Our video capture device is already using a linked list of buffers for transfer of video data via DMA.

I'd like to use these same buffers for screen and just post them when they are filled.  The attached code snippet 
outlines the basics of my approach, but it isn't working. 

To illustrate and try to figure this out I'm using the sync tutorial.  You'll see the #ifdef clause that contains the 
code I would like to execute and have it work.

The result of the #ifdef block is a blank screen.  Of course, using the original code works.  Basically I would like to 
allocate the memory and assign it to a screen buffer and have screen use that memory instead of its own allocation.

Docs suggest that this is known and supported method, but apparently I'm missing some detail.