Optimize monitor discovery and validation process

Refactored monitor discovery to a two-phase process, enabling
parallel capability fetching and reducing total discovery time.
Introduced caching of monitor capabilities to avoid redundant
I2C operations, improving performance during initialization
and runtime validation.

Added `DdcCiValidationResult` to encapsulate validation status
and cached capabilities. Replaced `ValidateDdcCiConnection`
with `FetchCapabilities` for capability retrieval, marking the
former as obsolete. Introduced `QuickConnectionCheck` for fast
runtime validation.

Updated `CanControlMonitorAsync`, `GetCapabilitiesStringAsync`,
and `InitializeMonitorCapabilitiesAsync` to leverage cached
data. Improved logging for better insights into discovery and
validation processes.

src/modules/powerdisplay/PowerDisplay.Lib/Drivers/DDC/DdcCiController.cs

@@ -47,7 +47,9 @@ namespace PowerDisplay.Common.Drivers.DDC
         public string Name => "DDC/CI Monitor Controller";
 
         /// <summary>
-        /// Check if the specified monitor can be controlled
+        /// Check if the specified monitor can be controlled.
+        /// Uses quick connection check if capabilities are already cached,
+        /// otherwise falls back to full validation.
         /// </summary>
         public async Task<bool> CanControlMonitorAsync(Monitor monitor, CancellationToken cancellationToken = default)
         {
@@ -55,7 +57,23 @@ namespace PowerDisplay.Common.Drivers.DDC
                 () =>
                 {
                     var physicalHandle = GetPhysicalHandle(monitor);
-                    return physicalHandle != IntPtr.Zero && DdcCiNative.ValidateDdcCiConnection(physicalHandle);
+                    if (physicalHandle == IntPtr.Zero)
+                    {
+                        return false;
+                    }
+
+                    // If monitor already has cached capabilities with brightness support,
+                    // use quick connection check instead of full capabilities retrieval
+                    if (monitor.VcpCapabilitiesInfo != null &&
+                        monitor.VcpCapabilitiesInfo.SupportsVcpCode(NativeConstants.VcpCodeBrightness))
+                    {
+                        return DdcCiNative.QuickConnectionCheck(physicalHandle);
+                    }
+
+                    // Fall back to full validation for monitors without cached capabilities
+#pragma warning disable CS0618 // Suppress obsolete warning - needed for backward compatibility
+                    return DdcCiNative.ValidateDdcCiConnection(physicalHandle).IsValid;
+#pragma warning restore CS0618
                 },
                 cancellationToken);
         }
@@ -345,10 +363,18 @@ namespace PowerDisplay.Common.Drivers.DDC
         }
 
         /// <summary>
-        /// Get monitor capabilities string with retry logic
+        /// Get monitor capabilities string with retry logic.
+        /// Uses cached CapabilitiesRaw if available to avoid slow I2C operations.
         /// </summary>
         public async Task<string> GetCapabilitiesStringAsync(Monitor monitor, CancellationToken cancellationToken = default)
         {
+            // Check if capabilities are already cached
+            if (!string.IsNullOrEmpty(monitor.CapabilitiesRaw))
+            {
+                Logger.LogDebug($"GetCapabilitiesStringAsync: Using cached capabilities for {monitor.Id} (length: {monitor.CapabilitiesRaw.Length})");
+                return monitor.CapabilitiesRaw;
+            }
+
             return await Task.Run(
                 () =>
                 {
@@ -492,7 +518,9 @@ namespace PowerDisplay.Common.Drivers.DDC
                         return monitors;
                     }
 
-                    // Get physical handles for each monitor
+                    // Phase 1: Collect all candidate monitors with their handles
+                    var candidateMonitors = new List<(IntPtr Handle, string DeviceKey, PHYSICAL_MONITOR PhysicalMonitor, string AdapterName, int Index, DisplayDeviceInfo? MatchedDevice)>();
+
                     foreach (var hMonitor in monitorHandles)
                     {
                         var adapterName = _discoveryHelper.GetMonitorDeviceId(hMonitor);
@@ -511,7 +539,6 @@ namespace PowerDisplay.Common.Drivers.DDC
                         }
 
