[IM] Update GetMinSubscriptionsPerFabric (#20207)

Author: erjiaqing

src/app/InteractionModelEngine.cpp

@@ -28,6 +28,7 @@
 #include <cinttypes>
 
 #include <lib/core/CHIPTLVUtilities.hpp>
+#include <lib/support/CodeUtils.h>
 
 extern bool emberAfContainsAttribute(chip::EndpointId endpoint, chip::ClusterId clusterId, chip::AttributeId attributeId);
 
@@ -1370,19 +1371,14 @@ bool InteractionModelEngine::HasActiveRead()
     }) == Loop::Break));
 }
 
-uint16_t InteractionModelEngine::GetMinSubscriptionsPerFabric() const
+uint16_t InteractionModelEngine::GetMinGuaranteedSubscriptionsPerFabric() const
 {
-    uint8_t fabricCount                  = mpFabricTable->FabricCount();
-    const size_t readHandlerPoolCapacity = GetReadHandlerPoolCapacityForSubscriptions();
-
-    if (fabricCount == 0)
-    {
-        return kMinSupportedSubscriptionsPerFabric;
-    }
-
-    size_t perFabricSubscriptionCapacity = readHandlerPoolCapacity / fabricCount;
-
-    return static_cast<uint16_t>(perFabricSubscriptionCapacity);
+#if CHIP_SYSTEM_CONFIG_POOL_USE_HEAP
+    return UINT16_MAX;
+#else
+    return static_cast<uint16_t>(
+        min(GetReadHandlerPoolCapacityForSubscriptions() / GetConfigMaxFabrics(), static_cast<size_t>(UINT16_MAX)));
+#endif
 }
 
 size_t InteractionModelEngine::GetNumDirtySubscriptions() const

src/app/InteractionModelEngine.h

@@ -87,8 +87,8 @@ class InteractionModelEngine : public Messaging::UnsolicitedMessageHandler,
      * Spec 8.5.1 A publisher SHALL always ensure that every fabric the node is commissioned into can create at least three
      * subscriptions to the publisher and that each subscription SHALL support at least 3 attribute/event paths.
      */
-    static constexpr size_t kMinSupportedSubscriptionsPerFabric = 2;
-    static constexpr size_t kMinSupportedPathsPerSubscription   = 2;
+    static constexpr size_t kMinSupportedSubscriptionsPerFabric = 3;
+    static constexpr size_t kMinSupportedPathsPerSubscription   = 3;
     static constexpr size_t kMinSupportedPathsPerReadRequest    = 9;
     static constexpr size_t kMinSupportedReadRequestsPerFabric  = 1;
     static constexpr size_t kReadHandlerPoolSize                = CHIP_IM_MAX_NUM_SUBSCRIPTIONS + CHIP_IM_MAX_NUM_READS;
@@ -260,7 +260,13 @@ class InteractionModelEngine : public Messaging::UnsolicitedMessageHandler,
      */
     bool TrimFabricForRead(FabricIndex aFabricIndex);
 
-    uint16_t GetMinSubscriptionsPerFabric() const;
+    /**
+     * Returns the minimal value of guaranteed subscriptions per fabic. UINT16_MAX will be returned if current app is configured to
+     * use heap for the object pools used by interaction model engine.
+     *
+     * @retval the minimal value of guaranteed subscriptions per fabic.
+     */
+    uint16_t GetMinGuaranteedSubscriptionsPerFabric() const;
 
 #if CONFIG_BUILD_FOR_HOST_UNIT_TEST
     //

src/app/clusters/basic/basic.cpp

@@ -275,7 +275,7 @@ CHIP_ERROR BasicAttrAccess::Read(const ConcreteReadAttributePath & aPath, Attrib
         constexpr uint16_t kMinCaseSessionsPerFabricMandatedBySpec = 3;
 
         capabilityMinima.caseSessionsPerFabric  = kMinCaseSessionsPerFabricMandatedBySpec;
-        capabilityMinima.subscriptionsPerFabric = InteractionModelEngine::GetInstance()->GetMinSubscriptionsPerFabric();
+        capabilityMinima.subscriptionsPerFabric = InteractionModelEngine::GetInstance()->GetMinGuaranteedSubscriptionsPerFabric();
 
         status = aEncoder.Encode(capabilityMinima);
         break;