When generating _CPC objects, some fields are mandatory by spec [1].
Some fields cannot be supported by a the Juno platform, which is used
to test the _CPC generation. Therefore, rely on the
PcdDevelopmentPlatformRelaxations Pcd to either:
- warn about the missing fields and and let the OS handle the
  missing information
- consider the missing fields as an error

_CPC fields that are exempted from checks when the Pcd is set:
- PerformanceLimitedRegister
- ReferencePerformanceCounterRegister
- DeliveredPerformanceCounterRegister

Also warn if one of the following optional fields is missing:
- HighestPerformance[Buffer|Integer]
- NominalPerformance[Buffer|Integer]

[1] Cf. non-optional fields in:
    ACPI 6.5, s8.4.6.1 '_CPC (Continuous Performance Control)'

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
---
 .../Library/Common/AmlLib/AmlLib.inf          |  3 ++
 .../Common/AmlLib/CodeGen/AmlCodeGen.c        | 36 ++++++++++++++++---
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/DynamicTablesPkg/Library/Common/AmlLib/AmlLib.inf b/DynamicTablesPkg/Library/Common/AmlLib/AmlLib.inf
index 6d0aa6ff934c..f269691864d4 100644
--- a/DynamicTablesPkg/Library/Common/AmlLib/AmlLib.inf
+++ b/DynamicTablesPkg/Library/Common/AmlLib/AmlLib.inf
@@ -71,6 +71,9 @@ [LibraryClasses]
   BaseLib
   MemoryAllocationLib
 
+[FixedPcd]
+  gEdkiiDynamicTablesPkgTokenSpaceGuid.PcdDevelopmentPlatformRelaxations
+
 [BuildOptions]
   *_*_*_CC_FLAGS = -DAML_HANDLE
 
diff --git a/DynamicTablesPkg/Library/Common/AmlLib/CodeGen/AmlCodeGen.c b/DynamicTablesPkg/Library/Common/AmlLib/CodeGen/AmlCodeGen.c
index 6f3f46e3b1ed..a04e652588d6 100644
--- a/DynamicTablesPkg/Library/Common/AmlLib/CodeGen/AmlCodeGen.c
+++ b/DynamicTablesPkg/Library/Common/AmlLib/CodeGen/AmlCodeGen.c
@@ -3532,6 +3532,28 @@ AmlCreateCpcNode (
     return EFI_INVALID_PARAMETER;
   }
 
+  /// The following fields are theoretically mandatory, but not supported
+  /// by some platforms.
+  /// - PerformanceLimitedRegister
+  /// - ReferencePerformanceCounterRegister
+  /// - DeliveredPerformanceCounterRegister
+  if (IsNullGenericAddress (&CpcInfo->PerformanceLimitedRegister) ||
+      IsNullGenericAddress (&CpcInfo->ReferencePerformanceCounterRegister) ||
+      IsNullGenericAddress (&CpcInfo->DeliveredPerformanceCounterRegister))
+  {
+    if (PcdGet64 (PcdDevelopmentPlatformRelaxations) & BIT0) {
+      /// Just warn if DEVELOPMENT_PLATFORM_RELAXATIONS is set.
+      DEBUG ((
+        DEBUG_WARN,
+        "Missing PerformanceLimited|ReferencePerformanceCounter|"
+        "DeliveredPerformanceCounter field in _CPC object\n"
+        ));
+    } else {
+      ASSERT (0);
+      return EFI_INVALID_PARAMETER;
+    }
+  }
+
   if ((IsNullGenericAddress (&CpcInfo->HighestPerformanceBuffer) &&
        (CpcInfo->HighestPerformanceInteger == 0)) ||
       (IsNullGenericAddress (&CpcInfo->NominalPerformanceBuffer) &&
@@ -3540,13 +3562,19 @@ AmlCreateCpcNode (
        (CpcInfo->LowestNonlinearPerformanceInteger == 0)) ||
       (IsNullGenericAddress (&CpcInfo->LowestPerformanceBuffer) &&
        (CpcInfo->LowestPerformanceInteger == 0)) ||
-      IsNullGenericAddress (&CpcInfo->DesiredPerformanceRegister) ||
-      IsNullGenericAddress (&CpcInfo->ReferencePerformanceCounterRegister) ||
-      IsNullGenericAddress (&CpcInfo->DeliveredPerformanceCounterRegister) ||
-      IsNullGenericAddress (&CpcInfo->PerformanceLimitedRegister))
+      IsNullGenericAddress (&CpcInfo->DesiredPerformanceRegister))
   {
     ASSERT (0);
     return EFI_INVALID_PARAMETER;
+  } else if ((IsNullGenericAddress (&CpcInfo->HighestPerformanceBuffer) &&
+              (CpcInfo->HighestPerformanceInteger == 0)) ||
+             (IsNullGenericAddress (&CpcInfo->NominalPerformanceBuffer) &&
+              (CpcInfo->NominalPerformanceInteger == 0)))
+  {
+    DEBUG ((
+      DEBUG_WARN,
+      "Missing Reference|Delivered performance field in _CPC object\n"
+      ));
   }
 
   CpcPackage = NULL;
-- 
2.25.1



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