                         // Match physical monitors with DisplayDeviceInfo
-                        // For each physical monitor on this adapter, find the corresponding DisplayDeviceInfo
                         for (int i = 0; i < physicalMonitors.Length; i++)
                         {
                             var physicalMonitor = physicalMonitors[i];
@@ -542,29 +569,71 @@ namespace PowerDisplay.Common.Drivers.DDC
                             string deviceKey = matchedDevice?.DeviceKey ?? $"{adapterName}_{i}";
 
                             // Use HandleManager to reuse or create handle
-                            var (handleToUse, reusingOldHandle) = _handleManager.ReuseOrCreateHandle(deviceKey, physicalMonitor.HPhysicalMonitor);
+                            var (handleToUse, _) = _handleManager.ReuseOrCreateHandle(deviceKey, physicalMonitor.HPhysicalMonitor);
 
-                            // Always validate DDC/CI connection, regardless of handle reuse
-                            // This ensures monitors that don't support DDC/CI (e.g., internal laptop displays)
-                            // are not included in the DDC controller's results
-                            if (!DdcCiNative.ValidateDdcCiConnection(handleToUse))
-                            {
-                                Logger.LogDebug($"DDC: Handle 0x{handleToUse:X} (reused={reusingOldHandle}) failed DDC/CI validation, skipping");
-                                continue;
-                            }
-
-                            // Update physical monitor handle to use the correct one
+                            // Update physical monitor handle
                             var monitorToCreate = physicalMonitor;
                             monitorToCreate.HPhysicalMonitor = handleToUse;
 
-                            var monitor = _discoveryHelper.CreateMonitorFromPhysical(monitorToCreate, adapterName, i, monitorDisplayInfo, matchedDevice);
-                            if (monitor != null)
-                            {
-                                monitors.Add(monitor);
+                            candidateMonitors.Add((handleToUse, deviceKey, monitorToCreate, adapterName, i, matchedDevice));
+                        }
+                    }
 
-                                // Store in new map for cleanup
-                                newHandleMap[monitor.DeviceKey] = handleToUse;
+                    // Phase 2: Fetch capabilities in PARALLEL for all candidate monitors
+                    // This is the slow I2C operation (~4s per monitor), but parallelization
+                    // significantly reduces total time when multiple monitors are connected.
+                    // Results are cached regardless of success/failure.
+                    Logger.LogInfo($"DDC: Phase 2 - Fetching capabilities for {candidateMonitors.Count} monitors in parallel");
+
+                    var fetchTasks = candidateMonitors.Select(candidate =>
+                        Task.Run(
+                            () =>
+                            {
+                                var capabilitiesResult = DdcCiNative.FetchCapabilities(candidate.Handle);
+                                return (Candidate: candidate, CapabilitiesResult: capabilitiesResult);
+                            },
+                            cancellationToken));
+
+                    var fetchResults = await Task.WhenAll(fetchTasks);
+
+                    Logger.LogInfo($"DDC: Phase 2 completed - Got results for {fetchResults.Length} monitors");
+
+                    // Phase 3: Create monitor objects for valid DDC/CI monitors
+                    // A monitor is valid for DDC if it has capabilities with brightness support
+                    foreach (var result in fetchResults)
+                    {
+                        // Skip monitors that don't support DDC/CI brightness control
+                        if (!result.CapabilitiesResult.IsValid)
+                        {
+                            Logger.LogDebug($"DDC: Handle 0x{result.Candidate.Handle:X} - No DDC/CI brightness support, skipping");
+                            continue;
+                        }
+
+                        var monitor = _discoveryHelper.CreateMonitorFromPhysical(
+                            result.Candidate.PhysicalMonitor,
+                            result.Candidate.AdapterName,
+                            result.Candidate.Index,
+                            monitorDisplayInfo,
+                            result.Candidate.MatchedDevice);
+
+                        if (monitor != null)
+                        {
+                            // Attach cached capabilities data - this is the key optimization!
+                            // By caching here, we avoid re-fetching during InitializeMonitorCapabilitiesAsync
+                            if (!string.IsNullOrEmpty(result.CapabilitiesResult.CapabilitiesString))
+                            {
+                                monitor.CapabilitiesRaw = result.CapabilitiesResult.CapabilitiesString;
                             }
+
+                            if (result.CapabilitiesResult.VcpCapabilitiesInfo != null)
+                            {
+                                monitor.VcpCapabilitiesInfo = result.CapabilitiesResult.VcpCapabilitiesInfo;
+                            }
+
+                            monitors.Add(monitor);
+                            newHandleMap[monitor.DeviceKey] = result.Candidate.Handle;
+
+                            Logger.LogInfo($"DDC: Added monitor {monitor.Id} with {monitor.VcpCapabilitiesInfo?.SupportedVcpCodes.Count ?? 0} VCP codes");
                         }
                     }
 
