[Bug 95206] Display port bandwidth regression

[Bug 95206] Display port bandwidth regression

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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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[Bug 95206] Display port bandwidth regression

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

--- Comment #1 from Alex Deucher <alexdeucher@gmail.com> ---
Please attach your xorg log and dmesg output.  Can you bisect?

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[Bug 95206] Display port bandwidth regression

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

--- Comment #2 from barneyman@gmx.de ---
Created attachment 123350
  --> https://bugs.freedesktop.org/attachment.cgi?id=123350&action=edit
dmesg kernel 4.5.2

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[Bug 95206] Display port bandwidth regression

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

--- Comment #3 from barneyman@gmx.de ---
Created attachment 123351
  --> https://bugs.freedesktop.org/attachment.cgi?id=123351&action=edit
Xorg.0.log kernel 4.5.2

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[Bug 95206] Display port bandwidth regression

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

--- Comment #4 from barneyman@gmx.de ---
Created attachment 123352
  --> https://bugs.freedesktop.org/attachment.cgi?id=123352&action=edit
xrandr output kernel 4.5.2

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[Bug 95206] Display port bandwidth regression

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--- Comment #5 from barneyman@gmx.de ---
Attachments added...

What do you mean with bisect?

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[Bug 95206] Display port bandwidth regression

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

--- Comment #6 from Felix Schwarz <felix.schwarz@oss.schwarz.eu> ---
"bisecting" is a way to find out which commit caused a specific regression.
This involves compiling the linux kernel from git and testing the compiled
versions. If you can find out which commit is the culprit chances are pretty
good that the problem can be fixed quickly.

To learn more about bisecting I suggest seaching for "git bisect".

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[Bug 95206] Display port bandwidth regression

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

--- Comment #7 from barneyman@gmx.de ---
Ok, understood. Didn't know that feature. I'll give it a try in the next days.

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[Bug 95206] Display port bandwidth regression

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

--- Comment #8 from Christian L. <barneyman@gmx.de> ---
Hi,

I've bisected the problem down to the following commit to the mainline kernel
(see below). Can I help with some further information?

092c96a8ab9d1bd60ada2ed385cc364ce084180e is the first bad commit
commit 092c96a8ab9d1bd60ada2ed385cc364ce084180e
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Thu Dec 17 10:23:34 2015 -0500

    drm/radeon: fix dp link rate selection (v2)

    Need to properly handle the max link rate in the dpcd.
    This prevents some cases where 5.4 Ghz is selected when
    it shouldn't be.

    v2: simplify logic, add array bounds check

    Reviewed-by: Tom St Denis <tom.stdenis@amd.com>
    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

:040000 040000 c27fa6e02534c3ef9f53c3c954fa936b1e414cb3
0be8f64ebe28383efa39ebce68247b148f4a78f2 M    drivers

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[Bug 95206] Display port bandwidth regression

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

--- Comment #9 from Alex Deucher <alexdeucher@gmail.com> ---
Created attachment 123611
  --> https://bugs.freedesktop.org/attachment.cgi?id=123611&action=edit
add debugging output

Can you attach the dmesg output with the attached debugging patch applied?

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[Bug 95206] Display port bandwidth regression

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

--- Comment #10 from Christian L. <barneyman@gmx.de> ---
Created attachment 123618
  --> https://bugs.freedesktop.org/attachment.cgi?id=123618&action=edit
dmesg with debugging patch

attached dmesg output of mainline kernel 4.5.0 with your debugging patch
applied

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[Bug 95206] Display port bandwidth regression

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--- Comment #11 from Alex Deucher <alexdeucher@gmail.com> ---
Created attachment 123629
  --> https://bugs.freedesktop.org/attachment.cgi?id=123629&action=edit
possible fix

This patch should fix it.

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--- Comment #12 from Christian L. <barneyman@gmx.de> ---
Your fix works like a charm. Thanks alot!

Applied the fix to 4.5.0 mainline and the problem is gone.

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[Bug 95206] Display port bandwidth regression

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--- Comment #13 from mrj@advancedcontrols.com.au ---
May be causing https://bugzilla.redhat.com/show_bug.cgi?id=1402293

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Alex Deucher <alexdeucher@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #14 from Alex Deucher <alexdeucher@gmail.com> ---
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ff0bd441bdfbfa09d05fdba9829a0401a46635c1

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mrj@advancedcontrols.com.au changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|FIXED                       |---
             Status|RESOLVED                    |REOPENED

--- Comment #15 from mrj@advancedcontrols.com.au ---
Sorry Alex, I wasn't clear.

It's the fixed version that (I've now verified) is causing the Radeon HD6320
problem as described here

https://bugzilla.redhat.com/show_bug.cgi?id=1402293

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--- Comment #16 from mrj@advancedcontrols.com.au ---
The (native) display resolution exhibiting the problem on the Radeon HD6320
(AMD G-series T56N) is 1366x768.

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--- Comment #17 from mrj@advancedcontrols.com.au ---
The good and bad kernels give identical xrandr outputs:

Screen 0: minimum 320 x 200, current 1366 x 768, maximum 8192 x 8192
XWAYLAND0 connected 1360x768+0+0 520mm x 320mm
   1360x768      59.98*+

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--- Comment #18 from Alex Deucher <alexdeucher@gmail.com> ---
(In reply to mrj from comment #15)
> Sorry Alex, I wasn't clear.
> 
> It's the fixed version that (I've now verified) is causing the Radeon HD6320
> problem as described here
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1402293

Please open a new bug as this one has been addressed.

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[Bug 95206] Display port bandwidth regression

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Alex Deucher <alexdeucher@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|---                         |FIXED

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--- Comment #19 from mrj@advancedcontrols.com.au ---
Alex, I've created Bug 99326.

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