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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - APL: Fails to detect second external monitor after a 4k monitor is attached to DUT"
href="https://bugs.freedesktop.org/show_bug.cgi?id=107113">107113</a>
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<th>Summary</th>
<td>APL: Fails to detect second external monitor after a 4k monitor is attached to DUT
</td>
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<th>Product</th>
<td>DRI
</td>
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<th>Version</th>
<td>DRI git
</td>
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<th>Hardware</th>
<td>x86-64 (AMD64)
</td>
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<th>OS</th>
<td>Linux (All)
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>major
</td>
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<th>Priority</th>
<td>medium
</td>
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<th>Component</th>
<td>DRM/Intel
</td>
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<th>Assignee</th>
<td>intel-gfx-bugs@lists.freedesktop.org
</td>
</tr>
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<th>Reporter</th>
<td>raviraj.p.sitaram@intel.com
</td>
</tr>
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<th>QA Contact</th>
<td>intel-gfx-bugs@lists.freedesktop.org
</td>
</tr>
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<th>CC</th>
<td>intel-gfx-bugs@lists.freedesktop.org
</td>
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<pre>Created <span class=""><a href="attachment.cgi?id=140465" name="attach_140465" title="dmesg log">attachment 140465</a> <a href="attachment.cgi?id=140465&action=edit" title="dmesg log">[details]</a></span>
dmesg log
When 2 external monitors are connected to an Apollolake system (with one of
them being a 4k monitor), the second monitor is not supported (no display on
second monitor).
>From dmesg logs, this behaviour is because the system memory bandwidth
limitations are hit in the code path.
[drm:skl_compute_wm] Requested display configuration exceeds system DDB
limitations
This is a regression issue post the backporting DRM Modifier patchset in
chromeos-4.4 kernel. Patchsets in question are as below
commit 10d5f23e8e3a29e6b4f9a7bd2de20f12dcee5df6
Author: Ben Widawsky <<a href="mailto:ben@bwidawsk.net">ben@bwidawsk.net</a>>
Date: Fri Nov 10 03:16:35 2017
commit ddaa724757cec3cf443fe6285d1679c6889e7694
Author: Ben Widawsky <<a href="mailto:ben@bwidawsk.net">ben@bwidawsk.net</a>>
Date: Tue Nov 07 01:25:37 2017
Before the above patches were merged, the planes are configured with x-tiling.
With x-tiling, APL was able to drive two external monitors (with atleast one
of them being 4k) at their native resolutions.
Post merging of above patches, the planes are configured with y-tiling. With
y-tiling, the display buffer allocation calculations fail for the second
external monitor.
Currently, during display buffer allocation, the total display buffer blocks
available for the platform are divided equally across the active crtc's. When
Y-tiling is enabled and two external displays are connected, crtc count
increases to 3 and the total available display buffer blocks per crtc becomes
1/3. With a 4k monitor attached to the system, the required minimum blocks to
be allocated exceeds the available per crtc blocks. Thus causing the issue.
Verified the issue with drm-tip kernel and observed the same behavior.
uname -a output:
Linux localhost 4.18.0-rc3-13760-g655f2c563a64-dirty #2 SMP PREEMPT Wed Jul 4
18:31:36 IST 2018 x86_64 Intel(R) Celeron(R) CPU N3350 @ 1.10GHz GenuineIntel
GNU/Linux
Attached is the drm log.
Steps to reproduce:
(1)Connect 4K monitor to Port1
(2)Connect 19x12 monitor to port 2
Observed behaviour: 19X12 can't display</pre>
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