MdePkg/Rng: Add GetRngGuid() to RngLib

The EFI_RNG_PROTOCOL can use the RngLib. The RngLib has multiple
implementations, some of them are unsafe (e.g. BaseRngLibTimerLib).
To allow the RngDxe to detect when such implementation is used,
add a GetRngGuid() function to the RngLib.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Kun Qin <kun.qin@microsoft.com>

MdePkg/Library/DxeRngLib/DxeRngLib.c

@@ -1,6 +1,7 @@
 /** @file
  Provides an implementation of the library class RngLib that uses the Rng protocol.
 
+ Copyright (c) 2023, Arm Limited. All rights reserved.
  Copyright (c) Microsoft Corporation. All rights reserved.
  SPDX-License-Identifier: BSD-2-Clause-Patent
 
@@ -207,3 +208,30 @@ GetRandomNumber128 (
 
   return TRUE;
 }
+
+/**
+  Get a GUID identifying the RNG algorithm implementation.
+
+  @param [out] RngGuid  If success, contains the GUID identifying
+                        the RNG algorithm implementation.
+
+  @retval EFI_SUCCESS             Success.
+  @retval EFI_UNSUPPORTED         Not supported.
+  @retval EFI_INVALID_PARAMETER   Invalid parameter.
+**/
+EFI_STATUS
+EFIAPI
+GetRngGuid (
+  GUID  *RngGuid
+  )
+{
+  /* It is not possible to know beforehand which Rng algorithm will
+   * be used by this library.
+   * This API is mainly used by RngDxe. RngDxe relies on the RngLib.
+   * The RngLib|DxeRngLib.inf implementation locates and uses an installed
+   * EFI_RNG_PROTOCOL.
+   * It is thus not possible to have both RngDxe and RngLib|DxeRngLib.inf.
+   * and it is ok not to support this API.
+   */
+  return EFI_UNSUPPORTED;
+}