fix: correct HID output report format for G923 LEDs

- First byte must be Report ID (0x00), not the command directly
- Buffer size must match device OutputReportByteLength
- Add diagnostic logging via OutputDebugString

Author: privaloops

Common Files/LogitechLED.cpp

@@ -4,6 +4,7 @@
 #include <setupapi.h>
 #include <hidsdi.h>
 #include <hidpi.h>
+#include <stdio.h>
 
 #pragma comment(lib, "hid.lib")
 #pragma comment(lib, "setupapi.lib")
@@ -21,9 +22,20 @@ static const USHORT G29_PID       = 0xC24F;
 static const BYTE LOGITECH_CMD_SET_LED = 0xF8;
 static const BYTE LOGITECH_LED_SUBCMD  = 0x12;
 
+// Forward declaration for logging (defined in DllMain.cpp)
+extern void LogMessage(const char* fmt, ...);
+
+// Simple log helper - uses OutputDebugString if LogMessage is not available
+static void LEDLog(const char* msg)
+{
+	OutputDebugStringA(msg);
+	OutputDebugStringA("\n");
+}
+
 LogitechLED::LogitechLED()
 	: m_deviceHandle(INVALID_HANDLE_VALUE)
 	, m_available(false)
+	, m_outputReportLength(0)
 {
 }
 
@@ -49,6 +61,7 @@ void LogitechLED::Close()
 		m_deviceHandle = INVALID_HANDLE_VALUE;
 	}
 	m_available = false;
+	m_outputReportLength = 0;
 }
 
 bool LogitechLED::IsAvailable() const
@@ -58,34 +71,43 @@ bool LogitechLED::IsAvailable() const
 
 bool LogitechLED::SetLEDs(BYTE ledMask)
 {
-	if (!m_available || m_deviceHandle == INVALID_HANDLE_VALUE)
+	if (!m_available || m_deviceHandle == INVALID_HANDLE_VALUE || m_outputReportLength == 0)
 		return false;
 
-	// Logitech extended HID report for RPM LEDs
-	// Format: [ReportID, CMD, SUBCMD, LED_MASK, 0, 0, 0, 1]
-	BYTE report[8] = { 0 };
-	report[0] = LOGITECH_CMD_SET_LED;  // 0xF8
-	report[1] = LOGITECH_LED_SUBCMD;   // 0x12
-	report[2] = ledMask & 0x1F;        // 5 LEDs, bits 0-4
-	report[3] = 0x00;
+	// Allocate buffer matching the device's expected output report length
+	BYTE* report = (BYTE*)calloc(m_outputReportLength, 1);
+	if (!report)
+		return false;
+
+	// First byte = Report ID (0x00 for default)
+	// Then the Logitech extended command
+	report[0] = 0x00;                        // Report ID
+	report[1] = LOGITECH_CMD_SET_LED;        // 0xF8
+	report[2] = LOGITECH_LED_SUBCMD;         // 0x12
+	report[3] = ledMask & 0x1F;              // 5 LEDs, bits 0-4
 	report[4] = 0x00;
 	report[5] = 0x00;
 	report[6] = 0x00;
 	report[7] = 0x01;
 
 	DWORD bytesWritten = 0;
-	BOOL result = WriteFile(m_deviceHandle, report, sizeof(report), &bytesWritten, NULL);
+	BOOL result = WriteFile(m_deviceHandle, report, m_outputReportLength, &bytesWritten, NULL);
 
 	if (!result)
 	{
-		// Device may have been disconnected
 		DWORD err = GetLastError();
+		char buf[256];
+		sprintf_s(buf, "LogitechLED: WriteFile failed, error=%lu, reportLen=%u, mask=0x%02X",
+			err, m_outputReportLength, ledMask);
+		LEDLog(buf);
+
 		if (err == ERROR_DEVICE_NOT_CONNECTED || err == ERROR_GEN_FAILURE)
 		{
 			Close();
 		}
 	}
 
