Hi, Thomas Thank you for reply.
On 2021/06/21 16:10, Thomas Zimmermann wrote:
Hi
Am 21.06.21 um 08:27 schrieb Tomohito Esaki:
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. 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. On the other hand, if we use vDRM, the image will be displayed at the next VSYNC, so it will be displayed after 1 VSYNC at most.
Also, since the compositor is a single point of failure, we may not want to make it dependent on it.
Best regards Tomohito Esaki
On Tue, 22 Jun 2021 13:02:59 +0900 Esaki Tomohito etom@igel.co.jp wrote:
Hi, Thomas Thank you for reply.
On 2021/06/21 16:10, Thomas Zimmermann wrote:
Hi
Am 21.06.21 um 08:27 schrieb Tomohito Esaki:
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,
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.
Thanks, pq
On 2021-06-22 9:57 a.m., Pekka Paalanen wrote:
On Tue, 22 Jun 2021 13:02:59 +0900 Esaki Tomohito etom@igel.co.jp wrote:
Hi, Thomas Thank you for reply.
On 2021/06/21 16:10, Thomas Zimmermann wrote:
Hi
Am 21.06.21 um 08:27 schrieb Tomohito Esaki:
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,
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).
On 2021/06/23 17:04, Michel Dänzer wrote:
On 2021-06-22 9:57 a.m., Pekka Paalanen wrote:
On Tue, 22 Jun 2021 13:02:59 +0900 Esaki Tomohito etom@igel.co.jp wrote:
Hi, Thomas Thank you for reply.
On 2021/06/21 16:10, Thomas Zimmermann wrote:
Hi
Am 21.06.21 um 08:27 schrieb Tomohito Esaki:
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,
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
Hi
Am 22.06.21 um 06:02 schrieb Esaki Tomohito:
Hi, Thomas Thank you for reply.
On 2021/06/21 16:10, Thomas Zimmermann wrote:
Hi
Am 21.06.21 um 08:27 schrieb Tomohito Esaki:
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. 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. On the other hand, if we use vDRM, the image will be displayed at the next VSYNC, so it will be displayed after 1 VSYNC at most.
Other commenters already addressed these points.
Also, since the compositor is a single point of failure, we may not want to make it dependent on it.
The kernel is also a single point of failure.
TBH I don't think this feature should be merged until there's a clear use case that cannot be solved in userspace idiomatically.
Best regards Thomas
Best regards Tomohito Esaki
dri-devel@lists.freedesktop.org
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Esaki Tomohito -
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Pekka Paalanen -
Thomas Zimmermann