Warning at drivers/gpu/drm/drm_atomic_helper.c

[Fabio Estevam]

On Mon, Jan 23, 2017 at 1:15 AM, Marek Vasut <marex at denx.de> wrote:

> IMHO this will no longer handle the fences correctly and you'll see
> rendering artifacts sometimes.

Ok, for 4.9.x stable we cannot apply your 782ea2a493ba90800 ("drm/imx: Switch
to drm_fb_cma_prepare_fb() helper") as it depends on many things that
only show up on 4.10-rc1, so we need to provide a different fix for
4.9.x.

What I did not figure out so far is why are we getting sometimes the
WARN_ON(!plane_state->fb); inside drm_atomic_helper_wait_for_fences()?

The way to reproduce this issue on 4.9.x:

1. Start kmscube
2. Stop kmscube with CTRL + C

Repeate this sequence several times (10 to 20 times) and the
WARN_ON(!plane_state->fb) will trigger about 20% of the times.