In the end it turned out to a small typo, s/scanout/active_scanout/ in consideration of reusing the cache. Though there are a number of very useful debug improvements along the way.
6a6efd3 sna/dri2: Track tracking of active-scanout for back buffer replacement
src/sna/sna_display.c | 36 +++++++++++++++++++++++++++++++++---
src/sna/sna_dri2.c | 27 +++++++++++++++++++++------
2 files changed, 54 insertions(+), 9 deletions(-)
Upstream: cgit.freedesktop.org