Cuttlefish's streaming server, which acts as a Wayland compositor
in order to receive display framebuffers from Crosvm, needs some
mechanism to tell which Wayland surface corresponds to which display
(a "display" is a "scanout" in virtio-gpu terminology).
Wayland object ids can not be directly used for this as all Wayland
objects share a single global id space (so the first created Wayland
wl_surface surface object may have id = 15).
Previously, the case of unchanging displays was handled by enforcing
the creation order of surfaces within Crosvm so that Cuttlefish's
streaming server (which is a Wayland compositor) could assume the
creation order corresponded to the display order. However, this still
experienced issues (b:186580833) when surfaces were destroyed and
later recreated when handling `set_scanout(..., resource_id = 0)`
commands.
There is also an ongoing effort to support adding and removing
displays at runtime in (see aosp/1671968) which experiences the
same issue. When surfaces are arbitrarily created and destroyed,
Cuttlefish's streaming server has no way to determine which Wayland
surface corresponds to which display.
To solve all of this, this change introduces an extension to allow
Wayland clients (Crosvm) to attach additional metadata (scanout_id)
to Wayland objects (surfaces) so that Wayland compositors (Cuttlefish's
streaming server) can exactly determine which surfaces correspond
to which displays. I will attempt to upstream this protocol (tracked
in b:191901112).
BUG=b:188904670
BUG=b:187351899
BUG=b:191901112
TEST=launch Cuttlefish with single display
TEST=launch Cuttlefish with multiple displays
TEST=launch Cuttlefish and hotplug some displays
Change-Id: I2aa4b714a49e4d85b6a3c705ba0d5bc1720b838e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2909903
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Dennis Kempin <denniskempin@google.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Dennis Kempin <denniskempin@chromium.org>
Commit-Queue: Jason Macnak <natsu@google.com>