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

Pekka Paalanen ppaalanen at
Mon Aug 11 06:44:01 PDT 2014

On Mon, 11 Aug 2014 14:14:56 +0100
Damien Lespiau <damien.lespiau at> wrote:

> On Mon, Aug 11, 2014 at 03:07:33PM +0300, Pekka Paalanen wrote:
> > > > there is some hardware than can do 2D compositing with an arbitrary
> > > > number of planes. I'm not sure what the absolute maximum number of
> > > > planes is, but for the discussion, let's say it is 100.
> > > > 
> > > > There are many complicated, dynamic constraints on how many, what size,
> > > > etc. planes can be used at once. A driver would be able to check those
> > > > before kicking the 2D compositing engine.
> > > > 
> > > > The 2D compositing engine in the best case (only few planes used) is
> > > > able to composite on the fly in scanout, just like the usual overlay
> > > > hardware blocks in CRTCs. When the composition complexity goes up, the
> > > > driver can fall back to compositing into a buffer rather than on the
> > > > fly in scanout. This fallback needs to be completely transparent to the
> > > > user space, implying only additional latency if anything.
> > > 
> > > This looks like a fallback that would use GL to compose the intermediate
> > > buffer. Any reason why that fallback can't be kicked from userspace?
> > 
> > It is not GL, and GL might not be available or desireable. It is still
> > the same 2D compositing engine in hardware, but now running with
> > off-screen target buffer, because it cannot anymore keep up with the
> > continous pixel rate that the direct scanout would need.
> I didn't mean this was GL, but just making the parallel, ie. we wouldn't
> put a GL fallback into the kernel.
> > If we were to use the 2D compositing engine from user space, we would
> > be on the road to OpenWFC. IOW, there is no standard API for the
> > user space to use yet, as far as I'm aware. ;-)
> > 
> > I'm just trying to avoid having to design a kernel driver ABI for a
> > user space driver, then design/implement some standard user space
> > API on top, and then go fix all compositors to actually use it instead
> > of / with KMS.
> It's no easy trade-off. For instance, if the compositor doesn't know
> about some of the hw constraints you are talking about, it may ask the
> kernel for a configuration that suddently will only allow 20 fps updates
> (because of the bw limitation you're mentioning). And the compositor
> just wouldn't know.

Sure, but it would still be much better than the actual fallback in the
compositor in user space, if we cannot drive the 2D engine from user

KMS works the same way already: if you have GL rendering that just
runs for too long, your final pageflip using it will implicitly get
delayed that much. Does it not?

> I can only speak for the hw I know, if you want to squeeze everything
> you can from that simple (compared to the one you're talking about)
> display hw, there's no choice, the compositor needs to know about the
> constraints to make clever decisions (that's what we do on Android). But
> then the appeal of a common interface is understandable.
> (An answer that doesn't actually say anything interesting, oh well),

Yeah... so it comes down to deciding at what point will the kernel
driver say "this won't fly, do something else". And danvet has a pretty
solid answer to that, I think.


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