[Bug 95206] Display port bandwidth regression - bugzilla-daemon

From: bugzilla-daemon@freedesktop.org
To: dri-devel@lists.freedesktop.org
Subject: [Bug 95206] Display port bandwidth regression
Date: Fri, 29 Apr 2016 11:40:16 +0000	[thread overview]
Message-ID: <bug-95206-502@http.bugs.freedesktop.org/> (raw)

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https://bugs.freedesktop.org/show_bug.cgi?id=95206

            Bug ID: 95206
           Summary: Display port bandwidth regression
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Radeon
          Assignee: dri-devel@lists.freedesktop.org
          Reporter: barneyman@gmx.de

Hi,

I have a mobile laptop (Dell Precision M4600) with an external monitor (Dell
U2414H) connected via display port.

Starting with mainline kernel 4.5.x I'm unable to connect the monitor with mode
(1920x1080p), instead the highest available option is 1920x1080i (interlaced)
which produces heavy flickering on the monitor and makes it unable to use at
all. Forcing the monitor into 1920x1080p (using xrandr --newmode, --addmode and
--mode) causes a total signal loss, however forcing the monitor into
1920x1080p@30hz makes the monitor available again. If I connect the monitor
using DVI/HDMI 1080p@60hz works without any problems. 

So I think the problem might be that the maximum bandwidth of the display port
is reduced compared to the previous version.

Using kernel 4.4.8 the monitor is connected with 1080p using DP as expected
again. So it's definitely a regression introduced in kernel 4.5.

As a side note: the Ubuntu 16.04 kernel (4.4.0-21.37) also has this display
port problem, but as far as I know the ubuntu devs have backported large
portions of the 4.5 radeon patches.

If you need any further information (dmesg, xrandr output), don't hesitate to
ask...

Kind regards,
Christian

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