lost screen resolution after Debian/Jessie upgrade

[Joachim Wuttke]

After update/upgrade within Debian/Jessie (affecting some 50 packages) 
and a reboot (after an uptime of about a month), the full resolution 
2560x1440 of my Dell U2713HM has gone. xrandr is listing at most 1920x1200.

Given the long history of my installation, I tried a radical cure, 
purging all xorg related packages, and reinstalling them. No effect, 
still limited to 1920x1200.

Then I tried

~$ cvt 2560 1440
# 2560x1440 59.96 Hz (CVT 3.69M9) hsync: 89.52 kHz; pclk: 312.25 MHz
Modeline "2560x1440_60.00"  312.25  2560 2752 3024 3488  1440 1443 1448 
1493 -hsync +vsync
~$ xrandr --newmode "2560x1440"  312.25  2560 2752 3024 3488  1440 1443 
1448 1493 -hsync +vsync
~$ xrandr --addmode HDMI1 2560x1440
~$ xrandr  --output  HDMI1 --mode 2560x1440
xrandr: Configure crtc 0 failed

The only trace of this in Xorg.0.log is the following:

[  3263.882] (II) intel(0): Allocated new frame buffer 2560x1440 stride 
10240, tiled
[  3264.048] (EE) intel(0): failed to set mode: Invalid argument
[  3264.080] (II) intel(0): Allocated new frame buffer 1920x1200 stride 
7680, tiled

According to lspci -v, my video device is the following:

00:02.0 VGA compatible controller: Intel Corporation Xeon E3-1200 v3/4th 
Gen Core Processor Integrated Graphics Controller (rev 06) (prog-if 00 
[VGA controller])
     Subsystem: Fujitsu Technology Solutions Device 11e7
     Flags: bus master, fast devsel, latency 0, IRQ 46
     Memory at f7800000 (64-bit, non-prefetchable) [size=4M]
     Memory at e0000000 (64-bit, prefetchable) [size=256M]
     I/O ports at f000 [size=64]
     Expansion ROM at <unassigned> [disabled]
     Capabilities: [90] MSI: Enable+ Count=1/1 Maskable- 64bit-
     Capabilities: [d0] Power Management version 2
     Capabilities: [a4] PCI Advanced Features
     Kernel driver in use: i915

Since I installed the full xorg dependency package, missing drivers is 
not a likely explanation.

Old and new log files at http://apps.jcns.fz-juelich.de/tmp/xorg-posting/.

How to proceed?

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