@@ -582,12 +651,13 @@ namespace PowerDisplay.Common.Drivers.DDC
         }
 
         /// <summary>
-        /// Validate monitor connection status
+        /// Validate monitor connection status.
+        /// Uses quick VCP read instead of full capabilities retrieval.
         /// </summary>
         public async Task<bool> ValidateConnectionAsync(Monitor monitor, CancellationToken cancellationToken = default)
         {
             return await Task.Run(
-                () => monitor.Handle != IntPtr.Zero && DdcCiNative.ValidateDdcCiConnection(monitor.Handle),
+                () => monitor.Handle != IntPtr.Zero && DdcCiNative.QuickConnectionCheck(monitor.Handle),
                 cancellationToken);
         }
 

src/modules/powerdisplay/PowerDisplay.Lib/Drivers/DDC/DdcCiNative.cs

@@ -40,6 +40,57 @@ namespace PowerDisplay.Common.Drivers.DDC
         public string DeviceKey { get; set; } = string.Empty;
     }
 
+    /// <summary>
+    /// DDC/CI validation result containing both validation status and cached capabilities data.
+    /// This allows reusing capabilities data retrieved during validation, avoiding duplicate I2C calls.
+    /// </summary>
+    public struct DdcCiValidationResult
+    {
+        /// <summary>
+        /// Gets a value indicating whether the monitor has a valid DDC/CI connection with brightness support.
+        /// </summary>
+        public bool IsValid { get; }
+
+        /// <summary>
+        /// Gets the raw capabilities string retrieved during validation.
+        /// Null if retrieval failed.
+        /// </summary>
+        public string? CapabilitiesString { get; }
+
+        /// <summary>
+        /// Gets the parsed VCP capabilities info retrieved during validation.
+        /// Null if parsing failed.
+        /// </summary>
+        public Models.VcpCapabilities? VcpCapabilitiesInfo { get; }
+
+        /// <summary>
+        /// Gets a value indicating whether capabilities retrieval was attempted.
+        /// True means the result is from an actual attempt (success or failure).
+        /// </summary>
+        public bool WasAttempted { get; }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="DdcCiValidationResult"/> struct.
+        /// </summary>
+        public DdcCiValidationResult(bool isValid, string? capabilitiesString = null, Models.VcpCapabilities? vcpCapabilitiesInfo = null, bool wasAttempted = true)
+        {
+            IsValid = isValid;
+            CapabilitiesString = capabilitiesString;
+            VcpCapabilitiesInfo = vcpCapabilitiesInfo;
+            WasAttempted = wasAttempted;
+        }
+
+        /// <summary>
+        /// Gets an invalid validation result with no cached data.
+        /// </summary>
+        public static DdcCiValidationResult Invalid => new(false, null, null, true);
+
+        /// <summary>
+        /// Gets a result indicating validation was not attempted yet.
+        /// </summary>
+        public static DdcCiValidationResult NotAttempted => new(false, null, null, false);
+    }
+
     /// <summary>
     /// DDC/CI native API wrapper
     /// </summary>
@@ -163,29 +214,131 @@ namespace PowerDisplay.Common.Drivers.DDC
         }
 
