead()`.
+
+An example ACPI data store structure can be seen here:
+
+|---------------------------|
+|     MetaDataPpttProcs     | ---> Proc0 ---> Proc1
+|---------------------------|
+|     MetaDataMadtGicC      | ---> Proc0 ---> Proc1
+|---------------------------|
+|  MetaDataInstalledTables  | ---> "RSDP" ---> "XSDT" ---> "HMAT" ---> "PPTT"
+|---------------------------|
-- 
Guid("CE165669-3EF3-493F-B85D-6190EE5B9759")