After I updated my kernel to
https://cgit.freedesktop.org/~agd5f/linux/log/?h=amd-staging-drm-next as of
today (latest commit at this time:    
1c630e83443a0f271c192ecfa0d94023661a0000) I noticed that my computer would no
longer wake up from S3 sleep - power LED goes on, but apart from that no
display and no reaction to any input (other than Num-Lock still switchable).

The difference to the ~2 weeks older kernel was 100% reproducable by just
doing:

- boot to console (no start of X11 required) and login as root
- echo "mem" >/sys/power/state
- wait till power LED blinks
- press some key to wake up the system
- observe no display output and no reaction to anything

GPU is a RX 460, CPU a Ryzen R7 1800X, board Asus X370 Pro, connected is one 4k
display via HDMI 2.0.

I bisected the git commits, and the one that causes the bug is:

>  7ae4acd21e9e264afb079e23d43bcf2238c7dbea
>  drm/amd/display: Match actual state during S3 resume.

After this, I went back to the current HEAD of amd-staging-drm-next and
(manually) reverted only commit 7ae4acd21e9e264afb079e23d43bcf2238c7dbea, and
this indeed results in a kernel that works fine with regards to resuming from
S3 sleep.

There are no noteworthy "dmesg"-emissions that accompany going to S3 sleep, but
the dmesg-output that I can collect ends slightly before the system actually
sleeps, so wether anything is emitted upon the wake-up attempt, I have no way
to know.