Full-motion zero-copy screen capture in Weston

[Pekka Paalanen]

On Mon, 3 Jun 2024 17:57:18 +0000
"Hoosier, Matt" <Matt.Hoosier at garmin.com> wrote:

> > What do you mean you can capture all virtual machines with KMS
> > writeback?
> >
> > What's the architecture there? How do virtual machines access KMS
> > hardware? Why would Weston be able to capture their outputs at all?
> >  
> 
> I hope to have more to say about this shortly.

I'm interested because I worry whether it will work. I don't see how it
could work with the traditional display controllers and DRM drivers.

Maybe some kind of hardware assisted plane "leasing" that works around
the DRM KMS software model limitations?

> > The disadvantage is that buffer allocations are accounted to the
> > compositor (which can manage its own memory consumption, sure), and
> > either the compositor allocates a new buffer for every frame
> > (possibly serious overhead) or it needs to wait for all consumers
> > to tell that they are done with the buffer before it can be used
> > again. Clients might hold on to the buffer indefinitely or just a
> > little too long, which is the risk.  
> 
> It seems to me this would be a risk of the existing Pipewire backend,
> no? At least, if I understood the buffering model of the dmabuf MR
> that Marius linked earlier.
> 

I haven't looked at any of that, so I can't say. I don't know if
pipewire is only allocate-send-forget, or does it include consumers
signalling they are done to allow re-use as well. Or maybe the sources
trust that rotating N buffers is always enough, and if a sink needs the
data for longer, it will make a copy in time.

It's a trade-off. Sometimes the overhead of allocate-send-forget with
the risk of OOM (KMS writeback might require a specific type of memory
that could be scarce), maybe even with an added copy to avoid
exhausting writeback-capable memory, may be justified.


Thanks,
pq
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