[Bug 107113] New: APL: Fails to detect second external monitor after a 4k monitor is attached to DUT

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=107113

            Bug ID: 107113
           Summary: APL: Fails to detect second external monitor after a
                    4k monitor is attached to DUT
           Product: DRI
           Version: DRI git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: major
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: raviraj.p.sitaram at intel.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Created attachment 140465
  --> https://bugs.freedesktop.org/attachment.cgi?id=140465&action=edit
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 <ben at bwidawsk.net>
Date: Fri Nov 10 03:16:35 2017

commit ddaa724757cec3cf443fe6285d1679c6889e7694
Author: Ben Widawsky <ben at bwidawsk.net>
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

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