SystemInfo.h: add some documentation

src/gpu_info_util/SystemInfo.h

@@ -19,11 +19,6 @@ namespace angle
 using VendorID = uint32_t;
 using DeviceID = uint32_t;
 
-constexpr VendorID kVendorID_AMD      = 0x1002;
-constexpr VendorID kVendorID_Intel    = 0x8086;
-constexpr VendorID kVendorID_Nvidia   = 0x10DE;
-constexpr VendorID kVendorID_Qualcomm = 0x5143;
-
 struct GPUDeviceInfo
 {
     GPUDeviceInfo();
@@ -47,7 +42,12 @@ struct SystemInfo
     SystemInfo(const SystemInfo &other);
 
     std::vector<GPUDeviceInfo> gpus;
+
+    // Index of the primary GPU (the discrete one on dual GPU systems) in `gpus`.
+    // Will never be -1 after a successful GetSystemInfo.
     int primaryGPUIndex = -1;
+    // Index of the currently active GPU in `gpus`, can be -1 if the active GPU could not be
+    // detected.
     int activeGPUIndex  = -1;
 
     bool isOptimus       = false;
@@ -61,8 +61,18 @@ struct SystemInfo
     std::string primaryDisplayDeviceId;
 };
 
+// Gathers information about the system without starting a GPU driver and returns them in `info`.
+// Returns true if all info was gathered, false otherwise. Even when false is returned, `info` will
+// be filled with partial information.
 bool GetSystemInfo(SystemInfo *info);
 
+// Known PCI vendor IDs
+constexpr VendorID kVendorID_AMD      = 0x1002;
+constexpr VendorID kVendorID_Intel    = 0x8086;
+constexpr VendorID kVendorID_Nvidia   = 0x10DE;
+constexpr VendorID kVendorID_Qualcomm = 0x5143;
+
+// Predicates on vendor IDs
 bool IsAMD(VendorID vendorId);
 bool IsIntel(VendorID vendorId);
 bool IsNvidia(VendorID vendorId);