Add target helper macro documentation (#208)

---
Signed-off-by: Kartik Nema <kartnema@qti.qualcomm.com>

Author: kartnema

docs/README.md

@@ -53,7 +53,8 @@
 - [6. Customizations & Extensions](#6-customizations--extensions)
   - [6.1. Extensions Interface](#61-extensions-interface)
     - [6.1.1. Registering Resource Callbacks](#611-registering-resource-callbacks)
-    - [6.1.2. Registering Custom Configs](#612-registering-custom-configs)
+    - [6.1.2. Fetching Target Information](#612-fetching-target-information)
+    - [6.1.3. Registering Custom Configs](#613-registering-custom-configs)
   - [6.2. Custom Configs](#62-custom-configs)
     - [6.2.1. Initialization Configs](#621-initialization-configs)
     - [6.2.2. Resource Configs](#622-resource-configs)
@@ -1105,8 +1106,41 @@ void resetCustomCpuFreqCustom(void* res) {
 URM_REGISTER_RES_TEAR_CB(0x00010001, resetCustomCpuFreqCustom);
 ```
 
+### 6.1.2. Fetching Target Information
 
-### 6.1.2. Registering Custom Configs
+Plugins can fetch target information using the "GET_TARGET_INFO" helper utility provided by URM. The following information can be retrieved:
+- CPU Masks
+- Logical to Physical Mappings
+- Number of cores / clusters on the target
+
+```cpp
+// Utility to fetch target-specific information
+uint64_t GET_TARGET_INFO(int32_t option,
+                         int32_t numArgs,
+                         int32_t* args);
+```
+
+Usage:
+
+For example, to get a cpu mask (which can be used, for example, for IRQ affinity use cases):
+```cpp
+    int32_t args[2] = {0, 0};
+    uint64_t mask = GET_TARGET_INFO(GET_MASK, 2, args); // all cores in silver cluster
+```
+
+- The first parameter in the args array, in this case is the logical cluster ID.
+Here, we use the logical identifier for the silver cluster, using logical identifiers in the code makes it target-architecture independent, hence portable.
+The clusters logical id's are ordered according to the cluster capacities. Hence: 0 represents silver / little, 1 represents gold / big and 2 represents prime.
+
+- The second parameter in the args array, in this case is the number of cores in the cluster to be considered for mask creation. 0 represents all cores within the cluster, a non-zero positive value represents the exact number of cores to be considered.
+
+To simply get a mask corresponding to the highest capacity cluster on the target, for all cores, the GET_MAX_CLUSTER query can be used, as follows:
+```cpp
+    int32_t args[2] = {GET_MAX_CLUSTER, 0};
+    uint64_t mask = GET_TARGET_INFO(GET_MASK, 2, args); // all cores in the highest capacity cluster
+```
+
+### 6.1.3. Registering Custom Configs
 
 Custom config files either can be placed in /etc/urm/custom or Registers a custom configuration (URM_REGISTER_CONFIG) YAML file. This enables target chipset to provide their own config files, i.e. allowing them to provide their own custom resources for example.
 

resource-tuner/core/Include/TargetRegistry.h

@@ -227,8 +227,8 @@ class CacheInfoBuilder {
 };
 
 // Utility to fetch target-specific information
-uint64_t getTargetInfo(int32_t option,
-                       int32_t numArgs,
-                       int32_t* args);
+uint64_t GET_TARGET_INFO(int32_t option,
+                         int32_t numArgs,
+                         int32_t* args);
 
 #endif

resource-tuner/core/TargetRegistry.cpp

@@ -818,9 +818,9 @@ CacheInfo* CacheInfoBuilder::build() {
     return this->mCacheInfo;
 }
 
-uint64_t getTargetInfo(int32_t option,
-                       int32_t numArgs,
-                       int32_t* args) {
+uint64_t GET_TARGET_INFO(int32_t option,
+                         int32_t numArgs,
+                         int32_t* args) {
     std::shared_ptr<TargetRegistry> targetRegistry = TargetRegistry::getInstance();
 
     switch(option) {