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Bug#792746: marked as done (Upgrading to 4.0.8-1 kernel makes external monitors unusable)

Your message dated Wed, 13 Jul 2016 01:59:07 +0100
with message-id <1468371547.4174.26.camel@decadent.org.uk>
and subject line Re: Bug#792746: Upgrading to 4.0.8-1 kernel makes external monitors unusable
has caused the Debian Bug report #792746,
regarding Upgrading to 4.0.8-1 kernel makes external monitors unusable
to be marked as done.

This means that you claim that the problem has been dealt with.
If this is not the case it is now your responsibility to reopen the
Bug report if necessary, and/or fix the problem forthwith.

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-- 
792746: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=792746
Debian Bug Tracking System
Contact owner@bugs.debian.org with problems
--- Begin Message --- 
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)

--- End Message ---
--- Begin Message --- 
Version: 4.6.3-1

This seems to be fixed in or before the above version.

Ben.

-- 

Ben Hutchings
Sturgeon's Law: Ninety percent of everything is crap.

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