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##

# 3.7.2.3.2 Data Transfer

The USB MSD Host Client Driver data transfer function is typically called by the SCSI Block Storage Driver Library. The USB_HOST_MSD_Transfer function allows the SCSI Block Storage Driver to transport SCSI commands to the mass storage device. The cdb parameter and the cdbLength parameter of the function specify the command and its size respectively. If the command requires the transport of data, then data must contain the pointer to the buffer and size specifies the amount of data expected to be transported. When the BOT transfer complete, the USB MSD Host Client Diver will call the callback function. The following code snippet shows an example of using the USB_HOST_MSD_Transfer function.

## Example:

    /* This code shows usage of the USB_HOST_MSD_Transfer function. The SCSI Block
     * Driver Library uses this function to send a SCSI Inquiry Command to the
     * device. Note how the commandCompleted flag in the SCSI instance object
     * tracks the completion of the transfer. This flag is updated in the transfer
     * callback. */
    void _USB_HOST_SCSI_TransferCallback
    (
        USB_HOST_MSD_LUN_HANDLE lunHandle,
        USB_HOST_MSD_TRANSFER_HANDLE transferHandle,
        USB_HOST_MSD_RESULT result,
        size_t size,
        uintptr_t context
    )
    {
        int scsiObjIndex;
        USB_HOST_SCSI_OBJ * scsiObj;
        USB_HOST_SCSI_COMMAND_OBJ * commandObj;
        USB_HOST_SCSI_EVENT event;

        /* Get the SCSI object index from the lunHandle */
        scsiObjIndex = _USB_HOST_SCSI_LUNHandleToSCSIInstance(lunHandle);

        /* Get the pointer to the SCSI object */
        scsiObj = &gUSBHostSCSIObj[scsiObjIndex];

        /* Pointer to the command object */
        commandObj = &scsiObj->commandObj;

        /* The processed size */
        commandObj->size = size;

        /* The result of the command */
        commandObj->result = result;

        /* Let the main state machine know that the command is completed */
        commandObj->commandCompleted = true;

        /* The rest of code is not shown here for the sake of brevity */
    }

    void USB_HOST_SCSI_Tasks(USB_HOST_MSD_LUN_HANDLE lunHandle)
    {
        switch(scsiObj->state)
        {
            /* For the sake of brevity, only one SCSI command is show here */
            case USB_HOST_SCSI_STATE_INQUIRY_RESPONSE:

                /* We get the SCSI Enquiry response. Although there isn't much
                 * that we can do with this data */
                _USB_HOST_SCSI_InquiryResponseCommand(scsiObj->commandObj.cdb);

                /* The commandCompleted flag will be updated in the callback.
                 * Update the state and send the command.   */
                scsiObj->commandObj.inUse = true;
                scsiObj->commandObj.commandCompleted = false;
                scsiObj->commandObj.generateEvent = false;

                result = USB_HOST_MSD_Transfer(scsiObj->lunHandle,
                        scsiObj->commandObj.cdb, 6, scsiObj->buffer, 36,
                        USB_HOST_MSD_TRANSFER_DIRECTION_DEVICE_TO_HOST,
                        _USB_HOST_SCSI_TransferCallback, (uintptr_t)(scsiObj));

                if(result == USB_HOST_MSD_RESULT_SUCCESS)
                {
                    scsiObj->state = USB_HOST_SCSI_STATE_WAIT_INQUIRY_RESPONSE;
                }

                break;

            default:
                break;

        }
    }