Bug#792746: linux: Upgrading to thinkpad T440s laptop to 4.0.8-1 kernel causes external monitors to stop working, with "fifo underrun" error

["Nathaniel J. Smith" <njs@pobox.com>]

Source: linux
Version: 4.0.8-1
Severity: important

On my thinkpad T440s (Intel graphics), I often use an external (HDMI) monitor
via a docking station. With the 3.16.7-ckt11 version of the kernel, this works
great. With 4.0.8-1 (current in testing), it doesn't work at all -- it detects
that the monitor is there, Xorg.0.log says:

[   103.360] (II) intel(0): switch to mode 3440x1440@60.0 on DP2-1 using pipe
1, position (1920, 0), rotation normal, reflection none
[   104.092] (II) intel(0): switch to mode 1920x1080@60.0 on eDP1 using pipe 0,
position (0, 0), rotation normal, reflection none

but then the kernel log says:

Jul 17 17:10:21 branna kernel: [   13.488687]
[drm:intel_set_cpu_fifo_underrun_reporting [i915]] *ERROR* uncleared fifo
underrun on pipe A
Jul 17 17:10:21 branna kernel: [   13.488707] [drm:ironlake_irq_handler [i915]]
*ERROR* CPU pipe A FIFO underrun

and the external monitor stays resolutely blank. The gnome display settings
tool thinks that it's active, and that it can be configured; each time I try
reconfiguring it I get the fifo underrun error again. But at no point does the
external monitor leave power-saving mode.

Curiously, if I switch to a text console (ctrl-alt-f1), then the text console
*does* appear on both the built-in and external displays.

The cause is definitely the kernel, because if I reboot and select the older
kernel from the grub menu, while keeping everything else the same, then it
works perfectly again.



-- System Information:
Debian Release: stretch/sid
  APT prefers testing
  APT policy: (900, 'testing'), (500, 'unstable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.16.0-4-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)