Full-motion zero-copy screen capture in Weston

[Pekka Paalanen]

On Tue, 28 May 2024 20:04:45 +0300
Marius Vlad <marius.vlad at collabora.com> wrote:

> On Tue, May 28, 2024 at 03:53:23PM +0000, Hoosier, Matt wrote:
> > Hi Marius,  
> Hi,
> > 
> > Okay, I guess that answers the bit about needing to screen-scrape to get content into Pipewire now.
> > 
> > But I'm still a little unclear about a couple things, if I were to try to build on this PW backend as a starting point:
> > 
> > First, it looks to me like when you use the PW backend to Weston, that
> > becomes your display. That is, rendering directly targets it. I was
> > hoping for a way to get it to broadcast the very same framebuffer(s)
> > that are getting scanned out for the current frame by the DRM
> > backend.  
> With https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/341
> the PipeWire output can mirror out the native DRM one. It aims at having
> a way to configure VNC, RDP and PipeWire to screen-share the output. 
> > 
> > Second, I'm don't see the path to getting this to leverage the
> > DRM_MODE_CONNECTOR_WRITEBACK hardware (like the weston_screen_recorder
> > does).  I think that any layering would be forced to
> > be offloaded to the GPU ahead of time. Maybe I missed some implication
> > of what you were pointing out here?  
> No sorry, I haven't really implied that, just pointed that out there's
> some work for PipeWire gaining dmabuf. 
> 
> Screen-sharing an output is done
> with multiple backends, and configuring Weston front-end is it with 
> https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/341.

Hi Matt, Marius,

multiple backends necessarily means that each backend renders its own
weston_output independently. While it's fine for most screen-sharing
use cases, I don't think it can ever be as efficient as KMS writeback
could be.

Getting a dmabuf-based video stream from DRM-backend using KMS
writeback needs integration in the DRM-backend. I don't know how that
would look like, but I suspect these ideas would be good:

- builds on and extends the existing writeback support instead of
  something new on the side

- does not use the DRM-backend "virtual output" API; this was a
  workaround for the lack of multi-backend support in the past and has
  the same disadvantages as multi-backend. (I don't think the DRM
  virtual output API has any reason to exist anymore, multi-backend
  should supersede it after converting the users.)

I also think that implementing the usual desktop portals in Weston
would be a good thing, and not something to avoid, if those can provide
a nice external interface for the use case.


Thanks,
pq
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