[Intel-gfx] DP ports on Dell docking station can't drive 4k resolution

[Pankaj Chavan]

Hi All,
My work recently updated our laptops to a Dell Lattitude E7450 laptop and accompanying Dell E-Port plus advanced docking station.  The machine hasBroadwell Core I7 processor and integrated graphics.  I've tried the latest Ubuntu 15.10 (kernel version 4.2.0-25) with updated drivers  from 01.org, and also running Linux Mint 17.3 based on the 14.4 Ubuntu LTS release, which runs LInux kernel version 3.19.0-32.
I have a 32" external 4k monitor connected to it through DP/miniDP cable. I have checked that the monitor is set up to use DP 1.2 protocol.
The problem is that when I connect the monitor through the dock using a DP cable, the monitor can't do 4k resolution. It tops out at 2560x1440 at 60Hz.  However, if I connect the monitor directly to the laptop (with or without the laptop being on the dock), it can drive the full 4k at 60Hz. Moreover, the laptop is able to drive the full 4k resolution on the Windows 7 operating system, so the issue is definitely some device driver related. Needless to say, I do not use Windows at all. Note that I've tried using xrandr on the command line, and also through the GUI widget that Ubuntu provides.

When ever I connect/disconnect a monitor, I do get a bunch of warnings about interrupt controller, and FIFO underruns.
Jan 25 13:03:53 XXXX kernel: [  536.304816] [drm:gen8_irq_handler [i915_bpo]] *ERROR* The master control interrupt lied (SDE)!
....Jan 25 11:41:27 XXXX kernel: [26659.784376] [drm:intel_cpu_fifo_underrun_irq_handler [i915_bpo]] *ERROR* CPU pipe B FIFO underrun

Any help in resolving this would be appreciated. Is there a pointer to an existing bug report/mailing list email?  I am sure I'll soon get tired of the two step process of plonking down the laptop on the dock, and then additionally connecting the display cable to the monitor.

Thanks in advance,
Pankaj
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freedesktop.org/archives/intel-gfx/attachments/20160129/8542fc0a/attachment-0001.html>