USB Generic HID Device Example Code - What is the benefit of enabling the double buffer?

[nbgatgi]

I'm using the LPCXpresso54608 SDK for MCUXpresso.  In particular, I'm running the generic HID device with FreeRTOS example code (link to my code/setup).

One of the settings is to enable the double buffer in usb_device_lpcip3511.h:

#define USB_DEVICE_IP3511_DOUBLE_BUFFER_ENABLE (1u)

The double buffer is enabled by default.  It seems to add a layer of complexity that I'm not sure I need, and I'm not so sure it is even working correctly with the example!

Could someone enlighten me on the purpose/intended use of the double buffer?

[danielchen]

Hi Nick:

Double buffering is used for more efficient transfers.

Data in one buffer are being processed while the next set of data is read into the other one.

I hope it helps.

Regards

Daniel

[nbgatgi]

I should have been more clear in my question.  I generally understand the purpose of double-buffering.  I'm looking for more of a walk-through of this particular implementation.  I can see the buffer index toggle, and see that every packet received goes into the alternate buffer, but within "USB_DeviceHidGenericCallback", the case "kUSB_DeviceHidEventRecvResponse" first calls "USB_DeviceHidSend" without toggling the buffer index, which echoes the data received the the IN-ENDPOINT (to the PC).  Right after that, the buffer index is toggled which generally makes sense to me.  THEN "USB_DeviceHidRecv" is called using the newly toggled buffer index.  WHAT IS THIS DOING?!?!