How to design a DRM KMS driver exposing 2D compositing?

Pekka Paalanen ppaalanen at
Tue Aug 12 00:10:21 PDT 2014

On Mon, 11 Aug 2014 17:35:31 +0200
Daniel Vetter <daniel at> wrote:

> Well for other drivers/stacks we'd fall back to GL compositing. pixman
> would obviously be terribly. Curious question: Can you provoke the
> hw/firmware to render into abitrary buffers or does it only work together
> with real display outputs?

Since we have been talking about on-line (direct to output) and
off-line (buffer target) use of the HVS (2D compositing engine), it
should be able to do both I think.

> So I guess the real question is: What kind of interface does videocore
> provide? Note that kms framebuffers are super-flexible and you're freee to
> add your own ioctl for special framebuffers which are rendered live by the
> vc. So that might be a possible way to expose this if you can't tell the
> vc which buffers to render into explicitly.

Right. I don't know the HVS details yet, but I'm hoping we can tell
it to render into a custom buffer, like the 3D core can.

This discussion is very helpful btw, I'm starting to see some possible


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