On 2021/06/23 17:04, Michel Dänzer wrote:

On 2021-06-22 9:57 a.m., Pekka Paalanen wrote:

quoted

On Tue, 22 Jun 2021 13:02:59 +0900
Esaki Tomohito [off-list ref] wrote:

quoted

Hi, Thomas
Thank you for reply.

On 2021/06/21 16:10, Thomas Zimmermann wrote:

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Hi

Am 21.06.21 um 08:27 schrieb Tomohito Esaki:  

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Virtual DRM splits the overlay planes of a display controller into
multiple
virtual devices to allow each plane to be accessed by each process.

This makes it possible to overlay images output from multiple
processes on a
display. For example, one process displays the camera image without
compositor
while another process overlays the UI.  

I briefly looked over your patches. I didn't understand how this is
different to the functionality of a compositor? Shouldn't this be solved
in userspace?  

I think when latency is important (e.g., AR, VR, for displaying camera
images in IVI systems), there may be use cases where the compositor
cannot be used.

Hi,

quoted

Normally, when the image is passed through the compositor, it is
displayed after 2 VSYNC at most, because the compositor combines the
image with VSYNC synchronization.

This is not a universal fact. You can write a Wayland compositor that
consistently reaches app-to-screen latency of less than one monitor
refresh cycle, while also using KMS planes.

I believe Weston succeeds in this already if you write the Wayland
application accordingly.

For a specific example, https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1620 allows app-to-screen latency as low as ~6 ms (including a fixed 2 ms buffer to avoid skipped frames). mutter doesn't use KMS planes yet, but if anything I'd expect that to help rather than hurt for latency (if the compositor doesn't need to draw anything).

Thank you for providing specific examples.

Best regards
Esaki