Setting Microsoft OS Descriptor

This section describes how to set the Microsoft OS Descriptor (MOD) for using Symbian MTP over USB services.

1. 
   Load the USB LDD–usbc.ldd and open a client of it.

   _LIT(KUsbLDDName, "eusbc"); //Name passed to User::LoadLogicalDevice()
   _LIT(KUsbLDDFreeName, "Usbc"); //Name passed to User::FreeLogicalDevice()
   
   TInt err = User::LoadLogicalDevice(KUsbLDDName); // Load the USB logical device
   
   if (err != KErrNone && err != KErrAlreadyExists)
      {
      User::Leave(err);
      }
   
   RDevUsbcClient ldd; //Create a client of the USB logical device
   
   User::LeaveIfError(ldd.Open(0)); //Open the client
   
   

2. 
   Set the OS string descriptor. OS string descriptor is stored in the USB device driver at the fixed index of 0xEE. It contains a request number that Windows uses to retrieve OS feature descriptors.

   /**
    * MTP OS string decsriptor definitions.
    */
   const TUint8 KMTPUsbOSStringIndex(0xEE); //OS string descriptor index
   _LIT16(KMTPUsbOSStringSignature, "MSFT100\xAE"); //qwSignature and bMS_VendorCode
   
   // Write the OS string descriptor to the LDD.
   User::LeaveIfError(ldd.SetStringDescriptor(KMTPUsbOSStringIndex, KMTPUsbOSStringSignature));
   
   

   qwSignature and bMS_VendorCode are two key fields which must be set. qwSignature is a Unicode character array which identifies the descriptor as an OS string descriptor. bMS_VendorCode is one byte field whose value is set by the vendor. This value is used by Windows to request the OS feature descriptor (compat ID and sub-compat ID).
3. 
   Route device-directed requests to the user binary. The user implementation must include the following to enable the USB Stack to route the device directed requests to the user binary.

   User::LeaveIfError(ldd.SetDeviceControl()); 
   

   Note: Only one instance of RDevUsbcClient can hold the device control at any given time.
4. 
   Provide the OS feature descriptor handler. Once a valid OS string descriptor has been returned, Windows retrieves the OS feature descriptor which it uses to load the appropriate USB driver. In this case, the OS feature descriptor contains the MTP compat ID and MTP sub-compat ID.

   // KMsOSFeatureDescriptor 
   _LIT8(KMsOSFeatureDescriptor, "\x28\x00\x00\x00\x00\x01\x04\x00\x01\x00\x00\x00\x00\x00\x00\x00" \
   //dwLength     |  bcdVersion | wIndex | bCount | Reserved      
   //4 characters |  2 chars    |  2     | 1      | 7
   
   "\x00\x01\x4D\x54\x50\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00");
   // bFirstInterfaceNumber | bInterfaceCount |compatibleID | subCompatibleID | Reserved
   // 1                     | 1               | 8           | 8               | 6
   
   const TInt KMODHeaderSize   = 0x10; //The first 16 characters of the feature descriptor
   const TInt KMODFunctionSize = 0x28;  
   
   TRequestStatus status;
   
   //aRequest is a structure for requests defined by a device creator
   //wLength is the field which specifies the length of the data to be transferred
   TInt length(aRequest.wLength); 
   
   if (length == KMODHeaderSize)
      {
      // The request is for the header. Return just the header
      ldd.Write(status, EEndpoint0, KMsOSFeatureDescriptor().Left(KMODHeaderSize), KMODHeaderSize, ETrue);
      }
   else if (length == KMODFunctionSize)
      {
      ldd.Write(status, EEndpoint0, KMsOSFeatureDescriptor(),KMODFunctionSize, ETrue);
      }