DisplayLink Provides USB GPU Support On Linux

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Ah. I did not know that.

That is very different from a USB video card... it's a USB _display_.

Very interesting.

Does the framebuffer need to be a real video card? Or can it be a software-driven thing?

[tettamanti]

Does the framebuffer need to be a real video card? Or can it be a software-driven thing?

I'm not sure. For a theoretical point of view it's possible, you just need a virtual video card to please the OS (_that_ one); in the embedded space the video subsystem is often just a chunk of system memory, with a DMA channel dedicated to sending the data off to the display controller; with DisplayLink only the last part would be different.
On desktop machine however you may want a real video card to accelerate the drawing operations (composition, spinning cubes...)