Flashing black screen

[Reycas]

Hi there,

Recently installed Stephenson's rocket in a Lenovo t420.
During installation, it didn't recognise the graphics card, an intel hd graphics 3000, and recommended to use aticonfig --initial.

desktop@steamos:~$ lspci -v -s 00:02.0
00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09) (prog-if 00 [VGA controller])
	Subsystem: Lenovo Device 21ce
	Flags: bus master, fast devsel, latency 0, IRQ 25
	Memory at f0000000 (64-bit, non-prefetchable) [size=4M]
	Memory at e0000000 (64-bit, prefetchable) [size=256M]
	I/O ports at 5000 [size=64]
	Expansion ROM at <unassigned> [disabled]
	Capabilities: <access denied>
	Kernel driver in use: i915

That didn't work, so I tried installing debian recommended packages for t420:
https://wiki.debian.org/InstallingDebianOn/Thinkpad/T420/jessie

Didn't solve the problem, when lightdm is launched and steam in big picture mode, the screen is flashing in black continuously. That doesn't happens if I launch gdm3, in that case the screen is working correctly, so I supose theres some issue with lightdm conf.

desktop@steamos:~$ sudo journalctl | grep lightdm [sudo] password for desktop: Jan 28 11:42:15 steamos lightdm[681]: pam_unix(lightdm-autologin:session): session opened for user steam by (uid=0) Jan 28 11:43:34 steamos lightdm[649]: ** (lightdm:649): CRITICAL **: session_run: assertion 'session->priv->display_server != NULL' failed Jan 28 11:43:34 steamos lightdm[1219]: pam_unix(lightdm-autologin:session): session opened for user desktop by (uid=0)

The log file is full of DEBUGs and one CRIT alert:
desktop@steamos:~$ sudo cat /var/log/lightdm/lightdm.log | grep CRIT [+80.30s] CRITICAL: session_run: assertion 'session->priv->display_server != NULL' failed

I can't see anything from /var/log/Xorg.0.log

Thanks