Well the current plan I had for this was to do it in userspace, I don't think the kernel
has any business doing it and I think for the simple USB case its fine but will fallover
when you get to the non-trivial cases where some sort of acceleration is required to move
pixels around. But in saying that its good you've done what something, and I'll try and spend
some time reviewing it.

There are some clear advantages in the kernel doing bits of this I think.
The kernel understands device to device DMA, and has a better idea than
userspace about things like buffer alignment internals. It also means
this ultimately can work without X running which is a plus for some
applications (I want a displaylink gadget for my phone but thats another
story 8)).

I'm also not sure how you deal with tiling etc, you can also start hitting rendering limits,
where a GPU can render to 4kx4k but you can plug in more USB devices, again I'm hoping to
solve this in userspace as well.

Tiling has to be handled by the recipient (at least when the fb is shared). The nastier end of
it that I don't see convered in the documentation is the handling of
fencing between cards. Eg if you wanted to do display on one card fed
into a second to do effects processing (think about TV type stuff)

Alan