Grab frames in a shared XImage

I have a BT878 grabber.
How is possible to set V4L for grabbing frames directly into a shared image XShmImage?

I mean, now I have the V4L that grabs rgb frames into a mmap buffer (that isn't shared user memory, right?).
Then I use memcpy from buffer to shmimage->data.

I wanna to avoid this time consuming copy!
- Grabbing frames directly in shmimage->data ?
- Using DMA instead of memcpy? (but how?)

thanks,
 Francesco