[gentoo-user] x11-misc/sddm-0.18.1-r6 exits inexplicably

[Michael]

I'm trying to understand why one PC in particular fails to start SDDM since an 
update to x11-misc/sddm-0.18.1-r6 and ends up with a blank page on VT7.  I 
suspect some pam module malfunction, but I don't understand why this happens 
or how to troubleshoot it.  Interestingly, if I restart the display-manager 
service manually, SDDM starts and no longer exits.  :-/

This is what syslog shows upon boot:
================================
Apr  9 09:20:04 localhost sddm-helper[4074]: pam_unix(sddm-greeter:session): 
session opened for user sddm(uid=104) by (uid=0)
Apr  9 09:20:04 localhost kernel: elogind-daemon[1349]: New session c1 of user 
sddm.
Apr  9 09:20:05 localhost chronyd[3554]: Selected source 192.169.1.1
Apr  9 09:20:05 localhost sddm-helper[4074]: pam_unix(sddm-greeter:session): 
session closed for user sddm
Apr  9 09:20:05 localhost kernel: elogind-daemon[1349]: Removed session c1.
==========================================

Checking if the display manager service has died I see rc-service reports it 
is running!  Then, ps reveals this much:

root      3757  0.1  0.1 137296 17092 ?        Ssl  12:21   0:00 /usr/bin/sddm
root      3898  5.7  0.5 1009092 85500 tty7    Ssl+ 12:21   0:02 /usr/bin/X -
nolisten tcp -auth /var/run/sddm/{77032685-d24f-482b-9634-49fce5f11fa0} -
background none -noreset -displayfd 17 -seat seat0 vt7

but all I get on VT7 is a blank screen with a cursor on the top left and no 
mouse or a graphic DM.


Then I login as root on a console and restart the service:
================================================
Apr  9 09:20:38 localhost login[3935]: pam_unix(login:session): session opened 
for user root(uid=0) by LOGIN(uid=0)
Apr  9 09:20:38 localhost kernel: elogind-daemon[1349]: New session 2 of user 
root.
Apr  9 09:20:38 localhost login[4087]: ROOT LOGIN  on '/dev/tty1'
Apr  9 09:20:41 localhost start-stop-daemon[4111]: Will stop /usr/bin/sddm
Apr  9 09:20:41 localhost start-stop-daemon[4112]: Will stop /usr/bin/sddm
Apr  9 09:20:41 localhost start-stop-daemon[4112]: Will stop PID 3787
Apr  9 09:20:41 localhost start-stop-daemon[4112]: Sending signal 15 to PID 
3787
Apr  9 09:20:41 localhost kernel: [drm] enabling PCIE gen 2 link speeds, 
disable with radeon.pcie_gen2=0
Apr  9 09:20:41 localhost kernel: [drm] PCIE GART of 1024M enabled (table at 
0x0000000000040000).
Apr  9 09:20:41 localhost kernel: radeon 0000:01:00.0: WB enabled
Apr  9 09:20:41 localhost kernel: radeon 0000:01:00.0: fence driver on ring 0 
use gpu addr 0x0000000080000c00
Apr  9 09:20:41 localhost kernel: radeon 0000:01:00.0: fence driver on ring 1 
use gpu addr 0x0000000080000c04
Apr  9 09:20:41 localhost kernel: radeon 0000:01:00.0: fence driver on ring 2 
use gpu addr 0x0000000080000c08
Apr  9 09:20:41 localhost kernel: radeon 0000:01:00.0: fence driver on ring 3 
use gpu addr 0x0000000080000c0c
Apr  9 09:20:41 localhost kernel: radeon 0000:01:00.0: fence driver on ring 4 
use gpu addr 0x0000000080000c10
Apr  9 09:20:41 localhost kernel: debugfs: File 'radeon_ring_gfx' in directory 
'1' already present!
Apr  9 09:20:41 localhost kernel: debugfs: File 'radeon_ring_cp1' in directory 
'1' already present!
Apr  9 09:20:41 localhost kernel: debugfs: File 'radeon_ring_cp2' in directory 
'1' already present!
Apr  9 09:20:41 localhost kernel: debugfs: File 'radeon_ring_dma1' in 
directory '1' already present!
Apr  9 09:20:41 localhost kernel: debugfs: File 'radeon_ring_dma2' in 
directory '1' already present!
Apr  9 09:20:42 localhost kernel: [drm] ring test on 0 succeeded in 1 usecs
Apr  9 09:20:42 localhost kernel: [drm] ring test on 1 succeeded in 1 usecs
Apr  9 09:20:42 localhost kernel: [drm] ring test on 2 succeeded in 1 usecs
Apr  9 09:20:42 localhost kernel: [drm] ring test on 3 succeeded in 4 usecs
Apr  9 09:20:42 localhost kernel: [drm] ring test on 4 succeeded in 4 usecs
Apr  9 09:20:42 localhost kernel: [drm] ib test on ring 0 succeeded in 0 usecs
Apr  9 09:20:42 localhost kernel: [drm] ib test on ring 1 succeeded in 0 usecs
Apr  9 09:20:42 localhost kernel: [drm] ib test on ring 2 succeeded in 0 usecs
Apr  9 09:20:42 localhost kernel: [drm] ib test on ring 3 succeeded in 0 usecs
Apr  9 09:20:42 localhost kernel: [drm] ib test on ring 4 succeeded in 0 usecs
Apr  9 09:20:43 localhost sddm-helper[4209]: pam_unix(sddm-greeter:session): 
session opened for user sddm(uid=104) by (uid=0)
Apr  9 09:20:43 localhost kernel: elogind-daemon[1349]: Existing logind 
session ID 2 used by new audit session, ignoring.
Apr  9 09:20:43 localhost kernel: elogind-daemon[1349]: New session c2 of user 
sddm.
=====

Thereafter SDDM runs on VT7 as expected without exiting.  The ps output shows 
more:

root      4187  0.0  0.1 137444 15656 ?        Ssl  12:22   0:00 /usr/bin/sddm
root      4197  3.8  0.5 1009092 83352 tty7    Ssl+ 12:22   0:01 /usr/bin/X -
nolisten tcp -auth /var/run/sddm/{9cb9ef8e-f25b-4272-9605-8da9ddbef75c} -
background none -noreset -displayfd 17 -seat seat0 vt7
root      4213  0.0  0.0  54060 12948 ?        S    12:22   0:00 /usr/libexec/
sddm-helper --socket /tmp/sddm-auth500f20d1-4477-420c-9eec-3a53462ef3e6 --id 2 
--start /usr/bin/sddm-greeter --socket /tmp/sddm-:0-ivjakp --user sddm --
greeter
sddm      4214  3.5  0.7 997596 111548 ?       Sl   12:22   0:01 /usr/bin/
sddm-greeter --socket /tmp/sddm-:0-ivjakp
sddm      4220  0.0  0.0   4696  2408 ?        S    12:22   0:00 dbus-launch 
--autolaunch dd882e08972cbb412f072c225d00d9b6 --binary-syntax --close-stderr
sddm      4221  0.0  0.0   4824  2040 ?        Ss   12:22   0:00 /usr/bin/
dbus-daemon --syslog-only --fork --print-pid 5 --print-address 7 --session

Any ideas why this is happening, on this PC only and how to fix it?