+	free(report);
 	return result == TRUE;
 }
 
@@ -121,13 +143,21 @@ bool LogitechLED::FindAndOpenDevice()
 		DIGCF_PRESENT | DIGCF_DEVICEINTERFACE);
 
 	if (deviceInfoSet == INVALID_HANDLE_VALUE)
+	{
+		LEDLog("LogitechLED: SetupDiGetClassDevs failed");
 		return false;
+	}
 
 	SP_DEVICE_INTERFACE_DATA interfaceData;
 	interfaceData.cbSize = sizeof(SP_DEVICE_INTERFACE_DATA);
 
+	int deviceCount = 0;
+	int logitechCount = 0;
+
 	for (DWORD i = 0; SetupDiEnumDeviceInterfaces(deviceInfoSet, NULL, &hidGuid, i, &interfaceData); i++)
 	{
+		deviceCount++;
+
 		// Get required buffer size
 		DWORD requiredSize = 0;
 		SetupDiGetDeviceInterfaceDetailA(deviceInfoSet, &interfaceData, NULL, 0, &requiredSize, NULL);
@@ -145,7 +175,7 @@ bool LogitechLED::FindAndOpenDevice()
 			continue;
 		}
 
-		// Open the device to check VID/PID
+		// Open the device
 		HANDLE handle = CreateFileA(
 			detailData->DevicePath,
 			GENERIC_READ | GENERIC_WRITE,
@@ -168,21 +198,39 @@ bool LogitechLED::FindAndOpenDevice()
 				 attrs.ProductID == G923_PID_PS ||
 				 attrs.ProductID == G29_PID))
 			{
-				// Check this is the right HID interface for output reports
-				// by verifying the output report length
+				logitechCount++;
+
 				PHIDP_PREPARSED_DATA preparsedData = NULL;
 				if (HidD_GetPreparsedData(handle, &preparsedData))
 				{
 					HIDP_CAPS caps;
 					if (HidP_GetCaps(preparsedData, &caps) == HIDP_STATUS_SUCCESS)
 					{
-						// We need an output report length that can hold our 8-byte command
+						char buf[512];
+						sprintf_s(buf,
+							"LogitechLED: Found Logitech VID=0x%04X PID=0x%04X "
+							"UsagePage=0x%04X Usage=0x%04X "
+							"InputReportLen=%u OutputReportLen=%u FeatureReportLen=%u "
+							"Path=%s",
+							attrs.VendorID, attrs.ProductID,
+							caps.UsagePage, caps.Usage,
+							caps.InputReportByteLength,
+							caps.OutputReportByteLength,
+							caps.FeatureReportByteLength,
+							detailData->DevicePath);
+						LEDLog(buf);
+
+						// We need an output report length that can hold our command
 						if (caps.OutputReportByteLength >= 8)
 						{
+							m_outputReportLength = caps.OutputReportByteLength;
 							HidD_FreePreparsedData(preparsedData);
 							free(detailData);
 							SetupDiDestroyDeviceInfoList(deviceInfoSet);
 							m_deviceHandle = handle;
+
+							sprintf_s(buf, "LogitechLED: Using device with OutputReportLen=%u", m_outputReportLength);
+							LEDLog(buf);
 							return true;
 						}
 					}
@@ -195,6 +243,11 @@ bool LogitechLED::FindAndOpenDevice()
 		free(detailData);
 	}
 
+	char buf[128];
+	sprintf_s(buf, "LogitechLED: Enumerated %d HID devices, found %d Logitech matches, none suitable",
+		deviceCount, logitechCount);
+	LEDLog(buf);
+
 	SetupDiDestroyDeviceInfoList(deviceInfoSet);
 	return false;
 }

Common Files/LogitechLED.h

@@ -35,6 +35,7 @@ class LogitechLED
 private:
 	HANDLE m_deviceHandle;
 	bool m_available;
+	USHORT m_outputReportLength;
 
 	// Find the G923 HID device path and open it
 	bool FindAndOpenDevice();