Created attachment 118588 [details]
output of dmesg

When logging in and my display is extended to two monitors, or switched between mirror/extended, and also when the screen blanks/sleeps, I get 1-5 seconds of horrible noise via DisplayPort audio out.

The noise is loud enough to damage ears and loudspeakers, if volume has been set to a normal listening level.

I bisected the kernel and found this commit to introduce the regression:

commit fdbc3b1f639bb2cbfb32c612b2699e0ba373317d
Author: Jani Nikula <email address hidden>
Date: Tue Nov 12 17:10:13 2013 +0200

    drm/i915/dp: set sink to power down mode on dp disable

To reproduce, make sure to connect some audio equipment to the audio output of the monitor and make sure you can actually hear audio (e.g. using the sound tests provided by your distribution). Then switch between mirrored and extended display mode, or switch resolutions, or make your laptop go to sleep. You should hear at least a short burst of random static.

WARNING: Do not try reproduce this with headphones, it will damage your ears!

Related Launchpad bug report:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1296224

I'm using a Dell T410s (BIOS 1.50) connected to a Dell U2711 via DisplayPort.

Side note: You asked me to test this on the latest drm-intel-nightly, which happens to be commit eb69e51 for me. The bug is reproducable there, however in addition to that the DP signal seems to be unstable. My monitor randomly blanks from time to time. This report is not about the blanking, rather it's just about the audio noise.

$ uname -a
Linux darkly 4.3.0-rc3-intel #16 SMP Fri Oct 2 10:01:54 CEST 2015 x86_64 x86_64 x86_64 GNU/Linux
$ uname -m
x86_64
$ uname -r
4.3.0-rc3-intel

$ lspci
00:00.0 Host bridge: Intel Corporation Core Processor DRAM Controller (rev 02)
00:02.0 VGA compatible controller: Intel Corporation Core Processor Integrated Graphics Controller (rev 02)
00:16.0 Communication controller: Intel Corporation 5 Series/3400 Series Chipset HECI Controller (rev 06)
00:19.0 Ethernet controller: Intel Corporation 82577LM Gigabit Network Connection (rev 06)
00:1a.0 USB controller: Intel Corporation 5 Series/3400 Series Chipset USB2 Enhanced Host Controller (rev 06)
00:1b.0 Audio device: Intel Corporation 5 Series/3400 Series Chipset High Definition Audio (rev 06)
00:1c.0 PCI bridge: Intel Corporation 5 Series/3400 Series Chipset PCI Express Root Port 1 (rev 06)
00:1c.1 PCI bridge: Intel Corporation 5 Series/3400 Series Chipset PCI Express Root Port 2 (rev 06)
00:1c.3 PCI bridge: Intel Corporation 5 Series/3400 Series Chipset PCI Express Root Port 4 (rev 06)
00:1d.0 USB controller: Intel Corporation 5 Series/3400 Series Chipset USB2 Enhanced Host Controller (rev 06)
00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev a6)
00:1f.0 ISA bridge: Intel Corporation 5 Series/3400 Series Chipset LPC Interface Controller (rev 06)
00:1f.2 SATA controller: Intel Corporation 5 Series/3400 Series Chipset 6 port SATA AHCI Controller (rev 06)
00:1f.3 SMBus: Intel Corporation 5 Series/3400 Series Chipset SMBus Controller (rev 06)
00:1f.6 Signal processing controller: Intel Corporation 5 Series/3400 Series Chipset Thermal Subsystem (rev 06)
03:00.0 Network controller: Intel Corporation Centrino Ultimate-N 6300 (rev 35)
05:00.0 SD Host controller: Ricoh Co Ltd MMC/SD Host Controller (rev 01)
05:00.1 System peripheral: Ricoh Co Ltd R5U2xx (R5U230 / R5U231 / R5U241) [Memory Stick Host Controller] (rev 01)
ff:00.0 Host bridge: Intel Corporation Core Processor QuickPath Architecture Generic Non-core Registers (rev 02)
ff:00.1 Host bridge: Intel Corporation Core Processor QuickPath Architecture System Address Decoder (rev 02)
ff:02.0 Host bridge: Intel Corporation Core Processor QPI Link 0 (rev 02)
ff:02.1 Host bridge: Intel Corporation 1st Generation Core Processor QPI Physical 0 (rev 02)
ff:02.2 Host bridge: Intel Corporation 1st Generation Core Processor Reserved (rev 02)
ff:02.3 Host bridge: Intel Corporation 1st Generation Core Processor Reserved (rev 02)