         /// <summary>
-        /// Validates the DDC/CI connection
+        /// Fetches VCP capabilities string from a monitor and returns a validation result.
+        /// This is the slow I2C operation (~4 seconds per monitor) that should only be done once.
+        /// The result is cached regardless of success or failure.
         /// </summary>
         /// <param name="hPhysicalMonitor">Physical monitor handle</param>
-        /// <returns>True if connection is valid</returns>
-        public static bool ValidateDdcCiConnection(IntPtr hPhysicalMonitor)
+        /// <returns>Validation result with capabilities data (or failure status)</returns>
+        public static DdcCiValidationResult FetchCapabilities(IntPtr hPhysicalMonitor)
+        {
+            if (hPhysicalMonitor == IntPtr.Zero)
+            {
+                return DdcCiValidationResult.Invalid;
+            }
+
+            try
+            {
+                // Try to get capabilities string (slow I2C operation)
+                var capsString = TryGetCapabilitiesString(hPhysicalMonitor);
+                if (string.IsNullOrEmpty(capsString))
+                {
+                    Logger.LogDebug($"FetchCapabilities: Failed to get capabilities string for handle 0x{hPhysicalMonitor:X}");
+                    return DdcCiValidationResult.Invalid;
+                }
+
+                // Parse the capabilities string
+                var capabilities = Utils.VcpCapabilitiesParser.Parse(capsString);
+                if (capabilities == null || capabilities.SupportedVcpCodes.Count == 0)
+                {
+                    Logger.LogDebug($"FetchCapabilities: Failed to parse capabilities string for handle 0x{hPhysicalMonitor:X}");
+                    return DdcCiValidationResult.Invalid;
+                }
+
+                // Check if brightness (VCP 0x10) is supported - determines DDC/CI validity
+                bool supportsBrightness = capabilities.SupportsVcpCode(NativeConstants.VcpCodeBrightness);
+
+                Logger.LogDebug($"FetchCapabilities: Handle 0x{hPhysicalMonitor:X} - BrightnessSupport={supportsBrightness}, VcpCodes={capabilities.SupportedVcpCodes.Count}");
+                return new DdcCiValidationResult(supportsBrightness, capsString, capabilities);
+            }
+            catch (Exception ex) when (ex is not OutOfMemoryException)
+            {
+                Logger.LogDebug($"FetchCapabilities: Exception for handle 0x{hPhysicalMonitor:X}: {ex.Message}");
+                return DdcCiValidationResult.Invalid;
+            }
+        }
+
+        /// <summary>
+        /// Validates the DDC/CI connection by checking if the monitor returns a valid capabilities string
+        /// that includes brightness control (VCP 0x10).
+        /// NOTE: This method performs a slow I2C operation. Prefer using FetchCapabilities() during
+        /// discovery phase and caching the result.
+        /// </summary>
+        /// <param name="hPhysicalMonitor">Physical monitor handle</param>
+        /// <returns>Validation result containing status and cached capabilities data</returns>
+        [System.Obsolete("Use FetchCapabilities() during discovery and cache results. This method is kept for backward compatibility.")]
+        public static DdcCiValidationResult ValidateDdcCiConnection(IntPtr hPhysicalMonitor)
+        {
+            // Delegate to FetchCapabilities which does the same thing
+            return FetchCapabilities(hPhysicalMonitor);
+        }
+
+        /// <summary>
+        /// Quick connection check using a simple VCP read (brightness).
+        /// This is much faster than full capabilities retrieval (~50ms vs ~4s).
+        /// Use this for runtime connection validation when capabilities are already cached.
+        /// </summary>
+        /// <param name="hPhysicalMonitor">Physical monitor handle</param>
+        /// <returns>True if the monitor responds to VCP queries</returns>
+        public static bool QuickConnectionCheck(IntPtr hPhysicalMonitor)
         {
             if (hPhysicalMonitor == IntPtr.Zero)
             {
                 return false;
             }
 
-            // Try reading basic VCP codes to validate connection
-            var testCodes = new byte[] { NativeConstants.VcpCodeBrightness, NativeConstants.VcpCodeContrast, NativeConstants.VcpCodeVcpVersion, NativeConstants.VcpCodeVolume };
-
-            foreach (var code in testCodes)
+            try
             {
-                if (TryGetVCPFeature(hPhysicalMonitor, code, out _, out _))
-                {
-                    return true;
-                }
+                // Try a quick brightness read to verify connection
+                return TryGetMonitorBrightness(hPhysicalMonitor, out _, out _, out _);
+            }
+            catch
+            {
+                return false;
+            }
+        }
+
+        /// <summary>
+        /// Try to get capabilities string from a physical monitor handle.
+        /// </summary>
+        /// <param name="hPhysicalMonitor">Physical monitor handle</param>
+        /// <returns>Capabilities string, or null if failed</returns>
+        private static string? TryGetCapabilitiesString(IntPtr hPhysicalMonitor)
+        {
+            if (hPhysicalMonitor == IntPtr.Zero)
+            {
+                return null;
             }
 
