Make HID device class driver ignore the previous HID report comparison buffer when the user sets it to NULL, disabling automatic report comparisons. Update HID device class driver documentation giving previous report buffer limitations.

Demos/Device/ClassDriver/MassStorageKeyboard/MassStorageKeyboard.c

@@ -40,6 +40,24 @@
 /** Buffer to hold the previously generated Keyboard HID report, for comparison purposes inside the HID class driver. */
 uint8_t PrevKeyboardHIDReportBuffer[sizeof(USB_KeyboardReport_Data_t)];
 
+/** LUFA HID Class driver interface configuration and state information. This structure is
+ *  passed to all HID Class driver functions, so that multiple instances of the same class
+ *  within a device can be differentiated from one another.
+ */
+USB_ClassInfo_HID_Device_t Keyboard_HID_Interface =
+ 	{
+		.Config =
+			{
+				.InterfaceNumber         = 1,
+
+				.ReportINEndpointNumber  = KEYBOARD_EPNUM,
+				.ReportINEndpointSize    = KEYBOARD_EPSIZE,
+
+				.PrevReportINBuffer      = PrevKeyboardHIDReportBuffer,
+				.PrevReportINBufferSize  = sizeof(PrevKeyboardHIDReportBuffer),
+			},
+    };
+	
 /** LUFA Mass Storage Class driver interface configuration and state information. This structure is
  *  passed to all Mass Storage Class driver functions, so that multiple instances of the same class
  *  within a device can be differentiated from one another.
@@ -60,24 +78,6 @@ USB_ClassInfo_MS_Device_t Disk_MS_Interface =
 			},
 	};
 
-/** LUFA HID Class driver interface configuration and state information. This structure is
- *  passed to all HID Class driver functions, so that multiple instances of the same class
- *  within a device can be differentiated from one another.
- */
-USB_ClassInfo_HID_Device_t Keyboard_HID_Interface =
- 	{
-		.Config =
-			{
-				.InterfaceNumber         = 1,
-
-				.ReportINEndpointNumber  = KEYBOARD_EPNUM,
-				.ReportINEndpointSize    = KEYBOARD_EPSIZE,
-
-				.PrevReportINBuffer      = PrevKeyboardHIDReportBuffer,
-				.PrevReportINBufferSize  = sizeof(PrevKeyboardHIDReportBuffer),
-			},
-    };
-
 /** Main program entry point. This routine contains the overall program flow, including initial
  *  setup of all components and the main program loop.
  */

LUFA/Drivers/USB/Class/Device/HID.c

@@ -157,12 +157,15 @@ void HID_Device_USBTask(USB_ClassInfo_HID_Device_t* const HIDInterfaceInfo)
 
 		memset(ReportINData, 0, sizeof(ReportINData));
 
-		bool ForceSend = CALLBACK_HID_Device_CreateHIDReport(HIDInterfaceInfo, &ReportID, ReportINData, &ReportINSize);
+		bool ForceSend         = CALLBACK_HID_Device_CreateHIDReport(HIDInterfaceInfo, &ReportID, ReportINData, &ReportINSize);
-
+		bool StatesChanged     = false;
-		bool StatesChanged     = (memcmp(ReportINData, HIDInterfaceInfo->Config.PrevReportINBuffer, ReportINSize) != 0);
 		bool IdlePeriodElapsed = (HIDInterfaceInfo->State.IdleCount && !(HIDInterfaceInfo->State.IdleMSRemaining));
 		
-		memcpy(HIDInterfaceInfo->Config.PrevReportINBuffer, ReportINData, ReportINSize);
+		if (HIDInterfaceInfo->Config.PrevReportINBuffer != NULL)
+		{
+			StatesChanged = (memcmp(ReportINData, HIDInterfaceInfo->Config.PrevReportINBuffer, ReportINSize) != 0);
+			memcpy(HIDInterfaceInfo->Config.PrevReportINBuffer, ReportINData, ReportINSize);
+		}
 
 		if (ReportINSize && (ForceSend || StatesChanged || IdlePeriodElapsed))
 		{

LUFA/Drivers/USB/Class/Device/HID.h

@@ -77,7 +77,14 @@
 					void*    PrevReportINBuffer; /** Pointer to a buffer where the previously created HID input report can be
 					                              *  stored by the driver, for comparison purposes to detect report changes that
 					                              *  must be sent immediately to the host. This should point to a buffer big enough
-					                              *  to hold the largest HID input report sent from the HID interface.
+					                              *  to hold the largest HID input report sent from the HID interface. If this is set
+												  *  to NULL, it is up to the user to force transfers when needed in the 
+												  *  \ref CALLBACK_HID_Device_CreateHIDReport() callback function.
+												  *
+												  *  \note Due to the single buffer, the internal driver can only correctly compare
+												  *        subsequent reports with identical report IDs. In multiple report devices,
+												  *        this buffer should be set to NULL and the decision to send reports made
+												  *        by the user application instead.
 					                              */
 					uint8_t  PrevReportINBufferSize; /** Size in bytes of the given input report buffer. This is used to create a
 					                                  *  second buffer of the same size within the driver so that subsequent reports