kms: Try to preserve existing output routing as weston would do

This is basically a backport from weston:
https://github.com/wayland-project/weston/commit/75487c2560dfca033d8d23649914efb4199356dd

It states that:

 - preserving the existing CRTC -> encoder -> connector routing will
   make the startup faster
 - the existing routing may be set according to some device limitations

Since the QtWayland implementation appears to be based off an old
version of weston (maybe pre-2.0), this patch might look different from
the latest weston implementation. But the idea stays the same.

FWIW, this works around the issue I've seen on Renesas R-Car E3 (aka
Ebisu) board. Without this patch, VGA1+HDMI1 setup would fail because
the device reported possible_crtcs=1 (meaning {crtcs[0]}) for the second
screen "HDMI1", but the crtcs[0] was already taken by the first screen
"VGA1".

  No usable crtc/encoder pair for connector "HDMI1"

With this patch, the crtcs[1] is paired with "VGA1" (as it is the
existing routing), so the crtcs[0] can be allocated for "HDMI1".

Change-Id: Ibd304a8b5efbe4a8aa94b2c5697fe2b399386280
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

0 files changed, 0 insertions, 0 deletions