-            return false;
+            try
+            {
+                // Get capabilities string length
+                if (!GetCapabilitiesStringLength(hPhysicalMonitor, out uint length) || length == 0)
+                {
+                    return null;
+                }
+
+                // Allocate buffer and get capabilities string
+                var buffer = Marshal.AllocHGlobal((int)length);
+                try
+                {
+                    if (!CapabilitiesRequestAndCapabilitiesReply(hPhysicalMonitor, buffer, length))
+                    {
+                        return null;
+                    }
+
+                    return Marshal.PtrToStringAnsi(buffer);
+                }
+                finally
+                {
+                    Marshal.FreeHGlobal(buffer);
+                }
+            }
+            catch (Exception ex) when (ex is not OutOfMemoryException)
+            {
+                Logger.LogDebug($"TryGetCapabilitiesString failed: {ex.Message}");
+                return null;
+            }
         }
 
         /// <summary>

src/modules/powerdisplay/PowerDisplay.Lib/Drivers/DDC/PhysicalMonitorHandleManager.cs

@@ -55,9 +55,10 @@ namespace PowerDisplay.Common.Drivers.DDC
             return _deviceKeyToHandleMap.ExecuteWithLock(dict =>
             {
                 // Try to reuse existing handle if it's still valid
+                // Use quick connection check instead of full capabilities retrieval
                 if (dict.TryGetValue(deviceKey, out var existingHandle) &&
                     existingHandle != IntPtr.Zero &&
-                    DdcCiNative.ValidateDdcCiConnection(existingHandle))
+                    DdcCiNative.QuickConnectionCheck(existingHandle))
                 {
                     // Destroy the newly created handle since we're using the old one
                     if (newHandle != existingHandle && newHandle != IntPtr.Zero)

src/modules/powerdisplay/PowerDisplay/Core/MonitorManager.cs

@@ -157,10 +157,19 @@ namespace PowerDisplay.Core
             IMonitorController controller,
             CancellationToken cancellationToken)
         {
-            // Verify if monitor can be controlled
-            if (!await controller.CanControlMonitorAsync(monitor, cancellationToken))
+            // Skip control verification if monitor was already validated during discovery phase
+            // The presence of cached VcpCapabilitiesInfo indicates the monitor passed DDC/CI validation
+            // This avoids redundant capabilities retrieval (~4 seconds per monitor)
+            bool alreadyValidated = monitor.VcpCapabilitiesInfo != null &&
+                                   monitor.VcpCapabilitiesInfo.SupportedVcpCodes.Count > 0;
+
+            if (!alreadyValidated)
             {
-                return null;
+                // Verify if monitor can be controlled (for monitors not validated in discovery phase)
+                if (!await controller.CanControlMonitorAsync(monitor, cancellationToken))
+                {
+                    return null;
+                }
             }
 
             // Get current brightness
@@ -207,6 +216,7 @@ namespace PowerDisplay.Core
 
         /// <summary>
         /// Initialize monitor DDC/CI capabilities.
+        /// If capabilities are already cached from discovery phase, only update derived properties.
         /// </summary>
         private async Task InitializeMonitorCapabilitiesAsync(
             Monitor monitor,
@@ -215,6 +225,15 @@ namespace PowerDisplay.Core
         {
             try
             {
+                // Check if capabilities were already cached during discovery phase
+                // This avoids expensive I2C calls (~4 seconds per monitor) for redundant data
+                if (monitor.VcpCapabilitiesInfo != null && monitor.VcpCapabilitiesInfo.SupportedVcpCodes.Count > 0)
+                {
+                    Logger.LogInfo($"Using cached capabilities for {monitor.Id}: {monitor.VcpCapabilitiesInfo.SupportedVcpCodes.Count} VCP codes");
+                    UpdateMonitorCapabilitiesFromVcp(monitor);
+                    return;
+                }
+
                 Logger.LogInfo($"Getting capabilities for monitor {monitor.Id}");
                 var capsString = await controller.GetCapabilitiesStringAsync(monitor, cancellationToken);