DynamicTablesPkg: Helper function to compute package length

Some AML object have a PkgLen which indicates the size of the
AML object. The package length can be encoded in 1 to 4 bytes.
The bytes used to encode the PkgLen is itself counted in the
PkgLen value. So, if an AML object's size increments/decrements,
the number of bytes used to encode the PkgLen value can itself
increment/decrement.

Therefore, a helper function AmlComputePkgLength() is introduced
to simply computation of the PkgLen.

Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>

DynamicTablesPkg/Library/Common/AmlLib/AmlEncoding/Aml.h

@@ -2,7 +2,7 @@
   AML grammar definitions.
 
   Copyright (c) 2010 - 2018, Intel Corporation. All rights reserved. <BR>
-  Copyright (c) 2019 - 2020, Arm Limited. All rights reserved.<BR>
+  Copyright (c) 2019 - 2021, Arm Limited. All rights reserved.<BR>
 
   SPDX-License-Identifier: BSD-2-Clause-Patent
 **/
@@ -326,5 +326,50 @@ AmlComputePkgLengthWidth (
   IN  UINT32  Length
   );
 
+/** Given a length, compute the value of a PkgLen.
+
+  In AML, some object have a PkgLen, telling the size of the AML object.
+  It can be encoded in 1 to 4 bytes. The bytes used to encode the PkgLen is
+  itself counted in the PkgLen value.
+  This means that if an AML object sees its size increment/decrement,
+  the number of bytes used to encode the PkgLen value can itself
+  increment/decrement.
+
+  For instance, the AML encoding of a DeviceOp is:
+    DefDevice := DeviceOp PkgLength NameString TermList
+  If:
+   - sizeof (NameString) = 4 (the name is "DEV0" for instance);
+   - sizeof (TermList) = (2^6-6)
+  then the PkgLen is encoded on 1 byte. Indeed, its value is:
+    sizeof (PkgLen) + sizeof (NameString) + sizeof (TermList) =
+    sizeof (PkgLen) + 4 + (2^6-6)
+  So:
+    PkgLen = sizeof (PkgLen) + (2^6-2)
+
+  The input arguments Length and PkgLen represent, for the DefDevice:
+    DefDevice := DeviceOp PkgLength NameString TermList
+                                    |------Length-----|
+                          |--------*PgkLength---------|
+
+  @param  [in]  Length  The length to encode as a PkgLen.
+                        Length cannot exceed 2^28 - 4 (4 bytes for the
+                        PkgLen encoding).
+                        The size of the PkgLen encoding bytes should not be
+                        counted in this length value.
+  @param  [out] PkgLen  If success, contains the value of the PkgLen,
+                        ready to encode in the PkgLen format.
+                        This value takes into account the size of PkgLen
+                        encoding.
+
+  @retval EFI_SUCCESS             The function completed successfully.
+  @retval EFI_INVALID_PARAMETER   Invalid parameter.
+**/
+EFI_STATUS
+EFIAPI
+AmlComputePkgLength (
+  IN  UINT32    Length,
+  OUT UINT32  * PkgLen
+  );
+
 #endif // AML_H_